4 of 12
POWERING UP THE CONTROL
After the PC705-2 is powered up, all the LCD segments are tuned on for 2
seconds, followed by a software version number. The control then displays the
outdoor temperature.
MIXING DEMAND
The PC705-2 obtains a mixing demand through the Taco Zone Control when
a zone calls for heat. When the PC705-2 receives a mixing demand, the Mixing
Demand pointer is displayed in the LCD.
MIXING OPERATION
The PC705-2 calculates the required supply water temperature based on the outdoor
temperature. The supply water temperature is then controlled by varying the speed of an
injection pump to maintain the target supply temperature. As the heating load increases,
the PC705-2 speeds up the injection pump and as the load decreases, the PC705-2 slows
down the injection pump. The bar graph in the bottom of the LCD indicates the speed of
the variable speed injection.
Warm Weather Shut Down (WWSD)
When the outdoor temperature is warmer than the WWSD setting, the PC705-2 turns
off the boiler and the variable speed injection pump. The PC705-2 has a freeze
protection feature that does not allow the supply water temperature to drop below 35
°
F
(2
°
C) as long as there is a mixing demand signal.
Maximum System Supply Temperature
The PC705-2 has a Maximum Supply setting that can be used to set a maximum system
supply water temperature. If the supply water temperature approaches the Maximum
Supply setting, the control reduces the speed of the injection pump.
SYSTEM PUMP OPERATION
The PC705-2 has an internal system pump contact. This contact turns on when the PC705-2 has a mixing demand and is not in a WWSD.
The system pump as well as the boiler pump may be controlled by this relay. By providing proper flow in the boiler loop, the boiler
temperature can be accurately controlled based on the mixing load.
Equation 1
System Flow Rate (US GPM) =
Design Heating Load (Btu/hr)
500 x
∆
Ts (
°
F)
Equation 2
∆
Ts (
°
F) =
Design Heating Load (Btu/hr)
500 x System Flow Rate (US GPM)
Equation 3
Design Injection Flow Rate (US GPM) =
System Flow Rate (US GPM) x Flow Ratio
Tb - Ts (
°
F)
∆
Ts (
°
F)
0.40
0.00
0.20
1.00
0.80
0.60
10
30
50
70
90
10
15
20
25
Flow Ratio
20
40
60
80
0.10
0.30
0.50
0.70
0.90
Fig. 2
Taco 007
Taco 007
Taco 007
Taco 007
Taco 007
Taco 007
Taco 007
Taco 0010
Taco 0012
Taco 0012
Taco 0012
Design Injection Flow Rate (US GPM)
Nominal Pipe
Diameter (inches)
TACO
Pump
Without Balancing Valve
—
4.0
4.5
7.3
9.5
10.0
14.0
23.0
35.0
41.0
45.0
With Balancing Valve
1.5
3.0
3.0
6.0
7.4
8.3
12.0
20.0
31.0
38.5
41.5
0.5
0.5
0.5
0.75
0.75
0.75
1
1.25
1.5
2
2
This table assumes there are 5 feet of pipe, 4 elbows, and 4 branch tees of the listed diameter.
Turns open of the
Balancing Valve (%)
20
100
50
100
100
100
100
100
100
100
100
Fig. 3
Sequence of Operation
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
90
(32)
°
F
(
°
C)
Supply w
ater temper
ature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Maximum System
Supply Setting
Zone 1 Zone 2 Zone 3 Zone 4
POWER
Zone 4
Zone 3
Zone 2
Zone 1
INPUT
120 VOLT CIRCULATORS
END
SWITCH
ZR
X
X
ZC
Mixing
Demand
PC705-2
Start
10 30 50 70 90
% Out
Item
C US
158033
OUTDR
°
F
VIEW
