Bosch 520-HN-N, 520-HN-L User Instructions

520 HN

Flow Modulated with Hydro-generated Ignition
Suitable for heating potable water only - Not approved for space heating purposes
(Intended for variable flow applications with steady cold water inlet temperatures)

520-HN-N
520-HN-L

6 720 644 942 (2010/09) US

Warning: If the information in this manual is not
followed exactly, a fire or explosion may result
causing property damage, personal injury or death.
Do not store or use gasoline or other flammable
vapor and liquids in the vicinity of this or any other
appliance.

Improper installation, adjustment, alteration,
service or maintenance can cause injury or
property damage. Refer to this manual. For
assistance or additional information consult a
qualified installer, service agency or the gas
supplier.
In the Commonwealth of Massachusetts this
product must be installed by a licensed plumber or
gas fitter.

Upon completion of the installation, these
instructions should be handed to the user of the
appliance for future reference.

What to do if you smell gas

• Close gas valve. Open windows.

• Do not try to light any appliance.

• Do not touch any electrical switch; do not use any
phone in your building.

• Immediately call your gas supplier from a neighbor’s
phone. Follow the gas supplier’s instructions.

• If you cannot reach your gas supplier, call the fire
department.

• Installation and service must be performed by a
qualified installer, service agency or the gas supplier.

6 720 644 942

2

Index

Index

1 Warning 2

2 Appliance details 3

2.1 520 HN specifications (Technical data) 3

2.2 Unpacking the 520 HN heater 4

2.3 General rules to follow for safe operation 4

2.4 Dimensions and installation clearances 5

3 Installation instructions 6

3.1 Introduction 6

3.2 Proper location for installing your heater 6

3.3 Venting 6

3.4 Combustion air requirements 9

3.5 Mounting Heater 9

3.6 Gas piping & connections 11

3.7 Measuring gas pressure 13

3.8 Water connections 13

3.9 Recirculation application 14

4 Operation instructions 14

4.1 For your safety, read before operating your water heater 14

4.2 Lighting instructions 15

4.3 To turn off appliance 15

4.4 Adjusting the water temperature 15

4.5 Power adjustment knob 16

4.6 Draining the appliance 16

5 Maintenance and service 17

5.1 Maintenance intervals 17

5.2 Water valve 17

5.3 Pilot 17

5.4 Main burners 18

5.5 Vent assembly 18

5.6 Mineral scale build-up 18

6 Troubleshooting 19

6.1 Introduction 19

6.2 No spark at pilot with water flow 19

6.3 Spark appears when hot water is turned on,
but pilot and burners will not ignite 19

6.4 Pilot lights, but burners will not come on. 20

6.5 Main burners go out during hot water use 20

6.6 Hot water temperature fluctuates at tap 21

6.7 Water is too hot 21

6.8 Water is not hot enough 22

6.9 Low hot water pressure 22

6.10 Noise when heater is running 22

6.11 Burners do not burn cleanly, yellow
flames when operating 22

7 Protecting the environment 23

8 Interior components and diagram parts list 24

8.1 Interior components 24

8.2 Components diagram 25

8.3 Parts list 26

9 Fifteen Year Limited Warranty 27

1 Warning

Fig. 1

Warning: The heater must be isolated
from the gas supply piping system
during any pressure testing of that
system at test pressures equal to or
more than 0.5 psig.

Caution: Any changes or
modifications not expressly approved
by the party responsible for compliance
could void the user’s authority to
operate the equipment.

HOT

BURN

D A N G E R

Watertemperatureover125°Fcan
causesevereburnsinstantlyor
deathfromscalds.

Children,disabledandelderlyare
athighestriskofbeingscalded.

Seeinstructionmanualbefore
settingtemperatureatwater
heater.

Feelwaterbeforebathingor
showering.

6720608643-16.1AL

6 720 644 942

Appliance details

3

2 Appliance details

2.1 520 HN specifications (Technical
data)

Approved in US/Canada

Capacity

520 HN: 4.36 GPM

Minimum input

520 HN: 30,735 Btu/hr

Maximum input

520 HN: 117 000 Btu/hr

Min. Output

520 HN: 23 906 Btu/hr

Maximum output

520 HN: 91 525 Btu/hr

Efficiency in %

Efficiency 80%

Gas Requirement

Gas connection - ¾” NPT

Inlet gas pressure under maximum operation:*

• Propane: 10.5” - 14” water column

• Natural Gas: 5.7” - 14” water column.

* To measure Gas Pressure, see Measuring Gas
Pressure, chapter 3.7.

Venting

• Natural Draft

• Vent diameter - 5" or 6"

• Minimum height (feet) - 6’ with no elbows

• Vertical termination through roof or into chimney.

• Sidewall termination only with AQ4 power vent kit.

Water

• Hot water connection - ½” NPT

• Cold water connection - ½” NPT

• Water valve material: Fiberglass reinforced polymide

(PA)

• Minimum water flow to activate heater: 0.5 gallon/

minute (1.9 l/m)

• Minimum recommended water pressure: 30 PSI

(2.07 bar)

• Connections:

– Bottom of heater

• Temperature rises:

1)

– 45F rise @ 4.1 gpm
– 50F rise @ 3.7 gpm
– 55F rise @ 3.4 gpm
– 65F rise @ 2.8 gpm
– 77F rise @ 2.4 gpm
– 90F rise @ 2.0 gpm

Dimensions

• Depth (in): 8.66” (220 mm)

• Width (in): 16.73” (425 mm)

• Height (in): 25.75” (655 mm)

• Weight: 35 pounds (16 kg).

Gas types

Natural Gas
LP Gas

Safety devices

• Flame failure device (ionization flame rod)

• Pressure relief valve (Available as accessory)

• Overheat prevention (temperature limiter).

Water protection

IP X4.

Accessories (Bosch part #)

• Freeze prevention kit (7709003775)

2)

• Pressure relief valve (FWL-2).

1) Depending on gas pressure and altitude.

2) The freeze prevention kit is designed to provide protec­tion for the water heater down to approximately 5°F for
short term conditions only. It will not protect the appli­ance in areas where the temperature is routinely
expected to be below freezing.

- The freeze prevention kit will not protect plumbing out­side the appliance from freezing. Precautions should be
taken.

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4

Appliance details

2.2 Unpacking the 520 HN heater

Before installing the unit, be certain you have the
correct heater for your type of Gas - Propane or
Natural Gas. Identification labels are found on the

shipping box, and on the rating plate which is located on
the right side panel of the cover.

B Installer must fill out checklist on back cover to

provide details in case service or warranty cov­erage is required.

Fig. 2 Rating plate

A Serial number
B Type of gas

The box includes:

• Hot and cold water connection fittings

• Mounting screws

• Product registration card

• Installation manual

• Incandescent particle tray.

Do not lose this manual, there is a charge for a
replacement.

Please complete and return the enclosed product
registration card.

The 520 HN is not approved or designed for:

• Manufactured (mobile) homes, RV's or boats

• Heating or other recirculating/pumping applications*

• Solar/preheat backup or high temperature booster
use

• Installation in bathroom, bedroom or other occupied
rooms normally kept closed.

* This includes domestic hot water circulator pump loop
systems that may be installed in home hot water system
prior to installing this unit. An approved recirculation
design can be found in chapter 3.9.

2.3 General rules to follow for safe
operation

1. You must follow these instructions when you install

your heater. In the United States: The installation must
conform with local codes or, in the absence of local
codes, the National Fuel Gas Code ANSI Z223.1/
NFPA 54.
In Canada: The Installation must conform with CGA
B149.(1,2) INSTALLATION CODES and /or local
installation codes.

2. Carefully plan where to install the heater. Proper

clearances must be followed.

3. The appliance must be isolated from the gas supply

piping system by closing its individual manual gas
shutoff valve (not supplied with heater) during any
pressure testing in excess of ½ Psig (3.5 kPa).
The appliance and its gas connection must be leak
tested before placing the appliance in operation.

4. Keep water heater area clear and free from

combustibles and flammable liquids. Do not locate the
heater over any material which might burn.

5. Correct gas pressure is critical for the optimum

operation of this heater. Gas piping must be sized to
provide the required pressure at the maximum output of
the heater, while all the other gas appliances are in
operation. Check with your local gas supplier, and see
chapter 3.6 and 3.7 to verify proper gas line sizing.

6. Should overheating occur or the gas supply fail to

shut off, turn off the gas supply at the manual gas shut
off valve, on the gas line. Note: manual gas shutoff valve
is not supplied with the heater.

7. Do not use this appliance if any part has been

underwater. Immediately call a qualified service
technician to inspect the appliance and to replace any
part of the control system and any gas control which
has been underwater.

i

BOSCH is constantly improving its
products, therefore specifications are
subject to change without prior notice.

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Appliance details

5

2.4 Dimensions and installation clearances

Fig. 3 Dimensions in Inches and (mm)

4 Heat exchanger
5 Burner
10 Flow control knob
11 Water valve
20 Gas connection
23 Ignition unit
26 Power adjustment knob
34 LED indicator for burner status
35 On/Off switch
36 Front cover
37 Mounting holes
38 Exhaust pipe union
39 Draft diverter with flue gas sensor
40 Gas valve

Fig. 4 Minimum clearances

39

38

4

5

23

11

40

H

6720608036-01.2Av

8,66"(220)

E

F

G

4,92"(125)

D

2,50"

(63.5)

1,97"

(50)

3,82"
(97)

A

B

37

10

26

35

34

20

36

C

Model 520 HN

TOP (A)

12 inches (306 mm)

FRONT (B)

4 inches (100 mm)

BACK

0 inches

SIDES

4 inches (100 mm)

BOTTOM (C)

12 inches (306 mm)

Dimensions inches (mm)

520 HN

A 16.73” (425)

B 25.75” (655)

C 13.15” (334)

D 5.11” (130)

E 21.25” (540)

F 2.55” (65)

G 1.18” (30)

H 3/4”

Table 1 Dimensions in inches (mm)

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Installation instructions

3 Installation instructions

3.1 Introduction

Please follow these instructions. Failure to follow
instructions may result in:

B Damage or injury.
B Improper operation.
B Loss of warranty.

If you are unable to perform the tasks required to install
this heater properly, please contact a locally licensed
plumber or gas technician.
Please contact Bosch Water Heating with any
questions.

3.2 Proper location for installing your
heater

Carefully select the location of the water heater. For
your safety and for proper heater operation, you must
provide combustion air to the heater and a proper
exhaust vent system.
Follow the guidelines below:

B 1. Common installation practice is to determine the

venting system layout and penetration first, then work
back to the heater.

B 2. Locate the heater where venting, gas and plumb-

ing connections are feasible and convenient.

B 3. The hot water lines should be kept short to save

energy. Centrally locating the water heater is best. It
is always best to have hot water lines insulated.

3.3 Venting

B Minimum vent pipe diameter: 5 inches
B Minimum vertical vent height: 6 feet, with no elbows
B Establish 12 inch rise before any elbow

The heater must be vented to the outside following all
local ordinances and specifications for installing a gas
appliance vent or chimney. The heater must be located
as close as practicable to a vertically rising chimney or
vent that has a listed vent cap at its termination point.
The venting system must be designed and constructed
so as to develop a positive flow adequate to remove flue
gasses to the outdoors. Consult the National Fuel Gas
Code if the vent will have elbows or share venting with
another natural draft appliance.

3.3.1 Horizontal venting

An AQ4 powervent, with a proof-of-draft safety interlock
device, is required and is available for sidewall venting.
Contact your dealer. In the Commonwealth of
Massachusetts power vented applications must utilize
proof-of-draft safety interlock device and comply with
regulations established for all side wall horizontally
vented gas fueled equipment.

3.3.2 Vertical venting

B A 5 inch diameter gas vent constructed of double

wall Type B gas vent is recommended. A 6 inch vent
is required in elevations greater than 2000 feet, see
Fig. 10. Under no circumstances must the vent pipe
be reduced in size.

Warning: The water in this water
heater is cold and always remains cold
except for the times that hot water is
being used. DO NOT INSTALL IN AN
AREA WHERE IT COULD FREEZE.
Drain the heater entirely if freezing
temperatures are anticipated in area
where heater is installed. See chapter

4.6 for draining instructions.
To prevent any freeze damage,
introduce short bursts of compressed
air (20-40psi) through these
connections to remove the residual
water in the horizontal pipes and water
valve.

Warning: Flammable materials,
gasoline, pressurized containers, or any
other items or articles that are potential
fire hazards must NOT be placed on or
adjacent to the heater. The appliance
area must be kept free of all
combustible materials, gasoline and
other flammable vapors and liquids.

Warning: Place the heater in a location
where water leaks will do NO DAMAGE
to adjacent areas.

Danger: Do not reduce the vent pipe
size. Do not put an elbow directly on top
of heater. Failure to follow venting
requirements may result in dangerous
exhaust gases entering living space.

Warning: Do not combination vent with
a power vented appliance.

Warning: Horizontally venting to a
vertically constructed vent stack along
an outside wall of a building is not
permissible.

Warning: Horizontally venting to a
sidewall vent terminator (without
powervent) is not permissible.

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Installation instructions

7

B An approved gas vent connector must be attached to

the top of the water heater and rise vertically at least
12" before entering into an approved gas vent
connector elbow.

B The minimum vertical gas vent height allowed is 6

feet; horizontal vent connectors and elbows are not
to be considered in the total gas vent height.

B All gas vent sections must be secured to each other

with sheet metal screws and be properly supported.

Horizontal runs:

Any gas vent section that is greater than 45 degrees
from the vertical is considered horizontal. Horizontal
sections must slope upwards at least ¼ inch for every
foot of its horizontal length and be properly supported.
Keep the horizontal section short and avoid too many
elbows. The maximum horizontal run allowed is half of
the total vertical vent height; horizontal vent connectors
and elbows are not to be considered in the total gas
vent height.

Vent termination:

The gas vent constructed of double wall Type B gas
vent must terminate above the roof surface with a listed
vent cap at a height that's in accordance with Fig. 7 and
table 2, provided they are at least 8 feet (2.4 m) from a
vertical wall or similar obstruction. All other gas vents
that are not able to terminate within the minimum
specified height allowed must terminate not less than 2
feet (0.6 m) above the highest point where it passes
through the roof and at least 2 feet (0.6 m) higher than
any vertical wall or similar obstruction within 10 feet (3.1
m).

Fig. 5 Flat roof

Fig. 6 Vent Adaptor for High Altitude Installations

Freeze prevention

In cold climates, components of a tankless water heater
can freeze and burst from negative draft. A leading
cause of negative draft is combustion appliances in the
building not being supplied with sufficient combustion
air. A wood stove or furnace can pull its combustion air
from the water heater‘s vent pipe, allowing the cold
incoming air to freeze the cold water in the heat
exchanger. Supplying more combustion air for all com­bustion appliances is the solution. A HVAC specialist
should be consulted to design solutions for providing
more combustion air.

In cold climates where backdraft is a problem,
the freeze prevention kit accessory (7709003775)
should be installed.

LISTED VENT CAP

MINIMUM 6
FEET (1.8M)

LISTED GAS VENT

ESTABLISH A ONE

FOOT RISE BEFORE

ANY ELBOWS

5” X 6” ADAPTOR

USED AT HIGH

ALTITUDE

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Installation instructions

Masonry chimney

Masonry chimneys shall be built and installed in
accordance with NFPS 211 or local codes. A minimum
5" diameter gas vent pipe (metal double wall Type B),
or an approved clay flue liner or a listed chimney lining
system must be used when venting into a naturally
drafting, internal masonry chimney. 6 inch is required in
elevations greater than 2000 feet, see Fig. 6. Local
codes may require the use of both gas vent and an
approved lining system when venting into a masonry
chimney. The Commonwealth of Massachusetts
requires the use of a listed liner. Lining systems include
approved clay flue lining, a listed chimney lining system
or other approved material that will resist corrosion,
erosion, softening, or cracking from exhaust flue gases
at temperatures up to 1800 degrees F. The lining
system must be listed for use with naturally drafting,
draft hood equipped gas appliances. Follow local
codes and refer to NFGC 54 and NFPA 58.

Existing interior masonry chimney

The metal gas vent pipe should be permanently
mounted inside the masonry chimney. Double wall Type
B gas vent is recommended. The masonry chimney may
have to be tile or metal lined before the insertion of the
gas vent pipe; check local codes for clarification. The
lining material must be listed for use only with naturally
drafting, draft hood equipped gas appliances. Follow
manufacturer's instructions for installation of listed
lining material. You may not vent any other fuel burning
appliances into any free space remaining in the
chimney. The minimum vertical gas vent length within
the masonry chimney must be no less than 5 ft (1.5 m);
the vent terminator must extend at least 3 feet (0.9 m)
above where the chimney meets the roofline and at least
2 feet (0.6 m) higher than any vertical wall or similar
obstruction within 10 feet (3.1 m). The top of the gas
vent must have an approved vent terminator. See Fig. 8.

Fig. 7 Pitch roof

Fig. 8 Masonry chimney

APPROVED VENT CAP

LOWEST DISCHARGE
OPENING

MINIMUM 6
FEET (1.8M)

X

12

ROOF

PITCH IS

X/12

LISTED GAS VENT

H (minimum) height

from roof to lowest
discharge opening

ESTABLISH A ONE

FOOT RISE BEFORE

ANY ELBOWS

GAS VENT TERMINATIONS FOR LISTED VENT CAPS

Roof pitch H (minimum) feet meters

Flat to 6/12 1.0 0.30

6/12 to 7/12 1.25 0.38

Over 7/12 to 8/12 1.5 0.46

Over 8/12 to 9/12 2.0 0.61

Over 9/12 to 10/12 2.5 0.76

Over 10/12 to 11/12 3.25 0.99

Over 11/12 to 12/12 4.0 1.22

Over 12/12 to 14/12 5.0 1.52

Over 14/12 to 16/12 6.0 1.83

Over 16/12 to 18/12 7.0 2.13

Over 18/12 to 20/12 7.5 2.27

Over 20/12 to 21/12 8.0 2.44

Table 2

6720608032-14.1Av

GAS
VENT

APPROVED
VENT CAP

VENT
CONNECTOR

Establish a
one foot
rise before
any elbows

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Installation instructions

9

3.4 Combustion air requirements

The Protankless water heater holds cold water in its
copper heat exchanger and water valve when not in use.
Because of this, any cold air that comes down through
the unit's vent pipe is capable of freezing these
components. This Installation Manual specifies the
minimum vertical vent pipe and the amount of
combustion air required for this unit. When all
requirements are followed, the unit will operate properly
and safely. However, there may still be a risk of freezing
due to negative draft if all the combustion appliances in
the structure are not being supplied with sufficient
combustion air. A wood stove or furnace can pull it's
combustion air from the Protankless vent pipe. This
allows the cold infiltrating air to potentially freeze the
cold water in the AquaStar heater. Damage from
freezing is not covered under the manufacturer's
warranty. Supplying more combustion air for all
combustion appliances is the solution. Follow the
instructions on venting and checking adequacy of
combustion air. A HVAC specialist should be consulted
to design solutions for providing more make-up air if
necessary. Observe the following instructions
concerning combustion air.

Appliances located in unconfined spaces:
a) An unconfined space is one whose volume is greater

than 50 cubic feet per 1000 Btu per hour of the
combined rating of all appliances installed in the space.
That would be 5850 cubic feet for the Protankless 520
HN alone.

b) Installations in structures that have been tightly
constructed (air infiltration rate of 0.40 ACH or less)
must be provided with combustion air per the National
Fuel Gas Code. Consult a HVAC specialist if your air
infiltration rate is questionable.

Appliances located in confined spaces:

The confined space must be provided with two
permanent openings, one commencing within 12
inches of the top and one commencing within 12 inches
of the bottom of the enclosure. Each opening must have
a minimum free area of one square inch per:

• 1000 Btu/hr if all air is taken from inside the building.

• 2000 Btu/hr if all air is taken from the outside by

horizontal ducts.

• 4000 Btu/hr if all air is taken from the outside by

direct openings or vertical ducts.

Or the confined space must be provided with one
permanent opening or duct that is within 12 inches of
the ceiling of the enclosure. This opening must have a
minimum free area of one square inch per:

• 3000 Btu/hr if all air is taken from the outside by a

direct opening or vertical duct.

Louvers, grills and screens have a blocking effect. If the
effective free area is not known, increase the sizes of

your openings by 400% if your louvers are wood and by
135% if your louvers are metal. Refer to the National
Fuel Gas Code for complete information. In buildings of
tight construction all air should be taken from outside.

3.5 Mounting Heater

The 520 HN is approved for installation on a combusti­ble wall provided the floor covering below the heater is
noncombustible. For installations in an alcove or closet,
maintain the minimum clearances to combustible and
non-combustible materials listed in Fig. 4.

Remove cover and inspect.

B Remove the flow control and power adjustment

knobs.

B Unsrew the cover fixing screws, see Fig. 10.
B Loosen the two Phillips head screws located on

bottom rear of cover. Remove cover by pulling it
outward and then lifting upwards.

B Ensure that the flue terminal is clear.
B After inspection, replace front cover and tighten

screws.

Install incandescent particle tray.

B Install incandenscent particle tray using screws

provided as shown in Fig. 9.

Fig. 9 Incandescent particle tray illustration

Warning: before starting installation:

B check that there are no loose parts

inside the appliance

B ensure that gas pipe, gas valve, and

burner have no damage and are
properly fitted.

B Read chapter 2.2 to verify proper gas

type and to verify all parts are included
in box.

i

Front cover must be removed in order to
inspect components visually (see
instructions below).

Incandescent particle tray

Screws

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10

Installation instructions

Mounting heater.

The heater must be mounted on a wall using
appropriate anchoring materials. If wall is a stud wall
sheathed with plasterboard, it is recommended that
support board(s), either 1x4’s or 1/2" (minimum)
plywood first be attached across a pair of studs and
then the heater should be attached to the support
boards. See Fig. 11.

B Install the two included L shaped hooks to wall studs

or support board 13 ¼” apart (See Fig. 11).

B Hang heater on two L shaped hooks (See Fig. 12).

Fig. 10 Remove front cover

Fig. 11 Support board

Fig. 12 Secure heater to wall

Warning:

B Do not install this appliance on a

carpeted wall.

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Installation instructions

11

3.6 Gas piping & connections

Before connecting the gas supply, check the rating
plate on the right side of the heater to be sure that the
heater is rated for the same gas to which it will be
connected.
In the United States: The installation must conform with
local codes or, in the absence of local codes, the
National Fuel Gas Code ANSI Z223.1/NFPA 54.
In Canada: The Installation must conform to CGA B149
INSTALLATION CODES and/or local installation
codes.

Mount regulator to gas inlet pipe as shown in Fig. 13.
The arrow on the back of the regulator indicates the
direction of gas flow and must point upwards toward
the appliance.

Fig. 13 Installation of Gas Pressure Regulator

Note: The 520 HN comes with a gas pressure
regulator. Failure to install or altering the gas pressure
regulator will be a violation of CSA certification of the
unit. The regulator supplied with the heater is preset for
the gas shown on the rating plate to the correct
pressure. It is an appliance level regulator designed for
(low inlet) pressure (less than 1/2 Psig or 14" W.C.).

GAS CONNECTIONS

B Install a manual gas shut off valve on the gas supply

line.

B Install a union when connecting gas supply.
B Attach the appliance regulator to the inlet gas pipe.
B The minimum diameter required for any appliance

connector used is ¾” NPT.

B National Fuel Gas Code requires that a sediment

trap (drip leg) be installed on gas appliances not so
equipped. The drip leg must be accessible and not
subject to freezing conditions. Install in accordance
with the recommendations of the serving gas
supplier.

When connections are made, check for gas leaks at all
joints. Apply some gas leak detection solution to all gas
fittings. Bubbles are a sign of a leak. A combustible gas
detector may also be used to detect for leaks.

HIGH ALTITUDE INSTALLATION

The pressure regulator provided with the heater has
been adjusted to deliver the proper gas pressure (as
indicated on the rating plate and in the manual) for
altitude up to 2000 feet (660 meters) above sea level.
On appliances being installed above 2000 ft (660
meters) elevation, the inlet gas pressure should be
adjusted at installation to the value shown below.
Note: The gas pressures specified below refer to
pressures taken at the pressure tap on the gas inlet pipe
just above the regulator. See chapter 3.7 for measuring
gas pressure.

MAXIMUM INLET GAS FLOW PRESSURE SETTING

GAS LINE SIZING

The gas supply piping should be sized according to the
applicable code for a maximum draw of 117,000 BTUH.
Measure the length of gas supply line and use the tables
in Fig. 14 or the gas line manufacturer’s sizing tables to
determine the pipe diameter necessary to
accommodate the BTU demand of the unit. If there are
more gas appliances drawing on the same line, size the
gas line according to the total maximum amount of BTU
draw for all appliances.

Note: Undersizing the gas line may result in diminished
output. See chapter 3.7 for the procedure to measure
gas pressure.

Warning: DO NOT connect to an
unregulated or high pressure propane
line or to a high pressure commercial
natural gas line.

Warning: The heater must be isolated
from the gas supply piping system
during any pressure testing of that
system at test pressures equal to or
more than 0.5 psig. If overpressure has
occurred, such as through improper
testing of the gas lines or malfunction of
the supply system, the gas valve must
be checked for safe operation.

6720608036-07.2Av

Danger: If you have a leak, shut off the
gas. Tighten appropriate fittings to stop
leak. Turn the gas on and check again
with a gas leak detection solution.
Never test for gas leaks using a match
or flame.

Altitude

Nat. Gas

inches W.C.

Liquid Propane

inches W.C.

2 000 ft - 4 500 ft 4.6” 8.4”

Table 3 Above 4.500 ft consult your local gas provider

6 720 644 942

12

Installation instructions

FOR NATURAL GAS

Maximum Capacity of pipe in Cubic Feet of Gas per Hour for Gas Pressure of 0.5 Psig or less and a Pressure drop
of 0.3 in Water Column (0.75mbar).(Based on a 0.60 Specific Gravity Gas) Btu numbers given in thousands.
Copper tubing is prohibited for use with Natural Gas in the Commonwealth of Massachusetts and not recommended
elsewhere.

Fig. 14

Follow boxed numbers for piping just one 520 HN (example: ¾” B.I. Natural Gas pipe for 20 ft (6.1m). will handle
190,000 btu’s (55.7 kWh). For multiple appliances combine the total btu input load and then refer to applicable
chart below.

* EHD = Equivalent Hydraulic Diameter. The greater the
value of EHD, the greater the gas capacity of the tubing

.

Maximum Capacity of Semi-Rigid copper Tubing in
Thousands of BTU per Hour of Undiluted Liquefied

Petroleum Gases (at 11 inches Water Column Inlet
Pressure).

(Based on a Pressure Drop of 0.5 Inch Water Column)

* Source National Fuel Gas Code NFPA 54, ANSI Z223.1

- No Additional Allowance is necessary for an ordinary
number of fittings

FOR LP GAS

Maximum Capacity of Pipe in Thousands of BTU per Hour of Undiluted Petroleum Gases (at 11 inches Water Column Inlet
Pressure) (Based on a Pressure Drop of 0.5 Inch Water Column).

* EHD = Equivalent Hydraulic Diameter. The greater the
value of EHD, the greater the gas capacity of the tubing.

Nominal

Iron

Pipe Inte rnal
Size Diameter

inches inch es 10 20 30 40 50 60 70 80 90 100 125 150 175 200

3/4 0.824 278 190 152 130 115 105 96 90 84 79 72 64 59 55

1 1. 049 520 350 285 245 215 195 180 170 160 150 130 120 110 100

1 1/4 1.380 1050 730 590 500 440 400 370 350 320 305 275 250 225 210

Length of Black Iron Pipe, Feet

Nominal

Iron pipe

size

inches

10 20 30 40 50 60 80 100 125 150 200

1/2 291 200 160 137 122 110 94 84 74 67 58

3/4 608 418 336 287 255 231 197 175 155 140 120

1 1145 787 632 5 41 480 434 372 330 292 265 227

Black Iron Pipe

Lenght of pipe, Feet

Copper

Outside Le ngth o f Tubing, Feet
diamete r
Inch 10 20 30 40 50 60 70 80 90 100

3/8 39 26 21 19

_ _ _ _ _ _

1/2 92 62 50 41 37 35 31 29 27 26

5/8 199 131 107 90 79 72 67 62 59 55

3/4 329 216 181 145 131 121 112 104 95 90

Tube
size,
inches EHD*

10 20 30 40 50 60

1/2 18 E HD 82 58 47 41 37 34

3/4 23 E HD 161 116 96 83 75 68

1 30 EHD 330 231 188 162 144 131

1 1/ 4 37 EHD 639 456 374 325 29 2 267

Length of Flexible Corrugated Stainless Steel Tubing (CSST), Feet

Tube
size
inches EHD*

10 20 30 40 50 60

1/2 18 EHD 129 91 74 64 58 53

3/4 23 EHD 254 183 151 131 118 107

1 30 EHD 521 365 297 256 227 207

1 1/4 37 EHD 971 661 528 449 397 359

Length of Flexible Corrugated Stainless Steel Tubing (CSST), Feet

6 720 644 942

Installation instructions

13

3.7 Measuring gas pressure

Connecting manometer

B Shut off gas.
B Remove front cover and locate inlet gas pressure

measuring point on the left side, see Fig. 15.

B Loosen screw from test point on the left side and

connect manometer tube on test point.

Fig. 15 Gas pressure measuring point

1 Inlet tap

Static pressure test

B Turn gas supply back on.
B Record static gas pressure reading on back page of

manual.

Operating pressure test

B Turn on all hot water taps served by the water heater.
B Turn flow control (right knob) fully clockwise.
B Turn power adjustment (left knob) fully counter-

clockwise.

B Turn power adjustment knob fully clockwise.
B Operate all other gas appliances on same gas piping

system at maximum output.

B Record operating gas pressure reading on back

page of manual.

Gas pressures lower than 5.7" W.C. for Natural Gas or

10.5" W.C. for LP Gas will result in insufficient degree

rise to the hot water being used and must be corrected.
See Gas Line Sizing under chapter 3.6.

3.8 Water connections

When facing the heater, the ½” cold water inlet is on the
bottom right and the hot water outlet is on the bottom
left.

B Install unions and shutoff or isolation valves

when connecting plumbing to the water
heater. This will facilitate any necessary
cleaning and servicing.

Although water piping throughout the structure may be
other than copper, we require that copper piping or
suitably rated stainless steel flex line piping be used for
at least three feet before and after the heater (follow
local codes if more stringent). Never sweat any rigid
piping directly to or beneath the water connections or
damage can occur to the internal water valve from
heating of the pipe. Plastics or other PEX type plumbing
line materials are not allowed for connecting directly to
the water heater. Keep water inlet and outlet pipes to no
less than ½" (12.7mm) diameter to allow the full flow
capacity.
It is recommended that all hot water piping be properly
insulated to avoid heat loss.
If the cold and hot connections to the heater are
reversed, the heater will not function. Be certain there
are no loose particles or dirt in the piping. Blow out or
flush the lines before connecting to the water heater.
Full port valves must be installed on both the cold water
supply and hot water outlet lines to facilitate servicing
the heater (see Fig. 16). For installation on a private well
system with the use of a pressure tank, the lowest
pressure range setting recommended is 30-50 psi
(2.07 and 3.45bar).

Connecting the pressure relief valve (PRV)

A listed pressure relief valve must be installed at the
time of installation. No valve is to be placed between
the PRV and the heater. No reducing coupling or
other restriction may be installed in the discharge line.
The discharge line must be a minimum of 4” above a
drain and installed such that it allows complete drainage
of both the PRV and the line.
The location of the PRV must be readily accessible for
servicing or replacement, and be mounted as close to
the water heater as possible. See Fig. 16. To install the
PRV, a suitable fitting connected to an extension on a
“T” fitting can be sweated to the hot water line.
Support all piping.

Fig. 16 Plumbing Connections and Pressure Relief

Valve

6720608032-13.1Av

1

6720608036-02.2Av

6 720 644 942

14

Operation instructions

3.9 Recirculation application

Since recirculation through the heater is not allowed,
the following drawing is provided to outline a
recirculation application using the Protankless water
heater with an Ariston mini tank water heater. This
schematic is for illustration only and must not be used
for actual installation without appropriate engineering
and technical advice from a professional properly
licensed in locality where the installation is made.

Fig. 17 Recirculation application

1 Full port shutoff valve
2 Circulator
3 Check valve
4 PRV
5 Expansion tank

A small electric mini tank water heater (4-6 gallon size)
must be used for this application and designed so the
pump will circulate the water through the mini-tank and
the building's hot water return loop only. A timed or
thermostatically controlled operation of the pump is
commonly done. The 520 HN must be plumbed in line
before the mini-tank water heater. Contact Bosch
Water Heating if further instruction is needed.

4 Operation instructions

4.1 For your safety, read before
operating your water heater

A. This appliance is equipped with electronic ignition for

lighting the pilot and main burners. When turning the
heater on, follow these instructions exactly.
B. Before operating the unit, set the On/Off button to
the On position. The On/Off button is located on
the front panel. Smell all around the appliance area for
gas. Be sure to smell next to the floor because some
gas is heavier than air and will settle on the floor.

WHAT TO DO IF YOU SMELL GAS

B Close gas valve. Open windows.
B Do not try to light any appliance.
B Do not touch any electric switch; do not use any

phone in your building.

B Immediately call your gas supplier from a neighbors

phone. Follow the gas supplier’s instructions.

B If you cannot reach your gas supplier, call the fire

department.

C. Use only your hand to push in the on/off control
button. Never use tools. Follow these instructions
exactly. If control button is jammed, close the gas
supply and call a qualified service technician.
Attempted forceful repair may result in a fire or
explosion.
D. Do not use this appliance if any part has been under
water. Immediately call a qualified service technician to
inspect the appliance and to replace any part of the
control system and any gas control which has been
under water.

6720608030-19.4Av

1

4

3

2

5

Warning: If you do not follow these
instructions exactly, a fire or explosion
may result causing property damage,
personal injury or loss of life.

6 720 644 942

Operation instructions

15

4.2 Lighting instructions

B 1. STOP! Read the previous safety information.
B 2. The gas valve must be shut off by putting the ON/

OFF switch to position OFF . Wait five (5)
minutes to clear out any gas. If you smell gas, STOP!
Follow “B” in chapter 4.1. If you do not smell gas, go
to the next step.

B 3. This water heater is equipped with a safety pilot

burner and an automatic ignition control system.

B 4. Set the ON/OFF button to the position ON .

In this position, the water heater is ready to use.

Fig. 18

B 5. Open hot water tap to a flow rate above the

minimum activation rates listed in table 4. The
automatic ignition system first ignites the safety pilot
burner which then ignites the main burner in about 4
seconds.

B 6. The green LED indicator is on when the main

burner is on.

B 7. The pilot flame will extinguish a few seconds after

the burners come on. The burners will remain on until
the hot water tap is turned off.

Note: The 520 HN operates in two modes. See
SETTING THE WATER TEMPERATURE.

4.3 To turn off appliance

B Set the ON/OFF switch to the position OFF .

Fig. 19

B Close installer supplied manual gas shutoff valve on

the supply line to the heater.

4.4 Adjusting the water temperature

The 520 HN has two knobs that regulate temperature.
One knob controls temperature by regulating water flow
through the heater. The other knob controls tempera­ture by adjusting the gas supply to the burners.

Fig. 20

1 Power adjustment knob
2 Flow control knob

Flow control knob

The flow control knob (see Fig. 20 & Fig. 21) adjusts
temperature by adjusting flow capacity. See table 4 for
details.

Fig. 21

i

Note: On initial installation,
existence of air in the gas supply
line and in the water line may
cause some ignition delay. In that
case, repeat the ignition process
until all the air has been purged.

6720608036-08.1Av

1 2

6 720 644 942

16

Operation instructions

B Turning the flow control knob clockwise restricts

water flow. This provides higher temperature
because the water is moving slower, spending more
time over the burner flames. Turning flow control
knob fully clockwise also lowers the activation rate
(water flow needed to turn the heater on) to 0.5 gpm.

B Turning the flow control knob counter-clockwise

increases water flow. This provides lower tempera­ture because the water is moving faster, spending
less time over the burner flames. Turning flow control
knob fully counter-clockwise also increases the acti­vation rate of the heater to 1.1 gpm.

4.5 Power adjustment knob

The power adjustment knob (see Fig. 20 & 22) adjusts
temperature by adjusting the amount of gas supplied to
the burners.
(note: change in flame height not always visible with
knob adjustment).

B Turning the power adjustment knob clockwise sup-

plies less gas to the burners and lowers the water
temperature.

Fig. 22

B Turning the power adjustment knob counter-clock-

wise supplies more gas to the burners and raises the
water temperature.

Fig. 23

Maximum temperature

The maximum temperature is achieved by turning the
(left) power adjustment knob fully counter-clockwise
and the (right) flow control knob fully clockwise.

4.6 Draining the appliance

If there is a risk of freezing, proceed as follows:

B Remove the retaining clip from water valve (pos. 1).
B Remove bushing and filter (pos. 2) from water valve.
B Empty the appliance of all water.

Fig. 24 Draining

1 Retaining clip
2 Threaded bushing
3 Filter

Knob position Clockwise

Counter-

clockwise

Degree rise 90°F 45°F

Activation rate 0.5 GPM 1.1 GPM

Max flow rate 2.0 GPM 4.3 GPM

Table 4 Flow control knob settings

6720608036-09.1Av

Warning: To prevent any freeze dam­age, introduce short bursts of com­pressed air (20-40psi) through the
outlet water connection to remove the
residual water in the horizontal pipes
and water valve.

6720608036-10.1Av

6720608032-12.1Av

1

3

2

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Maintenance and service

17

5 Maintenance and service

5.1 Maintenance intervals

The 520 HN requires periodic maintenance. The time
maintenance intervals below should keep the unit
operating for many years.

Every year

B Inspect inlet water filter screen ( 5.2)
B Inspect pilot assembly and flame ( 5.3)
B Inspect burner assembly ( 5.4)

Every 2 years

B Inspect heat exchanger fins for soot, debris or block-

age. Clean if necessary (5.5)

B Clean and lubricate water valve ( 5.2)

Every 3 to 5 years

B Rebuild water valve with parts kit (part# WVKITCT)

( 5.2)

B Clean pilot assembly and clean or replace orifice

( 5.3)

5.2 Water valve

The water valve is the main control that tells the heater
to fire. If the periodic maintenance is neglected, more
costly damage may occur.

Inspecting inlet water filter screen

Shut off the installer supplied cold water shut-off valve
to the heater. If one is not installed, install before
proceeding. Open the nearest hot water tap to drain the
plumbing lines. Position a bucket under the heater's
water valve assembly to catch any water that may drain
from the heater. Remove the filter retaining clip located
at base of water valve (see Fig. 24). Remove the
bushing to access the cylindrical filter inside. Remove
filter, clean with water and inspect for damage. If the
filter is damaged, it must be replaced.

Lubricating the water valve

Service bulletins are available on our website at
www.boschpro.com. Shut off the installer supplied cold
water shut-off valve to the heater. If one is not installed,
install before proceeding. Open the nearest hot water
tap to drain the plumbing lines. Position a bucket under
the heater's water valve assembly to catch any water

that may drain from the heater. Disconnect the inlet
water pipe going to the back of the water valve.
Disconnect the water pipe leading out of water valve to
the heat exchanger. Do this by removing the retaining
clip on water valve assembly and pulling the pipe free
from the water valve. Loosen the two set screws
underneath the water valve where it connects to the gas
valve. If the set screws are seized, STOP and call
Bosch tech support before proceeding. Forcing and
breaking seized set screws may damage the water valve
and gas valve assemblies. Remove the water valve by
pulling it to the right. Separate the two halves of the
water valve by removing the five screws on the left side
of the water valve. Apply lithium or faucet & valve grease
to the pushrod and o-ring. (See Fig. 28, num. 4).
Remove and inspect the venturi and clean if necessary.
(see Fig 28, num. 23).

Rebuilding the water valve

Several parts within the water valve must be replaced
every 3 to 5 years depending on water quality and
usage. For more detailed instruction, bulletin CT-23 can
be found on our website at www.boschpro.com. Failure
to rebuild the water valve can result in decreased
performance or water leaks in the heater.

5.3 Pilot

Inspecting pilot

The pilot should burn with a clean sharp blue flame. The
flame should fully engulf the bend of the flame sensor. If
it does not, clean the pilot assembly using the
procedure below.

Cleaning pilot assembly

Shut off gas supply to the heater using installer supplied
gas shutoff valve. Remove 2 screws mounting pilot
burner to front of burner assembly. Remove the filter
screen at the base of the pilot burner. Remove 8 mm
brass hex head pilot orifice from pilot electrovalve. If
heater is a NG model, soak orifice and pilot burner in
carburetor cleaner for 30 minutes. Dry, blow out with
compressed air and reinstall orifice. If heater is an LP
model, replace orifice. Reinstall the pilot burner
following removal instructions in reverse. Open gas
supply and return heater to service.

Warning: Failure to perform
recommended maintenance may result
in complete failure of the unit over time.
The warranty does not cover failures
due to improper or insufficient
maintenance.

Caution: Do not ream or poke orifice as
it will enlarge orifice opening.

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18

Maintenance and service

5.4 Main burners

The main burner flames should be blue, with a more
intense blue cone in the center core. Yellow flames
could be a sign of wrong size gas orifices, dirty burners
or a blockage on the heat exchangers fins. If some
burners have yellow flames while others have blue
flames, it is likely that dust, lint or spider webs have
partially clogged the burner venturis. To clean the
burners, contact a gas service person and refer to
service bulletin CT-13.

5.5 Vent assembly / heat exchanger

Inspect the draft hood and heat exchanger fins for signs
of soot build-up or any other foreign material such as
spider webs. Clean out any debris found in the vent
hood and/or heat exchanger fins. Signs of soot indicate
insufficient combustion air or exhaust draft. Check for
vent assembly blockage or combustion air blockage on
the underside of the unit.

5.6 Mineral scale build-up

The 520 HN, when operated at lower temperatures
settings, does not accumulate mineral build-up. If
however, the heater is used at higher temperature
settings and the water has a high mineral content,
mineral deposits (scale) may form within the heat
exchanger and periodic descaling may be necessary.
The heat exchanger should be flushed with a descaling
solution.

5.6.1 Descaling heat exchanger

1. Shut off the water supply to the water heater using
(installer supplied) shut-off valve.

2. Open hot water taps to drain and relieve pressure
from the plumbing system.

3. Drain water from the unit by disconnecting inlet and
outlet water connections.

4. Connect the line (A) from the outlet of the circulating
pump (installer supplied) to the inlet water fitting on the
water heater.

5. Using another line (B), connect to the water outlet
fitting on the water heater. Route the other end of this
line into a descaling reservoir.

6. Using a 3rd line (C) from the descaling reservoir,
connect to the inlet side of circulating pump. Install a
filter on the end of the line in the descaling reservoir.

7. Make sure all connections are "hand tight."

8. Fill tank with descaling solution so lines inside are
submersed. We recommend a straight white vinegar
solution. If using a commercial descalant, refer to
manufacturer's instructions on dilution with water.

9. Operate the circulating pump.

10. Make sure there are no leaks and the solution is

flowing from the descaling reservoir through the heat
exchanger and returning to the reservoir.

11. Run solution through the heat exchanger until the

solution returning to the descaling reservoir comes out
clear.

B Changing to a fresh solution may be necessary dur-

ing this process.

B Several hours may be necessary to flush the heater

thoroughly.

12. Disconnect all lines and drain all solution from heat

exchanger. Properly discard of solution.

13. Position a container below the hot water outlet and

connect cold water supply. Open cold water supply
shut-off valve and flush heater with clean water.

14. Shut cold water shut-off valve and reconnect hot

water supply to the water heater.

15. Open open water shut-off valves, and return the unit

to service.

Fig. 25

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Troubleshooting

19

6 Troubleshooting

6.1 Introduction

Many of the questions customers ask regarding
operation of this unit can be answered by following the
troubleshooting steps as outlined below. Visit our web
site at www.boschpro.com for more detailed
troubleshooting bulletins. For best results, perform each
step before proceeding to the next. The suggested
solutions may require that the cover be taken off. (See
chapter 3.5).

6.2 No spark at pilot with water flow

Refer to service bulletin CT-09

• 1. Ensure that the On/Off button on control box is
pushed to the "On" position. .

• 2. Ensure that cold water supply is attached to rear
of water valve and hot water outlet is on the left.

• 3. Measure voltage at hydrogenerator. Disconnect
the wire connector at hydrogenerator and measure
voltage on hydrogenerator side of connector.

– a. If the wires from the hydrogenerator are black

and red, the voltage should be at least 1.3VDC.

– b. If the wires from the hydrogenerator are blue

and brown, the voltage should be at least 2.8VAC.
If the voltage is inadequate, refer to bulletin CT-29
to clean the hydrogenerator.

• 4. Check for crossover:
– a. Turn off cold water supply to heater only. If no

shut off valve is present, install before continuing.
– b. Turn on each hot water fixture to hot only.
– c. Wait 5 minutes.
– d. If any water is flowing, even a trickle, there is a

crossover or the scald protection in a shower valve

is adjusted incorrectly. Consult plumber or faucet

valve manufacturer for repair or adjustment.

• 5. Rotate flow control (right knob) to full clockwise
position for the lowest activation rate of about .6 gal­lon per minute. As the knob is turned counter-clock­wise, the activation rate will rise to about 1.1 gallons
per minute.

• 6. Check for obstructions to water flow. Clean inlet
filter screen in water valve, whole house filters,
showerheads and faucet aerators. Obstructions
such as dirt, debris or mineral deposits will decrease
the water flow through the heater below the
activation rate.

• 7. Check for tight and clean wire connections in
heater:

– a. wire from ignition unit to spark electrode
– b. wires to flue gas sensor on draft diverter
– c. wires to overheat sensor on outlet pipe
– d. wires to overheat sensor on heat exchanger

– e. wires to microswitch
– f. ground wire connection between ignition unit

and fixing screw

• 8. If a powervent and spill switch are installed, check
the reset button on the spill switch (installed on the
draft diverter). Consult powervent manual for more
details.

– a. If the button clicks when pushed, the switch

was tripped. Check venting for restrictions and
exhaust hood damper for correct operation.

• 9. Check microswitch on bottom of gas valve. Dis­connect wire connector at microswitch and jump
connector on wiring cable. Run water through heater.

– a. If heater sparks, check continuity through micro-

switch connector with water flow. If circuit does
not close one of the following may be the cause;
microswitch is faulty, water valve needs mainte­nance or flow is inadequate.

• 10. If heater still does not spark, contact Bosch tech­nical support.

6.3 Spark appears when hot water is

turned on, but pilot and burners
will not ignite

Refer to service bulletin CT-10
If there is a spark from the spark electrode with water
flow, but the pilot does not ignite:

• With water flowing and heater sparking, apply flame
from a match or lighter to the rear of the pilot burner.
Avoid applying flame to flame sensor.

If main burners only light, repeat procedure and watch
for brief flame from pilot burner:

• 1. If pilot does not light:
– a. Measure voltage between pilot electrovalve wire

connection and ground when heater is sparking
(Do not disconnect wire); Voltage must be at least

1.1VDC. If less, contact Bosch Tech Support.

– b. Clean or replace pilot orifice. See Bulletin CT-

22.

– c. If heater has never fired, check for correct gas

type on rating label on right side of cover. If incor­rect, exchange heater for correct gas type. It is not
practical to convert a heater to a different gas
type.

– d. If heater has never fired or gas lines have

recently been serviced, there may be air in the gas
line. Turn water on, wait until sparking has
stopped, turn water off and back on again. Repeat
until pilot lights or have gas technician bleed the
gas line.

6 720 644 942

20

Troubleshooting

– e. Verify that gas shut off valves are open. Gas

must be supplied to heater.

– f. Measure gas pressure at inlet tap with manome-

ter. See service bulletin CT-04.

– g. If no gas is present, Maxitrol regulator supplied

with heater may be upside down or locked. Rein­stall correctly or unlock regulator (see Bulletin CT-

14). Measure gas pressure on gas line before
Maxitrol. If over 14", Maxitrol should lock up to pre­vent high gas pressure from damaging heater.

– h. Check for tight and clean wire connections to

pilot and burner electrovalves. Remove connector
and clean with pencil eraser if dirty or corroded.

– i. Check pilot electrovalve function: Turn off gas

supply to heater, remove wire connection from
electrovalve, connect positive terminal from a sin­gle cell battery (1.2-1.5VDC) to electrovalve termi­nal and a jumper wire from negative terminal of
battery to ground. When connection is complete,
there should be an audible "click" from the electro­valve. If no click is heard, electrovalve may be
faulty.

• 2. If pilot and burners light:
– a. Check for drafts around pilot burner. See Bulle-

tin CT-03 but apply smoke to pilot burner without
heater operating. If smoke is blown away from pilot

burner, find cause of draft and correct.
– b. If an AQ4 powervent is installed:
– I. Check for presence of time delay in powervent.

If there is no time delay, replace with newer style

AQ4 powervent with time delay.
– II. If powervent has time delay, check for correct

setting (2 seconds).
– III. Check for adequate minimum vent length. Refer

to AQ4 Installation Manual.

6.4 Pilot lights, but burners will not
come on.

1. Verify that gas type indicated in the rating sticker

located on the cover's right hand side, coincides with
the gas type you are using. NG is a natural gas unit and
LP is for liquid propane (See chapter 2.2).

2. Pilot flame should be blue in color and completely

engulfing the flame sensor. If not, the pilot orifice may be
dirty or clogged. See chapter 5.3 on how to clean the
pilot and orifice.

3. If sparking does not stop when pilot is lit, the flame

sensor may not be recognizing pilot flame. Verify that
wire connection clip on wire from the bottom of the
flame sensor is secure. If flame sensor is fully engulfed
by the pilot flame, it may be dirty. Clean entire surface
area of flame sensor with a pencil eraser or fine steel
wool.

4. Confirm that wire connection to the burner

electrovalve is secure (see components diagram for

location of burner electrovalve). Inspect terminals for
corrosion. To clean terminals, remove the spade
connectors and clean with a pencil eraser.

5. Ignition unit, burner electrovalve or flame sensor
maybe faulty. Measure voltage from the spade
connection of the burner electrovalve to ground with
wire connected (see components diagram for location
of burner electrovalve). After the pilot lights, voltage
should read at least 1 VDC. If voltage is proper, the
burner electrovalve may be faulty. If voltage is not
proper, ignition unit may be faulty.

Fig. 26 Electrical wiring diagram

2 Spark electrode
3 Flame sensor
18 Microswitch
23 Ignition unit
25 Burner electrovalve
28 Pilot electrovalve
31 Temperature limiter/ECO
32 Flue gas sensor
33 Gas valve
34 Main burner operation LED
35 On/Off button
50 Hydrogenerator
111 Temperature limiter/ECO

6.5 Main burners go out during hot
water use

1. Hot water flow rate is dropping below required

activation flow rate. Turn flow control knob all the way
clockwise. Fully open a hot water faucet and fill a quart
container. If the container fills in 36 seconds or less, the
flow rate (0.5 gallon per minute) is sufficient to activate
the water heater. If flow rate is not sufficient, the water
heater's inlet filter should be inspected and cleaned.
See chapter 5.2.

2. Close installer supplied cold water shut off valve (if

none installed, install before proceeding). Open every
hot water tap supplied by the heater. Wait 5 minutes
and check all taps. Any water running, even a trickle, is
a sign of a plumbing crossover. Consult local plumber

6 720 644 942

Troubleshooting

21

or service person for help correcting a plumbing
crossover.

3. The minimum water pressure required is 30psi. For
installation on a well system with use of a pressure tank,
the lowest pressure range setting recommended is 30­50 psi (2.07-3.45 bar).

4. Hot water is very hot out of the tap, requiring a lot of
cold water to be added to it in order to attain a useable
hot water temperature. The addition of too much cold
water will slow the flow within the tankless heater,
decreasing it below activation point, which shuts off the
burners. The end result is nothing but cold water
coming out of the outlet. Reduce the amount of cold
water to be mixed by turning the gas control knob
clockwise for lower hot water temperatures. See
chapter 4.4 for details in lowering temperature.

5. A temperature limiter (ECO) is tripped. The water
heater does not have a thermostat. If the inlet water is
preheated, the unit will overheat, stopping the flow of
gas. Supply heater inlet with cold water only.

6. Flue gas sensor is tripped. Lack of adequate
combustion air or venting improperly by reducing pipe
diameter, improper use of elbows or exceeding
maximum vent length will trip the flue gas sensor.
Confirm that venting and combustion air meet
requirements in this manual. Refer to service bulletin
CT-03.

7. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to overheat and trip the
temperature limiter (ECO). To descale the heat
exchanger, consult section 5.6 on page 18.

8. The water valve assembly, which allows gas flow to
the burners, requires periodic maintenance (every 2-5
years depending on water quality and use). See
section 5.2.

6.6 Hot water temperature fluctuates
at tap

1. Close installer supplied cold water shut off valve (if

none installed, install before proceeding). Open every
hot water tap supplied by the heater. Wait 5 minutes
and check all taps. Any water running, even a trickle, is
a sign of a plumbing crossover. Consult local plumber
or service person for help correcting a plumbing
crossover.

2. Check for a clogged inlet water filter screen. See

chapter 5.2.

3. Hot water is very hot out of the tap, requiring a lot of

cold water to be added to it in order to attain a useable
hot water temperature. The addition of too much cold
water will slow the flow within the tankless heater,
decreasing it below activation point, which shuts off the
burners. The end result is nothing but cold water
coming out of the outlet. Reduce the amount of cold
water to be mixed by turning the gas control knob

clockwise for lower hot water temperatures. See
chapter 4.5. If the problem persists, go to
www.boschpro.com for service bulletin CT-07.

4. The water heater is designed to modulate gas flow to
the burners when water flow varies. If the water
pressure in the home is erratic and the water flow is not
consistent while a tap is opened, then the temperature
of the hot water will fluctuate. The minimum water
pressure for the home must be 30psi or greater. For
installations on a private well system with the use of a
pressure tank, the lowest pressure range setting
recommended is 30-50psi (2.07-3.45 bar). The use of
a pressure reducing/regulating valve before the water
heater (directly after the pressure tank on well systems)
may be an effective way to maintain constant water
pressure to the water heater.

5. If the inlet water temperature is not consistent then
the hot water temperature from the water heater will
fluctuate. The water heater modulates its burners when
flow rate is changed. It does not have a thermostat and
will not adjust to changes in inlet water temperature.

6. If the inlet water temperature to the water heater is
above 70 F, the heater may produce outlet
temperatures that are too hot for most uses. A
temperature balance shower valve can automatically
mix in cold water to reduce very hot water temperatures.
This can slow the flow within the tankless water heater,
decreasing it below activation point, which shuts off the
burners. In the event of any temperature instability with
the use of a temperature balance shower valve, refer to
shower valve manufacturer instructions for internal
adjustment to minimize the amount of cold water the
valve is adding.

7. The gas pressure also needs to be stable and
adequate. Ensure that gas pressure meets
specifications in chapter 3.8. A gas pressure reading is
needed to proceed further. Contact your original
installer or a local gas technician to obtain this reading.

6.7 Water is too hot

1. Verify that gas type indicated in the rating sticker
located on the cover's right hand side coincides with
the gas type you are using. NG is a natural gas unit and
LP is for liquid propane (See chapter 2.2).

2. Adjust the power adjustment knob clockwise for
cooler temperatures. See section 4.4.

3. Inlet water is preheated. This model does not have a
thermostat and will not modulate flame height in order
to prevent overheating. It is designed for cold water
supply only. Supply the heater with cold water to the
inlet.

4. Increase flow rate. Restrictions in the water path can
slow the flow of water through the heater, resulting in
very hot outlet temperatures. Restrictive showerheads
and faucet aerators should be cleaned or upgraded
with less restrictive ones. In addition, the inlet filter on

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22

Troubleshooting

the water heater should be cleaned and inspected. See
chapter 5.2.

5. If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to boil and produce
extremely hot temperatures. To descale the heat
exchanger, section 5.6 on page 18.

6.8 Water is not hot enough

1. Verify that gas type indicated in the rating sticker
located on the cover's right hand side coincides with
the gas type you are using. NG is a natural gas unit and
LP is for liquid propane (see chapter 2.2).

2. Flow at one particular tap is too great or too many
fixtures are running at one time. Lower flow to stay
within the heater's specifications.

3. Ensure that the gas control knob is all the way
counter-clockwise for highest temperatures. In addition,
the flow control knob maybe set too high. Raise the
setting by turning the knob clockwise for higher
temperatures.

4. Close installer supplied cold water shut off valve (if
none installed, install before proceeding). Open every
hot water tap supplied by the heater. Wait 5 minutes
and check all taps. Any water running, even a trickle, is
a sign of a plumbing crossover. Consult local plumber
or service person for help correcting a plumbing
crossover.

5. Inspect the water path outside the heater for
obstructions. Make sure all showerheads, faucet
aerators and whole house filters are clear of debris.
Also, the water heater's inlet filter should be inspected
and cleaned. See chapter 5.2.

6. The water valve assembly, which allows gas flow to
the burners, requires periodic maintenance (every 2-5
years depending on water quality and use). See
section 5.2.

7. Gas pressure is too low. Ensure that gas pressure
meets specifications meets chapter 3.9. A gas pressure
reading is needed to proceed further. Contact your
original installer or a local gas technician to obtain this
reading.

6.9 Low hot water pressure

1. Confirm adequate flow rate through the unit. With
cold supply shut off to the unit, disconnect hot outlet
side to the unit. Place a gallon jug under hot water
outlet to catch water. Open cold water supply and
measure flow rate out of the unit. With temperature
adjustment knob set all the way clockwise, the flow rate
should be close to 2GPM (filling the jug in 30 seconds
or less). If the flow rate is not adequate, inspect and
clean inlet filter screen. See chapter 5.2.

2. Confirm that the incoming water pressure to the unit
is above the minimum required pressure of 30psi.

3. Inspect the water path outside the heater for
obstructions. Make sure all showerheads, faucet
aerators and whole house filters are clear of debris.

4. Be sure to run only one major fixture at a time with this
water heater. Opening too many taps at one time can
disperse water flow resulting in diminished flow or
pressure at all outlets.

6.10 Noise when heater is running

If the water has a high mineral content, the heat
exchanger may be scaled internally. This restricts the
water path, causing the water to boil resulting in noise
and overheating. To descale the heat exchanger,
consult section 5.6 on page 18.

6.11 Burners do not burn cleanly, yellow
flames when operating

1. Verify that gas type indicated in the rating sticker

located on the heater's right hand side, coincides with
the gas type you are using. NG is a natural gas unit and
LP is for liquid propane (see chapter 2.2).

2. For proper combustion at high elevations the inlet

gas pressure must be set at installation to the values in
table 3 on page 11 (while operating at maximum BTU).
A gas pressure reading is needed to proceed further.
Contact your original installer or a local gas technician
to obtain this reading.

3. Combustion air is inadequate. Ensure that adequate

combustion air is being supplied to the unit in
accordance with requirements in the installation
manual. See chapter 3.3.

4. Verify that the water heater is vented properly. Ensure

that the venting meets requirements in chapter 3.6.

5. If the burner flames burn yellow or orange the burners

must be removed from the unit and cleaned. Once the
burners are removed, the fins in the top of the heat
exchanger can be accessed from below and brushed or
vacuumed clean. Consult service bulletin CT-13 on
www.boschpro.com for removing and cleaning the
burners.

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Protecting the environment

23

7 Protecting the environment

Packing

The packing box may be fully recycled as confirmed by
the recycling symbol .

Components

Many parts in the heater can be fully recycled in the end
of the product life. Contact your city authorities for
information about the disposal of recyclable products.

Saving water resources:

B Make sure you close all the taps after any use. Avoid

leaving the taps dripping. Repair any leaking tap.

B Define the temperature you want. This way you have

the precise water flow needed (mixing cold water to
regulate temperature will increase the water flow with
consequent waste of water).

For increased safety shut off the appliance from
the main water supply if you are staying away
from home for a considerable time.
See Section 4.6.

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24

Interior components and diagram parts list

8 Interior components and diagram parts list

8.1 Interior components

Fig. 27 Functional scheme

4 Heat exchanger
5 Burner
9 Ignition unit
10 Flow control knob
11 Water valve
20 Gas connection
21 Flexible cold inlet

22 Flexible hot outlet
26 Power adjustment knob
30 Pilot burner
51 on/off button

4

5

11

20

10

22

26

51

9

30

6720608036-04.3AL

21

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Interior components and diagram parts list

25

8.2 Components diagram

Fig. 28 Components Diagram

12

50

40

10

11

8

6

7

21

22

42

35

31

33

36

39

45

20

19

15

14

13

16

17

18

43

46

47

36

48

49

2

1

3

5

4

28

29

30

6720608036-05.3 JF

53

23

24

38

41

34

32

37

27

25

26

44

51

52

54

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26

Interior components and diagram parts list

8.3 Parts list

Item Description Reference

1 Front cover 8 705 431 415 0
2 Shield 8 705 506 912 0
3 Trade mark badge 8 701 103 135 0
4 Flow control knob 8 702 000 297 0
5 Power adjustment knob 8 702 000 295 0
6 Draft Diverter 8 705 505 442 0
7 Exhaust pipe union Ø128 8 705 504 046 0
8 Angle bracket 8 708 003 192 0
10 Heat exchanger 8 705 406 386 0
11 Hydrogenerator 8 707 406 095 0
12 Cold water pipe 8 700 715 109 0
13 Main burner NG 8 708 120 620 0
13 Main burner LP 8 708 120 621 0
14 Connector 8 705 209 056 0
15 Pilot burner 8 708 105 655 0
16 Electrode/sensor group 8 708 107 013 0
17 Ignitor bracket 8 701 300 010 0
18 Pilot screen 8 700 507 066 0
19 Gas valve LP 8 707 011 922 0
19 Gas valve NG 8 707 021 011 0
20 Pilot orifice LP 8 708 200 321 0
20 Pilot orifice NG 8 708 200 322 0
21 Microswitch 8 707 200 020 0
22 Gas pressure regulator LP 8 707 406 083 0
22 Gas pressure regulator NG 8 707 406 084 0
23 Water valve 8 707 006 344 0
24 Watervalve pushrod 8 703 204 051 0
25 Diaphragm 8 700 503 084 0
26 Inlet water connection 8 700 306 110 0
27 Selector screw 8 708 500 304 0
28 Ignition unit 8 707 207 272 0
29 Cable 8 704 404 060 0
30 Flue gas sensor 8 707 206 446 0
31 Heat exchanger washer 8 700 103 658 0
32 Threaded bushing 8 700 306 148 0
33 Connector 8 700 306 187 0
34 Water Filter 8 700 507 059 0
35 Outlet Hose 8 700 703 139 0
36 Water connection clip 8 701 201 020 0
37 Plunger 8 703 502 039 0
38 Volumetric water governor 8 705 705 034 0
39 ECO/temperature limiter 8 707 206 017 0
40 Temperature limiter/ECO 8 707 206 435 0
41 Venturi 8 708 205 279 0
42 Hot water pipe 8 700 705 178 0
43 Gas supply pipe 8 700 715 351 0
44 Cover 8 705 500 105 0
45 Clip 8 701 300 015 0
46 Protection shield 8 701 000 397 0
47 Wire form spring 8 704 705 030 0
48 Pilot valve 8 708 501 249 0
49 Wire form spring 8 704 705 025 0
50 Servovalve 8 708 501 250 0
51 Wire form spring 8 704 705 024 0
52 Inlet Hose 8 700 703 114 0
53 Wire form spring (10x) 8 704 705 029 0
54 Protection shield 8 701 000 414 0

Table 5

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Fifteen Year Limited Warranty

27

9 Fifteen Year Limited Warranty

General

BOSCH heaters are warranted by the Manufacturer
(BOSCH) through Bosch Thermotechnology. Bosch
Thermotechnology will furnish a replacement heat
exchanger and will furnish a replacement of any other
part which fails in normal use and service within the
applicable periods specified below, in accordance with
the terms of this warranty. The replacement will be
warranted for the unexpired portion of the original
warranty. This warranty will be valid only for water
heaters in possession of the original purchaser as
recorded on the warranty card.

The Heat Exchanger

If the heat exchanger fails within fifteen (15) years after
the original installation and operation Bosch
Thermotechnology Corporation will furnish a
replacement heat exchanger. However, if the water
heater is installed in other than a single family dwelling
this heat exchanger warranty is limited to three (3) years
from date of original installation and operation.

Exceptions

This warranty will not apply:

• 1. to defects or malfunctions resulting from failure to
properly install, operate or maintain the unit in
accordance with the printed instructions provided;

• 2. to damage or abuse, accident, neglect or freezing
and other acts of nature;

• 3. to damage resulting from operation with either the
flame sensor rod or overheat sensor removed;

• 4. to failure of the heat exchanger resulting from the
operation of the water heater in a corrosive
atmosphere or at water temperatures exceeding the
maximum rating, or if the water heater is not supplied
with potable water;

• 5. to defects or damage cause by any attachment
or modification, including any energy-saving device

• 6. to damage resulting from scale deposits and/or
highly mineralized / unsoftened water supply.

All Other Parts

If any other part fails within three (3) years after original
installation and operation, Bosch Thermotechnology
Corporation will furnish a replacement part free of
charge.

Shipping costs

In addition to supplying the replacement part(s), Bosch
Thermotechnology Corporation will provide ground

service delivery for these parts. Expedited or upgraded
shipping will be charged to the customer.

Service Labor Costs

This warranty does not cover any labor costs
associated with service, removal or re-installation of
part(s). All such costs must be borne by the Purchaser.
Additionally, this warranty does not cover any labor
costs associated with service, removal or re-installation
of the original water heater or a replaced water heater.

How to Make a Claim

Any claim for warranty parts should be made to your
local dealer or distributor:

BOSCH THERMOTECHNOLOGY CORPORATION
50 Wentworth Avenue
Londonderry, NH 03053
Phone: [phone]

www.boschpro.com

In most cases, the dealer or distributor will be able to
promptly honor your claim and subsequently notify
Bosch Thermotechnology Corporation. However, all
replacements are made subject to validation by Bosch
Thermotechnology Corporation of in-warranty cover­age. The damaged or defective item must be made
available in exchange for the replacement.

Miscellaneous

No one is authorized to make any other warranties on
behalf of Bosch Thermotechnology Corporation. It is
expressly understood that the replacement warranty of
Bosch Thermotechnology Corporation shall be in lieu
of any and all other warranties, express or implied,
including warranties of merchantability or fitness for a
particular use or purpose, and further that Bosch
Thermotechnology Corporation shall not be liable for
any loss or damage directly or indirectly arising from the
use of the hot water heater, or for any consequential
damages arising from such use (including damages
from water leakage). Bosch Thermotechnology
Corporation sole liability with respect to any defect shall
be for the replacement of the defective part(s). Some
states do not allow such limitations and exclusions, so
the above may not apply to you.

This warranty gives specific legal rights. You may also
have other rights which vary from state to state.

i

Note: the water heater must be free of
damaging scale deposits and not subject
to gas pressures greater than those
shown on the rating plate, which must not
be altered, defaced or removed.

Recycled paper © 2010 Bosch Thermotechnology Corp.,

Londonderry, NH all rights reserved

Bosch Termotecnologia SA

Estrada de Cacia

3800 - 533 Cacia - PORTUGAL

BOSCH THERMOTECHNOLOGY CORP.

50 Wentworth Avenue

Londonderry, NH 03053

Tel. [phone]

www.boschpro.com

Installer Checklist, to be performed by installer upon installation

Serial Number ___ ___ ___ ___ ___ ___ ___ ___

(8 digit serial number is located on rating plate on right side panel)

Gas Pressure Reading* Static__________ Operating__________

Building Water Pressure __________ Range if on Well system __________

Average Winter Temperature __________

Average Summer Temperature __________

Installing Company _____________________________________________

Installer name _____________________________________________

Address _____________________________________________

Phone _____________________________________________

* See Chapter 3.7 and gas pressure table (to be filled out by installer)

Installation manual should be left with the owner after the installation is tested and completed

6720644942