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The PC705-2 is a microprocessor-based PID control designed to regulate the supply water tempera-
ture to a heating system by controlling a boiler and the speed of an injection pump. The PC705-2
controls the system supply water temperature based on the outdoor air temperature (outdoor reset). A
boiler sensor can be connected in order to prevent corrosion in the boiler due to flue gas condensa-
tion. If the PC705-2 is controlling the boiler supply water temperature, the control continuously adjusts
the boiler differential in order to optimize the firing cycles of the boiler, prevent large water temperature
swings, and increase the efficiency of the system. A wiring harness is provided to be easily connected
to the Taco Expandable (-EXP) Controls.
The PC705-2 also includes features such as an automatic reset ratio calculation, Warm Weather Shut
Down (WWSD), and a maximum supply water temperature setting.
Control Strategy . . . . . . . . pg. 1
Control Adjustments . . . . . . . . pg. 9
Variable Speed Injection . . pg. 3
Display Operation . . . . . . . . . . pg. 10
Sequence of Operation . . . pg. 4
Troubleshooting . . . . . . . . . . . pg. 11
Installation . . . . . . . . . . . . . pg. 5
Error Messages . . . . . . . . . . . . pg. 12
Applications . . . . . . . . . . . . .pg. 6
Settings . . . . . . . . . . . . . . . . .
pg. 9
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
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
the outdoor temperature drops, the building heat loss increases.
Instruction Sheet
PC705-2 Variable Speed Injection Mixing Control
Heat
Loss
Increases
Outdoor
Temperature
Drop
Constant
Room Temperature
Control Strategy
Plant ID#: 9300-1057
SUPERSEDES:
March
1
, 200
4
EFFECTIVE: March
2
, 20
16
102-098
Mixing
Demand
PC705-2
Start
10 30 50 70 90
% Out
Item
C US
158033
OUTDR
°
F
VIEW
Input
Outdoor Sensor
Included
To
Taco
Zone
Control
Input
Supply Sensor
Included
Item
1
H1
194A
Power:
24 V
±
10% 50/60 Hz 3 VA
Var. Pmp:
120 V (ac) 2.4 A 1/6 hp
Boiler:
24 V (ac) 5 A 1/6 hp pilot 240 VA
Pump:
120 V (ac)
2
3
Out
Com Sup
Signal wiring
must be rated
at least 300V
Made in Canada
PC705-2
C US
158033
Press & Hold:
Item, to view settings
, to test
Press & Release:
Item + + to adjust menu
10 30 50 70 90
Boil
4
Input
Boiler Sensor
Included
Do not apply power
Mixing
Demand
Start
% Out
Output
Variable Speed
Injection Pump
(through back of control)
Output
System Pump
and/or Boiler Pump
(through back of control)
VIEW
OUTDR
°
F
