Viessmann Vitocrossal 300, CU3A Technical Manual

Technical Data Manual

Model Nos. and pricing: see Price List

For operation with natural gas and liquid propane gas
Floor mounted, gas-fired condensing boiler
Heating input 19 to 199 MBH

5.6 to 58 kW

VITOCROSSAL 300

Product may not be exactly as shown

Vitocrossal 300
CU3A Series 26 to 57

Floor mounted, gas-fired condensing
boiler with modulating MatriX gas burner.
For room air dependent or independent
operation.

5673 647 - 03 07/2015

H

Product Information

Benefits

Equipped with the industry’s first intelligent combustion
management system and powerful control technology,
the new generation Vitocrossal 300 CU3A gas-fired
Floor mounted condensing boiler delivers unparalleled
performance, reliability and comfort.

The benefits at a glance:

Vitocrossal 300 CU3A 26 to 57 Technical Data

■ Best value in its class with new industry-leading
technology and the most standard features.

■ Highest Efficiency up to 98% at full modulation and up
to 95% A.F.U.E.

■ Lasting performance with industry-leading Viessmann
made SA240 316 Ti stainless steel Inox-Crossal heat
exchanger constructed to CSA B51 and ASME Section IV.

■ Low Emission Combustion with Viessmann made
stainless steel MatriX burner. Factory calibrated.

■ Fast Installation and Reduced Maintenance with Lambda
Pro, industry-first intelligent combustion management
system: Adjusts automatically to gas type and quality.
No fuel conversion kit required.

■ Powerful and User-friendly
Viessmann Vitotronic on-board multi-function outdoor
reset boiler and system control for multi-temperature
space and DHW heating.

■ Expanded Application Range with increased capacity up
to 199 MBH (58 kW). Multi-boiler installation up to
1592 MBH (464 kW) with up to 8 boilers.

■ Greater Venting Flexibility with increased vent length [up
to 198 ft. (60 m)] and multiple venting options.

- Horizontal or vertical sealed combustion coaxial
venting (field supplied).

- Horizontal, vertical or hybrid sealed combustion
double-pipe venting (field supplied).

- Horizontal or vertical single-wall venting (field
supplied).

■ Compact, Esthetic Product Design and zero clearance
to combustibles.

■ Extremely Quiet Operation; quieter than most
refrigerators.

■ True High-Altitude Operation up to 10,000 ft. (3,000 m)
with simple electronic adjustment.

■ Efficiency up to 98% through intensive condensation.
The flue gas temperature is only approximately 9-27ºF
(5-15ºC) above boiler return temperature.

2

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5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Cross-Section

Vitocrossal 300 CU3A 26 to 57

Product Information

Vitocrossal 300 CU3A boiler efficiency dependent
on system heating water return temperatures and
load conditions

Product may not be exactly as shown

Legend

A Digital Vitotronic boiler control unit
B Water-cooled stainless steel combustion chamber
C Modulating MatriX gas burner – for extremely clean

combustion

D Stainless steel Inox-Crossal heat exchanger
E Highly effective thermal insulation
F Flue gas collector with condensate drain pipe
G Combustion air intake for direct vent (two-pipe)

systems

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3

3

Technical Data

Vitocrossal 300 CU3A 26 to 57 Technical Data

Technical Data

Boiler Model No. CU3A 26 35 45 57

Natural Gas / Liquid Propane Gas
CSA input

CSA output / DOE heating capacity*1

Net AHRI Rating MBH

Heat exchanger surface area ft.

Min. gas supply pressure

Natural gas
Liquid propane gas

Max. gas supply pressure *3

Natural gas
Liquid propane gas

A.F.U.E. %95 95 95 95
Weight lbs

Boiler water content USG

Max. operating pressure

at 210ºF (99ºC)

Boiler water temperature

- Adjustable high limit (AHL) range
space heating (steady state)

DHW production

- Fixed high limit (FHL)

Boiler connections

Boiler heating supply and return
Pressure relief valve
Boiler drain

Gas valve connection

MBH
(kW)
MBH
(kW)

(kW)

(m2)

“w.c.
“w.c.

“w.c.
“w.c.

(kg)

(L)

psig

(bar)

ºF

(ºC)

ºF

(ºC)

ºF

(ºC)

NPTM

NPTF

NPTM

NPTF

2

19-94

(5.6-27.5)

17.7-87

(5.2-25.5)

76

(22)

16.7
(1.5)

4

10

14
14

269

(122)

13.5
(51)

30
(2)

68-194
(20-90)

194
(90)
210
(99)

1a”

¾”

1”

¾”

25-125

(7.3-36.6)

23.3-116
(6.8-34)

101
(30)

20.7
(1.9)

4

10

14
14

275

(125)

13.0
(49)

30
(2)

68-194
(20-90)

194
(90)
210
(99)

1a”

¾”

1”

¾”

43-160

(12.6-47)

40-149

(11.7-43.7)

129
(38)

34.1
(3.2)

4

10

14
14

352

(160)

18.8
(71)

30
(2)

68-194
(20-90)

194
(90)
210
(99)

1a”

¾”

1”

¾”

43-199

(12.6-58)

40-185

(11.7-54.2)

161
(47)

34.1
(3.2)

4

10

14
14

352

(160)

18.8
(71)

30
(2)

68-194
(20-90)

194
(90)
210
(99)

1a”

¾”

1”

¾”

*1 Output based on 140ºF (60ºC), 120ºF (49ºC) system supply/return temperature.

*2 Net AHRI rating based on piping and pick-up allowance of 1.15.

*3 If the gas supply pressure exceeds the maximum gas supply pressure value, a separate gas pressure regulator

must be installed upstream of the heating system.

Note: For high altitude installation at 10,000 ft. the input for model Vitocrossal 300 CU3A will have an altitude
de-ration of 18%.

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Vitocrossal 300 CU3A 26 to 57 Technical Data

Technical Data

Technical Data

Boiler Model No. CU3A 26 35 45 57

Dimensions

Overall depth

Overall width

Overall height
(with control interface open)
Overall height

Flue gas *4

Temperature (at boiler return
temperature of 86ºF (30ºC)

- at rated full load

- at rated partial load

Temperature (at boiler return
temperature of 140ºF (60ºC)

Max. condensate flow rate *5

for NG and LPG
T

S/TR =104/86ºF (40/30ºC) USG/h

Condensate connection hose

Boiler flue gas connection *6 Ø

Combustion air supply connection outer

Sound Rating (A scale)

- at maximum input

- at minimum input
Standby losses *7 BTU/hr

inches

(mm)

inches

(mm)

inches

(mm)

inches

(mm)

ºF (ºC)
ºF (ºC)

ºF (ºC)

(L/h)

nozzle

Ø in.

in. (mm) 3 (80) 3 (80) 4 (110) 4 (110)

Ø in. (mm) 3 (80) 3 (80) 3 (80) 3 (80)

dB
dB

W/hr

27

684

26

660

67

1707

61.5

1562

113 (45)

90 (32)

167 (75)

0.9

3.43

27

684

26

660

67

1707

61.5

1562

113 (45)

90 (32)

167 (75)

1.2

4.62

31b

801

26

660

67

1707

61.5

1562

113 (45)

90 (32)

167 (75)

1.6

5.95

31b

1707

61.5

1562

113 (45)

90 (32)

167 (75)

7.92

¾¾ ¾ ¾

48
32

1128

330

55
33

1000

292

53
33

1120

328

801

26

660

67

2.1

58
33

995
291

*4 Measured flue gas temperature with a combustion air temperature of 68°F (20°C).

*5 Based on maximum input rate.

*6 For side wall vent installations (coaxial system):

Do not exceed max. equivalent length specified in the Installation Instructions of the Vitocrossal 300 CU3A
Venting System.
Do not attempt to common-vent Vitocrossal 300 CU3A with any other appliance.
Side wall co-axial vent installation must include Viessmann protective screen!
For details refer to the Installation Instructions for the Vitocrossal 300 CU3A Venting System.

*7 Standby losses measured from the boiler temperature sensor well based on a boiler water temperature of 158°F
(70°C) and a room temperature of 68°F (20°C).

For information regarding other Viessmann System Technology componentry, please reference documentation
of respective product.

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5

Dimensional Information

Models 300 CU3A 26 to 57

Vitocrossal 300 CU3A 26 to 57 Technical Data

Legend

A Overall boiler height (boiler control open)
B Overall boiler height (boiler control closed)
C Safety header connection
D Boiler supply
E Gas connection
F Boiler return
G Boiler drain
H Vent connection
I Condensate drain connection
J Combustion air connection

Model 26 to 35 Model 45 and 57

a in. (mm)

b in. (mm)

c in. (mm)

d in. (mm)

Note: All height dimensions of the boiler have a tolerance of +.6 in. (+15 mm) due to the factory installed adjustable feet.

27 (684)

17 (430)

23b (595)

4

¾ (120)

a in. (mm)

b in. (mm)

c in. (mm) 28 (712)

d in. (mm)

31b (801)

21b (545)

5b (140)

6

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5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Recommended Minimum Service Clearances

Note: The Vitocrossal 300 CU3A series boiler conforms

on the special venting system label.

The back or side clearance shall be increased if

piping installation is required.

Minimum Clearances

to the zero inches vent clearance to combustibles
testing requirements dictated by the boiler
Harmonized Standard ANSI Z21.13. CSA/4.9
and therefore is listed for zero clearance to
combustibles when vented with a single-wall UL/
ULC certified special venting system. The zero
inches vent clearance to combustibles for the
Vitocrossal 300 CU3A boiler supercedes the
clearance to combustibles listing that appears

Clearances

Top Front Rear Left Right Vent pipe

0 0 AL CL 0 0 0 0

AL = Alcove
CL = Closet

The Vitocrossal CU3A series boiler is approved for
closet and alcove installation with the clearances to
combustibles listed above.

Recommended minimum service clearances

For typical Vitocrossal CU3A series boiler installations,
Viessmann recommends installing the boiler with the
clearances shown in the illustration on the left.

See the Vitocrossal 300 CU3A Venting
System Installation Instructions for details.

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7

Waterside Flow

Pressure Drop (heating water side)

Pressure drop

Vitocrossal 300 CU3A 26 to 57 Technical Data

Flow rate

Legend

A Boiler models CU3A 26 to 35 kW
B Boiler models CU3A 45 to 57 kW

The Vitocrossal 300 is only suitable for fully pumped hot
water heating systems.

∆T = 20ºF (10 K) ∆T = 30ºF (15 K) ∆T = 40ºF (20 K)

Boiler models CU3A

26 9.9 (2.2) 6.6 (1.5) 4.9 (1.1)

35 13.3 (3.0) 8.9 (2.0) 6.6 (1.5)

45 17.0 (3.9) 11.4 (2.6) 8.5 (2.0)

57 22.7 (5.2) 15.0 (3.4) 11.4 (2.6)

Note: This boiler does not require a flow switch.

Flow rate

USGPM (m

3

/h)

Flow rate

USGPM (m3/h)

Flow rate

USGPM (m3/h)

8

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

On-Board Vitotronic 200, HO1B Control

Modular structure

The control unit is integrated into the boiler. The control
unit comprises a standard unit, electronics modules and
a programming unit.

Standard unit:

■ ON/OFF switch

■ Optolink laptop interface

■ Operating and fault indicators

■ Reset button

■ Fuses

Boiler Control

Programming unit:

■ Easy operation through:
– Plain text display with graphic ability
– Large font and black & white depiction for good
contrast
– Context-sensitive help text
– Removable programming unit; can be mounted
on the wall with separate accessory

■ With digital time switch

■ Control keys for:

– Navigation
– Confirmation
– Help and additional information
– Menu

■ Setting the:
– Room temperature
– Reduced room temperature
– DHW temperature
– Heating program
– Time programs for central heating, DHW heating
and DHW recirculation
– Economy mode
– Party mode
– Holiday program
– Heating curves
– Codes
– Actuator tests
– Test mode

■ Displaying the:
– Boiler water temperature
– DHW temperature
– Operating details
– Diagnostic details
– Fault messages

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9

Boiler Control

Design and Function

Vitocrossal 300 CU3A 26 to 57 Technical Data

Functions

■ Weather-compensated control of the boiler water
and/or supply temperature

■ Control of one heating circuit without mixer and two
heating circuits with mixer (with accessory mixing
valve extension modules)

■ Demand-dependent heating circuit pump and burner
off control

■ Adjustment of a variable heating limit

■ Anti-seizing pump protection

■ Integral diagnostic system

■ Maintenance display

■ DHW temperature control with priority

■ Auxiliary function for DHW heating (short-term

heating to a higher temperature)

■ 0-10V External signal input

■ 0-10V Pump speed control output signal

Control Characteristics

PI characteristics with modulating output.

Time Switch

Digital time switch.

■ Individual and 7-day program

■ Automatic daylight savings time changeover

■ Automatic function for DHW heating and DHW

recirculation pump

■ Time, day and standard switching times for space
heating, DHW heating and the DHW recirculation
pump are factory-set

■ Switching times are individually programmable,
i.e. up to four switching periods per day

Shortest switching interval: 10 minutes

Power backup: 14 days.

Setting the Operating Programs

The heating system frost protection (see
frost protection function) applies to all heating programs.
You can select the following heating programs with the
program keys:

■ Heating and DHW

■ DHW only

■ Standby mode

External heating program changeover.

Frost protection

■ The frost protection function will be started when the
outside temperature drops below approx. 34°F (1°C).
With the frost protection function, the heating circuit
pump will be switched ON and the boiler water is
maintained at a lower temperature of approx. 68°F
(20°C).
The DHW tank will be heated to approx. 68°F (20°C).

■ The frost protection function will be stopped when the
outside temperature rises above approx. 37°F (3°C)
(default settings).

Summer Operation

Heating program “w”
The burner is only activated upon a call for domestic hot

water from the DHW storage tank (controlled by DHW
tank temperature sensor).

10

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Design and Functions (continued)

Boiler Control

Technical Data

Rated supply voltage: 120VAC
Rated frequency: 60 Hz
Rated current: Less than 12 A
Max. ambient temp.

- at operation: 32 to 104ºF (0 to 40ºC)
Installation in living spaces or
boiler rooms (standard ambient
conditions)

- when storing
or transporting: -4 to +158ºF (-20 to +70ºC)

Max. operating temp.
setting (space heating): 194ºF (90ºC)

DHW production: 194ºF (90ºC)

Setting fixed high limit: 210ºF (99ºC)
(not adjustable)

Adjustment range of
DHW tank set-point
temperature: 50 to 149ºF (10 to 65ºC)

Heating curve settings

- Heating curve slope: 0.2 to 3.5

- Heating curve shift: -12 to +33ºC

-13 to 40 K

Heating Curve Adjustment (slope and shift)

The control unit regulates the boiler water temperature
(= supply temperature of heating circuit without mixing
valve) and the supply temperature of the heating circuit
with mixing valve (in conjunction with the accessory kit
for a heating circuit with mixing valve) according to the
outdoor temperature. The boiler water temperature is
automatically raised by 0 to 72 F / 0 to 40 K higher than
the currently required set supply temperature
(in the factory default setting the differential temperature
is 14.4 F / 8 K). See Service Instructions for coding
address “9F” in coding level 2.

The supply temperature that is required to achieve a
given room temperature depends on the heating system
and the thermal insulation of the building that is being
heated.

The adjustment of the two heating curves is used to
match the boiler water temperature and the supply
temperature to these conditions. The boiler water
temperature is limited upwards by the fixed high limit
and the temperature set for the electronic high limit.
The supply temperature cannot rise above the boiler
water temperature.

Boiler water or supply

temperature

Room
set-point

temperature

Outdoor temperature

Legend

A Low temperature heating system,
e.g. radiant floor heating

B Medium temperature heating system,
e.g. cast iron radiation, staple-up radiant floor heating

C High temperature heating system,
e.g. fintube radiation, fan coils

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11

Accessories

Sensors

Vitocrossal 300 CU3A 26 to 57 Technical Data

Boiler Temperature Sensor (supplied)
The boiler temperature sensor is connected at the control

unit for weather-responsive operation and is built into the
boiler.

Specification sensor type: Viessmann NTC 10K ohms
at 77ºF (25ºC)

Max. ambient temp.

- at operation: 32 to 266ºF (0 to 130ºC)

- when storing

or transporting: - 4 to+158ºF (-20 to+70ºC)

DHW Tank Temperature Sensor

- integrated DHW temperature sensor

Immersion Temperature Sensor (accessory)
To capture the low loss temperature
(if the LLH is used)

Specification
Lead length fully wired: 12 ft. (3.75 m),

Permissible ambient temp.

- during operation: 32 to 194°F (0 to 90°C)

- when storing

or transporting: - 4 to+158°F (-20 to+70°C)

(supplied)

Outdoor Temperature Sensor

Sensor location:

- North or northwest wall of building

- 6.6 to 8.2 ft. (2 to 2.5 m) above ground or in case

of a multi-storey building approximately halfway up
the second floor

Electrical connection:

- 2-wire cable, max. cable length 115 ft. (35 m)
with a wire size of min. AWG 16 copper

- Cable to the outdoor sensor must not be laid near
line voltage wiring (120/240V)

Max. ambient temp.

- at operation: 32 to 194ºF (0 to 90ºC)

- when storing

or transporting: - 4 to+158ºF (-20 to+70ºC)

(supplied)

Optional Accessories

Supply Temperature Sensor

(strap-on sensor, included with mixing valve actuator
accessory kit),

- when storing
or transporting: -4 to+158°F (-20 to+70ºC)

Installed with a strapping band.
Cable length 19.7 ft. (6 m), ready to plug in.
Max. ambient temp.

- at operation: 32 to 212ºF (0 to 100ºC)

- when storing

or transporting: -4 to+149ºF (-20 to+70ºC)

KM BUS Expansion Module

To connect 2 to 9 devices (mixing valve actuator,
Vitotrol, input module, etc.) to the single KM BUS
connection of the boiler.

Specification
Lead length 10 ft. (3.0 m), fully wired

Permissible ambient temp.

- during operation 32 to 104°F (0 to 40°C)

- during storage and
transport: - 4 to+149°F (-20 to+65°C)

12

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Optional Accessories (continued)

Accessories

Room Temperature Sensor,

Separate room temperature sensor as supplement to the
Vitotrol 200A and 300A; to be used if the Vitotrol 200A
or 300A cannot be installed inside the main living room
or in a suitable position where the unit can capture and
adjust the temperature.
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelf units,
recesses, immediately by a door or heat source
(e.g. direct sunlight, fireplace, TV set, etc.).
Connect the room temperature sensor to the Vitotrol
200A or 300A.
Electrical connection:

- 2-wire cable with a wire size of min. AWG 18 copper

- The cable length between the control unit, remote

control unit and room temperature sensor must not
exceed 98 ft. (30 m)

Max. ambient temp.

- at operation: 32 to 104ºF (0 to 40ºC)

- when storing
or transporting: -4 to+149ºF (-20 to +65ºC)

Mixing Valve Extension Module

Mixing Valve Actuator Kit

The mixing valve actuator is mounted directly on the
Viessmann ¾ to 2½” mixing valve.
The mixing valve actuator is a motor-driven control unit.
The rotational direction is reversible.

Rated voltage: 120VAC
Rated frequency: 60 Hz

Power consumption: 4W max. ambient temperature

- at operation: 32º F to 104ºF (0º C to 40ºC)

application in living areas and installation sites
(normal ambient conditions)

-storage & shipping: -4ºF to 149ºF (-20ºC to 65ºC)

LON Communication Module

Electronic PCB for data exchange with the Vitotronic
200-H, Vitocom 200 and for connecting to a higher level
building management system.

LON Connecting Cable

(for data exchange between control units)
Cable length 23 ft. (7 m), fully wired

Rated voltage: 120VAC
Rated frequency: 60 Hz
Rated current: 2 A
Power consumption: 4 W
Max. ambient temp.

- at operation: 32 to 104ºF (0 to 40ºC)

- when storing

or transporting: -4 to+149ºF (-20 to+65ºC)
Relay output for
heating circuit pump: 2 A, 120VAC
Actuator torque: 3 Nm
Time of 90º: 120 seconds

5673 647 - 03

LON Terminal End Resistor

To terminate the LON BUS at the first and the last control
unit.

13

Accessories

Optional Accessories (continued)

Vitotrol 200A

One Vitotrol 200A can be used for every heating circuit
in a heating system. The Vitotrol 200A can regulate one
heating circuit and up to two remote controls may be
connected to the control unit.

KM BUS Participant
The Vitotrol 200A remote control regulates the required
set room temperature in standard mode, from any room
in the house.

- Party and economy mode can be enabled via keys

- Only for heating circuit with mixing valve
Fitted room temperature sensor for room temperature
hook-up

Vitocrossal 300 CU3A 26 to 57 Technical Data

Connection:

- 2-core lead 18 AWG, length max. 164 ft. (50 m)
(even if connecting
several remote control units)

- Never route this lead immediately next to 120/208/460 V
cables

- LV plug as standard delivery must be installed in the
main living room (lead room).

Specification:
Power supply via KM BUS
Power consumption: 0.2 W

Max. ambient temp.

- at operation: 32 to 104ºF (0 to 40ºC)

- when storing
or transporting: -4 to+149ºF (-20 to+65ºC)

Set room temp. range 37 to 99ºF (3 to 37ºC)

The set room temperature for reduced mode is adjusted
at the control unit.

WS function:
Installation anywhere in the building.

RS function:
Note: Never activate the RS function for underfloor
heating circuits (inertia). In heating systems with
a heating circuit without mixing valve and heating
circuits with mixing valve, the RS function must
only affect the heating circuit with mixing valve.

Room temperature hook-up:
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelf units,
recesses, immediately by a door or heat source (e.g.
direct sunlight, fireplace, TV set, etc.).
The integral room temperature sensor captures the actual
room temperature and effects any necessary corrections
of the supply temperature as well as a rapid heat-up
at the start of the heating operation (if appropriately
programmed).

14

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Optional Accessories (continued)

Vitotrol 300A

One Vitotrol 300A can be used for every heating circuit
in a heating system. The Vitotrol 300A can regulate up
to three heating circuits. Up to two remote controls may
be connected to the control unit.

KM BUS Participant
The Vitotrol 300A remote control regulates the required
set room temperature for one heating circuit in standard
and reduced mode, the heating program and the switching
times for central heating, DHW heating and the DHW
recirculation pump.

Displaying:

- Room temperature

- Outside temperature

- Heating program

- Operating condition

- Solar yield as graphic display

Settings:

- Set room temperatures for standard mode
(day temperature) and reduced mode (night temperature)
via the standard display

- Heating program, switching times for heating circuits,
DHW heating and DHW recirculation pump plus further
settings via plain text menu on the display

- Party and economy mode can be enabled via the menu

- Only for heating circuit with mixer:
Fitted room temperature sensor for room temperature
hook-up

Note: For room temperature hook-up, the Vitotrol 300A
must be installed in the main living room (lead room).

Accessories

RS function:
Installation in the main living room on an internal wall
opposite radiators. Never install inside shelving units,
in recesses, or immediately by a door or heat source
(e.g. direct sunlight, fireplace, TV set, etc.). The fitted
room temperature sensor captures the actual room
temperature and effects any necessary correction of
the flow temperature.

WS function:
Installation anywhere in the building.

Connection:

- 2-core lead 18 AWG, length max. 164 ft. (50 m)
(even if connecting several remote control units)

- Never route this lead immediately next to 120/208/460V
cables

- LV plug as standard delivery must be installed in the main
living room (lead room).

Specification:
Power supply via KM BUS
Power consumption: 0.2 W

Max. ambient temp.

- at operation: 32 to 104ºF (0 to 40ºC)

- when storing
or transporting: -4 to+149ºF (-20 to+65ºC)

Set room temp. range 37 to 99ºF (3 to 37ºC)

The set room temperature for reduced mode is adjusted
at the control unit.

5673 647 - 03

15

Accessories

Accessories for Vitocrossal 300 CU3A

Vitocrossal 300 CU3A 26 to 57 Technical Data

Fuel Conversion Label Kit (NG>LPG)

(included in boiler technical literature set)
The Vitocrossal 300 CU3A boiler comes factory set for
operation with natural gas. All Vitocrossal 300 CU3A
models can be field converted to operate with liquid
propane gas (as well as back to natural gas as required).
The kit includes instructions and labels for field conversion.

Low-Loss Header

When used in conjunction with the Vitocrossal 300
CU3A boiler, the low-loss header acts as hydraulic break,
decoupling boiler and system circuits from each other.

Viessmann recommends the use of a low-loss header
in cases where the system head and flow rates are
unknown.

The temperature sensor connection [TS] typically located
at the top of the low-loss header ensures low return
temperatures to the Vitocrossal 300 CU3A boiler at all
times, increasing operational efficiency.

IMPORTANT

Use only a Viessmann temperature sensor (accessory).
Do not use any other manufacturer’s temperature sensor.

Low-loss header design (Typical)

Legend

AB Air Bleed
BR Boiler Return
BS Boiler Supply
BY Bypass (with laminar flow)
D Debris and/or air
DV Drain Valve
SC Sensor Cable
SR System Return
SS System Supply
TS Viessmann Temperature Sensor
SW Sensor Well

In addition, the low-loss header helps eliminate air and
debris [D] from the heating system.
See illustrations for design and principle of operation.

16

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Vitocrossal 300 CU3A 26 to 57 Technical Data

Accessories for Vitocrossal 300 CU3A

Low-Loss Header (continued)

Principle of Operation

Accessories

IMPORTANT

When installing a low-loss header, the system mixed
supply temperature (T3) must be calculated as follows:

Vprimary < Vsecondary
T1 > T3
T2 = T4
Qprimary = Qsecondary

T1 167° F (75° C)
Vsecondary=Vprimary+Vbypass

Model No. Max. system flow rate

Type 80/60 17.6 USGPM (4 m

Type 120/80 35.2 USGPM (8 m

Type 160/80 *1 44 USGPM (10 m

Type 200/120 *1 80 USGPM (18 m

3

/h)

3

/h)

3

/h)

3

/h)

Legend

T1 Boiler supply temperature
T2 Boiler return temperature
T3 System supply temperature
T4 System return temperature
Vprimary Boiler circuit flow rate
Vsecondary Heating circuit flow rate
Vbypass Bypass flow rate
Qprimary Heat supplied by boiler
Qsecondary Heat consumed by system

*1 Floor-mounted version (typically used for
multiple-boiler installations).

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17

Standard Equipment/Management System

Standard Equipment

Combustion Management System

Vitocrossal 300 CU3A 26 to 57 Technical Data

The Vitocrossal 300 CU3A gas-fired condensing combi­boiler with Inox-Crossal heat exchanger, modulating
MatriX burner for liquid propane gas and natural gas
(available on all models), comes standard with:

■ boiler control unit with outdoor
temperature sensor, power/pump module:

- installation fittings with 30 psig pressure relief valve

- Temperature and pressure gauge field installation
on supply pipe.

- The boiler comes prewired and fully piped

internally for field connections.

- Side wall vent installations must include
Viessmann protective screen!

- Enclosure finish: black steel, powder-coated

Viessmann silver

- The Vitocrossal 300 CU3A comes ready for use
with natural gas and can be fuel converted to liquid
propane gas in the field.

The combustion management system utilizes the physical
correlation between the level of the ionization current
and the air factor λ. For all gas qualities, the maximum
ionization current results with air factor λ.
The ionization signal is evaluated by the combustion
management system, and the air factor is adjusted to
between λ=1.24 and 1.44. This range provides for an
optimum combustion quality. Thereafter, the electronic

To achieve an optimum combustion control, the system
regularly performs an automatic self-calibration; also after
a power failure (shutdown). For this, the combustion is
briefly regulated to max. ionization current (equals air factor
λ=1). The automatic calibration is performed shortly after
the burner starts and lasts approximately 5 seconds. During
the calibration, higher than normal CO2 emissions may

occur briefly.
gas valve regulates the required gas volume based on the
prevailing gas quality.
To check the combustion quality, the CO2 content or the
O2 content of the flue gas is measured. The actual values
enable the prevailing air factor to be determined. The
relationship between the CO2 or O2 content and air factor
λ is illustrated in the table below.

Air factor λ - CO

Air factor λ

/O2 content

2

O

2 content (%) CO

content (%) for natural gas CO2 content (%) for liquid propane gas

2

1.20 3.8 9.6 11.3

1.24 4.4 9.2 10.9

1.27 4.9 9.0 10.6

1.30 5.3 8.7 10.3

1.34 5.7 8.5 10.0

1.37 6.1 8.3 9.8

1.40 6.5 8.1 9.6

1.44 6.9 7.8 9.3

1.48 7.3 7.6 9.0

The Vitocrossal 300 CU3A boilers come equipped with Lambda Pro, the industry’s first intelligent combustion
management system. The boiler adjusts automatically to any gas type and quality without the need for a fuel
conversion kit.

18

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Vitocrossal 300 CU3A 26 to 57 Technical Data

Hydraulic Connection

Installation

System design

Viessmann condensing boilers can generally be installed

in any pumped hot water heating system (closed system).

Minimum system pressure 14 psi.

The boiler water temperature is limited to 194ºF (90ºC).

To minimize distribution losses and for increased

efficiency we recommend that you size the heat

distribution system to a max. supply temperature of

158ºF (70ºC). This can be achieved through the use of

a low temperature heating system (such as a radiant

floor).

2

For apartments with less than 860 ft

space or for low energy houses with low heat demand

we recommend, due to the immediate capturing of the

room-influencing factors, the utilization of the Vitocrossal

300 CU3A in conjunction with the Vitotrol 200A / 300A

and room dependant control.

The use of a low loss header is not necessarily required

for single boiler CU3A installations. Use of a low loss

header when used with the CU3A offers additional

benefits as they can be an effective method of air

separation, debris removal, temperature sensing and in

installations where the flow rates are unknown.

(80 m2) living

Installation Examples

Please note that in the following piping layout examples
all pumps external to the boiler are field supplied.

IMPORTANT

The examples on the following pages depict possible

piping layouts of the Vitocrossal 300 CU3A boiler

equipped with Viessmann System Technology.

Please note that the following examples are simplified

conceptual drawings only!

Piping and necessary componentry must be field verified.

A low water cut-off (LWCO) must be installed where

required by local codes.

Proper installation and functionality in the field is the

responsibility of the heating contractor.

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19

Installation Examples

System Layout 1

Vitocrossal 300 CU3A with a direct-connected heating circuit

Vitocrossal 300 CU3A 26 to 57 Technical Data

Legend

A Vitocrossal 300 CU3A boiler with
Vitotronic 200, KW6B outdoor reset control

B Outdoor temperature sensor !
C Vitotrol remote (optional)
D Heating circuit
E Heating circuit pump
F DHW pump sA
G DHW storage tank
H DHW tank temperature sensor %
I Expansion tank
J DHW recirculation pump
K Temperature and pressure relief valve
L Pressure relief valve

* Function based on coding of address 33

20

28/20 *

Installation of ...

■ radiator heating circuit (high-temp. circuit)

■ DHW production

20

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

System Layout 2

Vitocrossal 300 CU3A with...

- DHW storage tank

- one heating circuit with mixing valve and system separation

Installation Examples

Legend

A Vitocrossal 300 CU3A boiler with
Vitotronic 200, KW6B outdoor reset
control

B Outdoor temperature !
C Vitotrol remote (optional)
D Underfloor heating circuit
E Mixing valve temperature sensor ?
F Pressure relief valve
G Plate heat exchanger for system

separation
H Accessory kit for heating circuit with
mixing valve

I DHW circulating pump sA
J DHW storage tank
K DHW tank temperature sensor %
L Expansion tank
M DHW recirculation pump 28/20 *
N Temperature and pressure relief valve
O Heating circuit pump 20
P Heating circuit pump

* Function based on coding of address 33

Installation of ...

■ underfloor heating circuit with 3-way mixing valve

and system separation (low-temp. circuit)

■ DHW production

IMPORTANT

System separation is required of underfloor heating

systems employing non-oxygen diffusion barrier tubing.

All components on the secondary side of the heat

exchanger must be made of corrosion-resistant materials.

IMPORTANT

Sizing of field-supplied pump P is critical for proper boiler

operation in this system layout. See page 8 for the supply

head pressure drop of the boiler. An undersized pump

may cause short-cycling and/or improper operation of the

boiler.

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21

Installation Examples

System Layout 3

Vitocrossal 300 CU3A with...

- DHW storage tank

- one direct-connected heating circuit

- two heating circuit with a mixing valve

Vitocrossal 300 CU3A 26 to 57 Technical Data

Legend

A Vitocrossal 300 CU3A boiler with
Vitotronic 200, KW6B outdoor reset
control

B Outdoor temperature sensor !
C Vitotrol remote (optional)
D Under floor heating circuit
E Radiator heating circuit
F Heating circuit pump
G Mixing valve temperature sensor ?
H Accessory kit for heating circuit with mixing valve
I Domestic hot water storage tank
J DHW tank temperature sensor %
K Expansion tank
L Temperature and pressure relief valve
M Pressure relief valve
N DHW recirculation pump
O Boiler pump 20
P DHW circulating pump sA

* Function based on coding of address 33

22

28/20 *

Installation of different heating circuits...

■ radiator heating circuit (high-temp. circuit)

■ under floor heating circuit with 3-way mixing valve

(low-temp. circuit)

■ DHW production

The 3-way mixing valve, built-in to achieve the low-

temperature level of the under floor heating circuit, is

controlled by an accessory kit for a heating circuit with

mixing valve.

IMPORTANT

System separation is required of underfloor heating
systems employing non-oxygen diffusion barrier tubing.
All components on the secondary side of the heat
exchanger must be made of corrosion-resistant materials.

IMPORTANT

Sizing of field-supplied pump P is critical for proper boiler
operation in this system layout. See page 8 for the supply
head pressure drop of the boiler. An undersized pump
may cause short-cycling and/or improper operation of the
boiler.

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

System Layout 4

Vitocrossal 300 CU3A with...

- direct-connected heating circuit

- one heating circuit with system separation

Installation Examples

Legend

A Vitocrossal 300 CU3A boiler with Vitotronic 200,
KW6B outdoor reset control

B Outdoor temperature sensor !
C Vitotrol remote (optional)
D Under floor heating circuit
E Radiator heating circuit
F Plate heat exchanger for system separation
G Heating circuit pump
H Mixing valve temperature sensor ?
I Accessory kit for heating circuit with mixing valve
J DHW storage tank
K DHW tank temperature sensor %
L Expansion tanks
M Temperature and pressure relief valve
N Pressure relief valve
O DHW recirculation pump 28/20 *
P Boiler pump 20
Q DHW circulating pump sA

* Function based on coding of address 33

Installation of different heating circuits...

■ radiator heating circuit (high-temp. circuit)

■ under floor heating circuit with 3-way mixing valve

(low-temp. circuit)

■ DHW production

The 3-way mixing valve, built-in to achieve the low-

temperature level of the under floor heating circuit, is

controlled by an accessory kit for a heating circuit with

mixing valve I. The boiler pump

radiator heating circuit and the plate heat exchanger

for system separation.

20 supplies the

IMPORTANT

The pressure drop (secondary circuit) of the plate heat

exchanger F must be less/equal to the pressure drop of

the 3-way mixing valve. This must be taken into account

when sizing the plate heat exchanger.

5673 647 - 03

23

Installation Examples

System Layout 5

Vitocrossal 300 CU3A with...

- DHW storage tank

- low-loss header

- multiple heating circuits with mixing valves

- one heating circuit without mixing valve

Vitocrossal 300 CU3A 26 to 57 Technical Data

When designing a system as illustrated above, please contact your local Viessmann Sales Representative for assistance.

Legend

A Vitocrossal 300 CU3A boiler with Vitotronic
200, KW6B outdoor reset control

B Outdoor temperature !
C Boiler pump 20
D Vitotronic 200-H, HK1B *1
E Mixing valve temperature sensor ?
F Heating circuit pumps
G Accessory kit for heating circuit with mixing valve
H Heating circuits with mixing valves
I Viessmann temperature sensor for low-loss header ?
J Low-loss header (if required)
K DHW circulating pump sA
L DHW tank temperature sensor %
M DHW storage tank
N Expansion tank
O DHW recirculation pump sK
P Heating circuit pump 28/20 *
Q Vitotrol remote (optional)
R Radiator heating circuit
S Temperature and pressure relief valve
T Pressure relief valve

* Function based on coding of address 33

*1 Contact your local Sales Representative for details.

Vitotronic 200-H, HK1B mixing valve control with

LON communication module can be used instead

of the accessory kit and Vitocontrol.

IMPORTANT

Please note location of expansion tank N.

IMPORTANT

System separation is required of underfloor heating
systems employing non-oxygen diffusion barrier tubing.
All components on the secondary side of the heat
exchanger must be made of corrosion-resistant materials.

24

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

System Layout 6

Multiple (up to eight) Vitocrossal 300 CU3A with...

- multiple heating circuits with mixing valves

- low-loss header

Installation Examples

Legend

A Vitocrossal 300 CU3A boiler
B Vitotronic 300K, MW2C
C Low-loss header/common supply temperature sensor
D Low-loss header
E DHW storage tank
F High temperature heating circuit
G Low temperature heating circuit

When designing a multiple Vitocrossal 300 CU3A system
as shown above, please reference applicable multiple
Vitocrossal 300 CU3A technical documentation, and
contact your local Viessmann Sales Representative for
further assistance.

This installation example depicts a possible piping layout

for multiple Vitocrossal 300 CU3A boilers equipped with

Viessmann System Technology. Please note that this

example is based on a simplified conceptual drawing only!

Piping and necessary componentry must be field verified.

A low water cut-off (LWCO) must be installed where

required by local codes.

Proper installation and functionality in the field is the

responsibility of the heating contractor.

WARNING

If a DHW storage tank other than a Viessmann
Vitocell 100 or 300 tank is used, the installer must
verify proper operation of the Viessmann DHW tank
temperature sensor with the original manufacturer
of the tank. Viessmann strongly recommends the
installation of a temperature tempering valve in the
DHW supply line.

5673 647 - 03

25

Installation Examples

Electrical Connections

Vitocrossal 300 CU3A 26 to 57 Technical Data

26

WARNING

DISCONNECT POWER BEFORE
SERVICING BOILER.

If any of the original wires as
supplied with the appliance must
be replaced, it must be replaced
with the exact equivalent.

CAUTION

Label all wires prior to disconnection
when servicing controls. Wiring errors
can cause improper and dangerous
operation. Verify proper operation
after servicing.

* Only one function/connection may
be assigned to each connection.

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

Electrical Connections (continued)

Legend

1 Outdoor Temperature Sensor
2 Supply Temperature Sensor/Low Loss Header
3/1 Boiler Temperature Sensor #1
3/2 Boiler Temperature Sensor #2
5 DHW Temperature Sensor
11 Ionization Electrode
15 Flue Gas temperature Sensor
20 Boiler Pump
21 DHW Pump
28/20 Programmable Pump Output*
35 Gas Valve
40 Power Supply
40A Accessory Power Output
54 Ignition Transformer
96 Powered Accessory Connection
100 Fan Motor
100A Fan Motor Control
111 Not Used
145 KM BUS
147 Pump Modulation Output 0-10VDC
Multi Powered Accessory Connection
156A Switched Output F2
156B Switched Output F1
DE1 Digital Input 1 (Dry Contact)

DE2 Digital Input 2 (Dry Contact)

DE3 Digital Input 3 (Dry Contact)

0-10V 0-10VDC Input
157 Fault Alarm/DHW Recirculation Pump*

190 Gas Modulation Coil

Installation Examples

* See wiring diagram

A Boiler Control
B External Accessory Connection Board
C Power/Pump Connection Interface
D Integrate EA1 Module
E KM-BUS for External Devices
F Electrical Junction Box
G Field Wiring Connections

A1 Main Board
A2 Internal Power Supply Unit
A3 Optolink
A4 Burner Control Unit
A5 Programming Unit
A6 Coding Card
A7 Connection Adaptor
A8 LON Communication Module
S1 ON/OFF Switch
S2 Reset Button

X.. Electrical Interface

5673 647 - 03

27

System Design

System Design Considerations

Vitocrossal 300 CU3A 26 to 57 Technical Data

IN THE COMMONWEALTH OF MASSACHUSETTS...

- this product shall be installed by a licensed plumber or
gas fitter.

- the flexible connector (if used) cannot exceed 36”.

- any level type shutoff used must be of tee handle
type.

Boiler location

As a direct vent appliance, the Vitocrossal 300 CU3A can
be installed for room air independent operation (sealed
combustion) regardless of size and ventilation method
of the room in which it is located.
The Vitocrossal 300 CU3A can be installed, for example,
in the main living area of a house, in non-ventilated utility
rooms, cupboards, closets and alcoves with no clearance
required from combustible materials, as well as in attics
with a direct outlet for the flue gas/fresh air system.
Follow all local and national codes.

Flue gas system

Viessmann PP(s) (Polypropylene) concentric flue gas/
fresh air systems for room air independent operation
(sealed combustion) and side wall venting are tested to
ANSI Z21.13 - CSA 4.9 (latest edition) and are certified
together with the Vitocrossal 300 CU3A boiler as a
constructional unit.
The Vitocrossal 300 CU3A boiler may also be vented,
using an AL29-4C® special stainless steel, single-wall, room
air dependent or independent venting system (UL listed for
category IV), polypropolene PP(s) or CPVC single wall room
air dependent or independent venting system (listed
to ULCS636 or UL1738) for CAT IV appliances.
For a more detailed description of the direct vent and
single-wall vent system, please refer to the Vitocrossal 300
CU3A Venting System Installation Instructions.

Flue gas temperature protection

Flue pipes used for the Vitocrossal 300 CU3A are suitable
for max. flue gas temperatures of up to 230°F (110°C).
No flue gas temperature protection is required as the
maximum permissible flue gas temperature is not
exceeded in any operating condition or in the event of
malfunctioning.

Total permissible hardness of the fill and top-up water

Total heating output Specific heating volume

MBH <5 USG per 3412 BTU

170

>170 to  682

>682 to 170

>2050 2 ppm 0.11 gpg 2 ppm 0.11 gpg 2 ppm 0.11 gpg

ppm - parts per million
gpg - grains per gallon

300 ppm 17.5 gpg 200 ppm 11.7 gpg 2 ppm 0.11 gpg

200 ppm 11.7 gpg 150 ppm 8.8 gpg 2 ppm 0.11 gpg

150 ppm 8.8 gpg 2 ppm 0.11 gpg 2 ppm 0.11 gpg

Low water cut-off

A low water cut-off may be required by local codes.

If the boiler is installed above the radiation level, a low

water cut-off device of approved type must be installed

in all instances. An approved type low water cut-off

device must be provided by the heating contractor.

Do not install an isolation valve between the boiler and

the low water cut-off.

Water connections

Vitocrossal 300 CU3A boilers can be used in any fully

pumped hot water heating system.

Minimum system pressure is 14 psig.

Chemical corrosion protection products

Corrosion does not typically occur in sealed heating

systems which have been correctly installed and are

correctly operated.

Many manufacturers of plastic pipes recommend the

use of chemical additives. In this case, only those

commercially available corrosion protection products

approved for boilers with domestic hot water heating

via single-wall heat exchangers (instantaneous plate

heat exchangers or DHW tanks) must be used.

Water quality

Treatment for boiler feed water should be considered in

areas of known problems, such as where a high mineral

content and hardness exist. In areas where freezing might

occur, an antifreeze may be added to the system water

to protect the system. Please adhere to the specifications

given by the antifreeze manufacturer. Do not use

automotive silicate based antifreeze.

Please observe that an antifreeze/water mixture may

require a backflow preventer within the automatic water

feed and influence components such as diaphragm

expansion tanks, radiation, etc. Maximum antifreeze

content is 50% for the Vitocrossal 300 CU3A boiler.

Do not use antifreeze other than specifically made for

hot water heating systems. System also may contain

components which might be negatively affected by

antifreeze.

Check total system frequently when filled with antifreeze.

Advise system operator/ultimate owner that system is

filled with a glycol mix. The heating contractor must

provide a MSDS (Material Safety Data Sheet) for the

antifreeze used to the system operator/ultimate owner.

 5 USG per 3412 BTU to

<13 USG per 3412 BTU

 13 USG per 3412 BTU

28

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

System Design Considerations (continued)

System Design

System layout

■ The max. boiler water temperature for space heating
and DHW production is 194°F (90°C). To minimize
distribution losses, Viessmann recommends that the
heating and domestic hot water systems be based
on a maximum boiler supply temperature of 194°F
(90°C).

■ Due to the low return temperatures required for gas
condensing, no mixing valves should be used in the
heating circuit whenever possible. If mixing valves
are required, e.g. for multi-circuit systems or
underfloor heating systems, only 3-way mixing valves
must be used.
Do not use 4-way mixing valves with condensing
boilers.

Underfloor heating systems

For underfloor heating systems Viessmann recommends
the use of plastic tubing with an oxygen diffusion barrier
in order to prevent the diffusion of oxygen through
tubing. If plastic tubing without an oxygen diffusion
barrier is used in underfloor heating systems, Viessmann
recommends that such systems be separated from the
boiler with a heat exchanger.
Underfloor heating systems and heating circuits
containing a very large volume of water must be
connected to the boiler via a 3-way mixing valve; please
refer to the applicable installation example in this manual.

Oxygen diffusion barrier underfloor tubing

The boiler warranty does not cover leaks resulting from
corrosion caused by the use of underfloor plastic tubing
without an oxygen diffusion barrier. Such systems must
have the non-oxygen diffusion barrier tubing separated
from the boiler with a heat exchanger. Viessmann
recommends the use of underfloor plastic tubing with
an oxygen diffusion barrier.

Warranty

Our warranty does not cover damages resulting from
the following:

- installation or service by unqualified and unlicensed
personnel.

- attempting to perform any repair work on the boiler
other than that mentioned in the boiler literature.

- tampering with or attempting, without Viessmann
permission, to readjust the factory settings of the;

-combination gas valve

-combustion air opening of the burner blower

- leaks resulting from corrosion caused by the use of
underfloor plastic tubing without an oxygen diffusion
barrier. For detailed warranty information, please read
warranty sheet supplied with product.

5673 647 - 03

29

Vitocrossal 300 CU3A 26 to 57 Technical Data

30

5673 647 - 03

Vitocrossal 300 CU3A 26 to 57 Technical Data

5673 647 - 03

31

Vitocrossal 300 CU3A 26 to 57 Technical Data

(recycled and recyclable) paper.

Technical information subject to change without notice. Printed on environmentally friendly

5673 647 - 03