Taco PC702-1 User Manual

Instruction Sheet

Constant

Room Temperature

Outdoor

Temperature

Drop

Heat
Loss

Increase

SUPERSEDES : 102-106, dated June 1, 2000

Plant ID# 9300-1059

The PC702-1 is a microprocessor-based control designed to regulate the supply water temperature
from a two stage boiler system based on the outdoor temperature or DHW requirements. A wiring
harness is provided to be easily connected to the Taco Expandable (-EXP) Controls.

OUTDR

VIEW

°F

Interstage

1 2

DHW

Boiler

Delay

Demand

Demand

®

CUS

PC702

158033

Start

Item

The PC702-1 includes functions such as automatic reset ratio calculation, Warm Weather Shut Down
(WWSD), Minimum Boiler setting, adjustable interstage delay setting, optional boiler rotation, and an
optional automatic boiler differential. The control has a digital, liquid crystal display (LCD) that nor­mally displays the Boiler Supply temperature, but can display other temperatures and settings.

Control Strategy . . . . . . . . . . . . pg. 1 Control Adjustments . . . . pg. 6

Sequence of Operation . . . . . . . pg. 2 Troubleshooting . . . . . . . pg. 7

Installation . . . . . . . . . . . . . . . . . pg. 4 Error Messages. . . . . . . . . . . pg. 8

Settings . . . . . . . . . . . . . . . . . . . pg. 5 Technical Data . . . . . . . . . . . pg. 8

PC702–1 Two Stage Boiler

Reset & DHW Control

102-106

EFFECTIVE: March 1, 2004

DHW
Demand

Output

Boiler

Output

Boiler

Press & Hold:

Item,

to view settings

, to test

Press & Release:

Item +

+

to adjust menu

OUTDR

Start

Item

1

2

Rotation On / Off

Stage
1 1

Power : 24 V ±10% 50/60 Hz 3 VA
Relays: 120 V (ac) 5 A 1/6 hp pilot 240 VA
Demand: 20 to 260 V (ac) 2 VA

Delay

Interstage

PC702-1

Signal wiring

3

4

must be rated

Stage

at least 300V

2 2

VIEW

DHW

Demand

Do not apply power

5

Com

Jul 00

31000265

°F

1 2

Boiler

Demand

CUS

67

Sup

158033

Out

®

Made in
Canada

H1192A

To Taco
Zone
Control

Input

OutdoorSensor

Included

Input

Boiler Sensor

Included

Control Strategy

OUTDOOR RESET

In order to properly control a hot water heating system, the heat supplied
to the building must equal the heat lost by the building.

• The heat supplied to a building is proportional to the temperature of the
supply water and the surface area of the heating element. A small surface
area such as baseboard radiators requires a higher water temperature
than a larger surface area such as radiant floors.

• The heat lost from a building is dependent on the outdoor temperature,
as well as other factors. As the outdoor temperature drops, the building
heat loss increases.

Reset Ratio

Supply Water Temperature

Time

Differential = 10°F (5°C)

165°F(74°C)

B

o

i

l

e

r

o

f

f

B

o

i

l

e

r

o

n

155°F (68°C)

160°F (71°C)

B

o

i

l

e

r

o

f

f

B

o

i

l

e

r

o

n

Operation of a hot water heating system can generally be improved by modulating the
supply water temperature as the outdoor temperature changes. Using this approach, the
heat lost from the building is matched by the heat input to the building. The PC702-1 Boiler
Reset Control utilizes a reset ratio to set the relationship between outdoor temperature
and supply water temperature. The reset ratio determines the amount the supply water
temperature is raised for every 1° drop in outdoor air temperature, and it is determined
from the starting point and the system design conditions. In order for the control to
automatically determine the reset ratio, a starting point and design conditions must be
established. These two points are set by the following 4 adjustments:

• Boiler starting temperature

• Outdoor starting temperature

• Boiler design supply water temperature

• Outdoor Design Temperature

See Settings - Step Four for a complete description of each setting.

Reset

Starting

90

(32)

Ratio

Point

70

(21)

Design

Conditions

50

(10)30(-1)10(-12)

Outdoor air temperature

-10

(-23)

210
(99)

190
(88)

170
(77)

150
(65)

130

(54)

11 0

(43)

Supply water temperature

90

(32)

70

(21)

°

F

°C)

(

Reset Ratio Starting Point

The first point used to establish the reset ratio calculation is the starting point. It is a combination of an adjustable boiler starting water
temperature setting and an adjustable outdoor starting temperature setting.

Design Conditions

The second point to establish the reset ratio calculation is the design conditions. This point represents the required water temperature
during the coldest day of the year.

Warm Weather Shut Down (WWSD)

When the outdoor air temperature is warmer than the Warm Weather Shut Down setting, no additional heat is required in the building;
therefore, the heating system can be shut down. This setting has no effect on the reset ratio calculation.

BOILER OPERATION

The supply water temperature from the boiler(s) can be controlled by cycling the boiler(s) on and off. Modulation of the operating
temperature in hot water heating systems not only provides more comfort but also offers significant energy savings. The cooler the
boiler(s) runs, the more efficient it is due to less heat losses up the flue and reduced boiler jacket losses.

Differential

An on / off boiler must be operated with a differential in order to prevent
short cycling. When the supply water temperature drops below the
bottom rail of the differential, the boiler is turned on. The boiler is then
kept on until the supply water temperature rises above the top rail of
the differential. If the differential is too wide, there can be large supply
water temperature swings; however, if the differential is too narrow, the
boiler short cycles and operates inefficiently. This control can auto­matically calculate the boiler differential in order to achieve an appro­priate balance between temperature swings and boiler efficiency. This
also permits the control to adapt to changing loads and conditions.

Minimum Boiler Supply

Most boilers require a minimum supply water temperature in order to prevent corrosion
from flue gas condensation. The control should therefore only modulate the boiler supply
water temperature down to the boiler manufacturer’s minimum recommended operating
temperature. Some boilers are designed to condense and should be operated at low
water temperatures as much as possible for maximum efficiency.

Domestic Hot Water (DHW) Boiler Supply

Most indirect DHW tanks require a relatively high supply water temperature in order to
produce adequate amounts of DHW. The control should therefore override the reset ratio
when there is a demand for DHW and raise the system supply temperature to a
temperature that is suitable for DHW generation.

70

90

(21)

(32)

WWSD

Temperature

Reset

Ratio

50

(10)30(-1)10(-12)

Outdoor air temperature

Minimum

-10

(-23)

°F
°C)

(

210
(99)

190
(88)

170
(77)

150
(65)

130

(54)

11 0

(43)

Supply water temperature

90

(32)

70

(21)

Sequence of Operation

POWERING UP THE CONTROL

After the PC702-1 is powered up, all the LCD segments are turned on for 2 seconds, followed by a software version number. The control
then displays the outdoor temperature.

2 of 8

BOILER DEMAND

X

X

END

ZC

Zone 1 Zone 2 Zone 3 Zone 4

ZR

Zone 4Zone 3Zone 2Zone 1

The PC702-1 obtains a boiler demand through the Taco Zone Control when
a zone calls for heat. When the PC702-1 receives a boiler demand, the

Boiler Demand

pointer is displayed in the LCD.

DHW DEMAND

The PC702-1 obtains a DHW demand when a voltage between 24 and 240
V(ac) is applied across the two DHW demand wires (black wires) that come
out of the back of the control. When the PC702-1 receives a DHW demand,

DHW Demand

the

pointer is displayed in the LCD.

OUTDR

VIEW

°

F

1 2

Start

Interstage

DHW

Boiler

Delay

Demand

Demand

Item

®

CUS

PC702

158033

SWITCH

WARM WEATHER SHUT DOWN (WWSD)

When the outdoor temperature rises above the WWSD setting, the PC702-1 displays the WWSD segment in the LCD and turns off the
boiler(s). If the PC702-1 receives a boiler demand from the Taco Zone Control while in a WWSD, the boiler(s) remain off. If the
PC702-1 receives a DHW demand while in a WWSD, the boiler(s) are fired to satisfy the DHW demand.

BOILER OPERATION

The boiler(s) operates around a differential that is either manually set or automatically controlled by the PC702-1. The control includes
a minimum on and off time for the heat source(s), in order to minimize short cycling. The automatic boiler differential increases system
efficiency by adjusting to changing loads.

POWER

INPUT120 VOLT CIRCULATORS

Boiler Demand

While the PC702-1 has a boiler demand, the system supply water temperature is controlled by turning the boiler(s) on and off. The
PC702-1 calculates the target supply water temperature based on the outdoor temperature, the reset ratio settings, and the boiler
minimum setting.

DHW Demand

While the PC702-1 has a DHW demand, the system supply water temperature is controlled by turning the boiler(s) on and off. The
PC702-1 targets a supply water temperature of at least 180°F (83°C).

Maximum Boiler Supply

The PC702-1 does not allow the boiler target temperature to exceed the boiler design
temperature setting plus 10°F (6°C) to a maximum of 225°F (107°C). When operating at

Temperature

Maximum

WWSD

the maximum, the control displays the MAX segment in the LCD when either that boiler
supply or boiler target temperature are being viewed.

Minimum Boiler Supply

Minimum

The PC702-1 has a minimum boiler setting which selects a minimum boiler target
temperature in order to prevent corrosion from flue gas condensation. During mild
outdoor conditions, the boiler(s) cycles around the minimum boiler setting. This pro-

Reset

Ratio

grammed function has an OFF setting for condensing and electric boilers. When
operating at the minimum setting, the control displays the MIN segment in the LCD when
either that boiler supply or boiler target temperature are being viewed.

90

(32)

50

70

(10)30(-1)10(-12)

(21)

Outdoor air temperature

-10

(-23)

°

F

°C)

(

ROTATION

The PC702-1 has an optional boiler rotation function that is selected using the DIP switch located next to terminal 1 in the wiring
chamber. The PC702-1’s boiler rotation function is fixed at 48 hours. The firing order of the boilers changes whenever one stage
accumulates 48 hours more running time than the other stage. After each rotation, the stage with the least running hours is the first to
fire, and the stage with the most running hours is the last to fire. This function ensures that both stages receive equal amounts of use.
When the
NOTE: If the boiler used is a lo-hi fire boiler, set the

Rotation On / Off

DIP switch is set to the

Off

position, Stage 1 is always the first stage to fire.

Rotation On / Off

DIP switch to the

Off

position.

210
(99)

190
(88)

170
(77)

150
(65)

130

(54)

11 0

(43)

Supply water temperature

90

(32)

70

(21)

STAGING

The PC702-1 controls up to two stages in order to supply the required target temperature. After the first stage is turned on in the firing
sequence, the control waits a minimum amount of time before turning on the next stage. This minimum amount of time is set using the
interstage delay setting. After the interstage delay has expired, the PC702-1 examines the current supply water temperature to
determine when the next stage is to fire. The control uses Proportional, Integral and Derivative (PID) logic.

Proportional - compares the actual supply temperature to the boiler target temperature. The colder the supply water temperature, the
sooner the next stage is turned on.

Integral - compares the actual supply temperature to the boiler target temperature over a period of time.

Derivative - determines how fast or slow the supply water temperature is changing. If the supply temperature is increasing slowly, the
next stage is turned on sooner. If the supply temperature is increasing quickly, the next stage is turned on later, if at all.

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