Taco PC705-2 User manual

1 of 12
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, 2004EFFECTIVE: March 2, 2016
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
H1194A
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)
23
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

2 of 12
Reset Ratio
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 PC705-2
Variable Speed Injection Mixing 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:
• Mixing starting temperature
• Outdoor starting temperature
• Mixing design supply water temperature
• Outdoor design temperature
See Settings - Step Four for a complete description of each setting.
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 mixing starting water
temperature setting and the 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 a boiler can be controlled by cycling
the boiler on and off. Modulation of the boiler’s operating temperature in
hot water heating systems not only provides more comfort but also offers
significant energy savings. The cooler the boiler 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 automatically calcu-
lates 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 tempera-
ture. Some boilers are designed to condense and should be operated
at low water temperatures as much as possible for maximum efficiency.
MIXING OPERATION
The full range of water temperatures required through a heating season
can be provided with a standard (non-condensing) boiler by incorporating
a modulating mixing device into the system. Mixing valves or variable
speed injection pumps are commonly used to modulate both the system
supply water temperature and the boiler return water temperature. The
modulation of water temperatures improves comfort in the building and
also protects the boiler from cool return water.
Design
Conditions
Reset
Ratio
Starting
Point
Outdoor Temperature
Supply Water Temperature
90
(32)
70
(21)
50
(10)
30
(-1)
10
(-12)
-10
(-23)
°F
(°C)
70
(21)
90
(32)
110
(43)
130
(54)
150
(65)
170
(77)
190
(88)
210
(99)
Supply Water Temperature
Time
Differential = 10°F (5°C)
165°F(74°C)
Boileroff
Boileron
155°F (68°C)
160°F (71°C)
Boileroff
Boileron
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
90
(32)
WWSD
Temperature
°F
(°C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
Boiler
Minimum
Boiler
Loop
System
Loop Mixing
Device

3 of 12
Variable Speed Injection Pump
The PC705-2 uses a pump to inject hot water from the boiler loop into the cool system return water. This pump is operated by the
PC705-2 at different speeds in order to inject hot boiler water at different rates into the cool system return water. For residential and
small commercial systems Taco wet rotor circulators are suitable for use as variable speed injection pumps.
Boiler Loop Pump and a Boiler Supply Sensor
The use of a boiler loop pump and boiler supply sensor allows the PC705-2 to regulate the boiler supply water temperature as well
as the system supply water temperature. If the system supply water temperature approaches a maximum setting, the control can
back off the speed of the injection pump in order to protect the system. The same also occurs when the boiler supply water
temperature approaches a minimum boiler setting.
The rate of response to sudden temperature changes by the variable speed injection system can be typically 2 to 4 times faster
than by a tempering valve or mixing valve. This faster response provides greater protection for both the system and the boiler.
PIPING OF VARIABLE SPEED INJECTION SYSTEMS
Since mixing by a variable speed injection pump is a relatively new concept, there are a few piping details which should be considered.
When the injection pump is turned off, there must be no heat
transfer from the boiler loop to the system loop. In order to
avoid this unwanted heat transfer, standard primary-second-
ary piping techniques are used as shown in Figure 1. This
piping arrangement requires that the injection piping be at least
one pipe diameter smaller than the piping of the boiler and
system loops. There must be no more than 4 pipe diameters
between the tees in the boiler and system loops (Note 1) in
order to prevent ghost flow when the variable speed injection
pump is off and either the boiler pump or system pump is on.
Also, there must be at least 6 pipe diameters of straight pipe on
either side of the tees (Note 2) in order to prevent the momen-
tum of water in the boiler and system loops from pushing flow
through the injection loop. Finally, there should be a minimum
1 foot drop to create a thermal trap (Note 3) in order to prevent
convective heat transfer through the injection loop.
Design Procedure
1) Determine the design operating temperatures of the heating system loop and boiler. (Ts and Tb from figure 1.)
2) Determine the flow rate and design temperature drop (∆Ts) in the system loop when all zones are turned on. If one of these
variables is unknown, use Equation 1 or 2 to calculate the other variable.
3) Compute Tb - Ts. Look up the flow ratios on Figure 2.
4) The design injection flow rate for direct injection is calculated from Equation 3. If the injection flow rate is greater than 45 US GPM,
it may be necessary to use a 4-way mixing valve instead of a variable speed injection system.
5) Decide whether or not to include a balancing valve in the injection piping. A balancing valve allows adjustment when the injection
pump is larger than needed. A balancing valve also provides the possibility of manual operation of the heating system by turning
the injection pump fully on and adjusting the balancing valve to obtain the desired system supply water temperature.
6) The injection piping size and model of pump to install can now be looked up in Figure 3. Don't oversize the injection system. If
the heating system is not able to get enough heat, the boiler's operating temperature can be increased. This may be done
automatically by the PC705-2 when the boiler sensor is used.
Boiler Protection
Cool water is often returned to the boiler from low temperature radiant floor heating systems or when the
heating system is recovering from night setback. This cool boiler return water may cause the boiler to operate
at such a low temperature that the flue gases condense. Alternatively, when the boiler surfaces are hot due
to previous loads such as domestic hot water generation, the large temperature difference (∆T) between the
boiler and its return water can cause the boiler to become thermally shocked. Proper protection of the boiler
under these circumstances requires a modulating mixing device that can temporarily reduce the heating load.
This is normally accomplished by closing a valve or reducing the speed of an injection pump.
Maximum System Supply
Some systems, such as hydronic radiant floor heating, usually operate at water temperatures that are below
the minimum boiler supply temperature. This is due to the large surface area of the floors which radiate a
significant amount of heat at low water temperatures. Floor heating systems also have a maximum floor
surface temperature limit for occupant health reasons and to protect the materials within the floor. In such
systems a modulating mixing device is normally required to limit the supply water temperature.
Mix
Mix
Variable Speed Injection
Fig. 1
Balancing
Valve
Boiler
Loop
System
Loop
(Ts) (Tb)
Variable Speed
Injection Pump
∆Ts
Tb = Boiler supply temperature
Ts = System supply temperature
∆Ts = System temperature drop (Typically 20°F for
convectors and 10°F for radiant floor heating)
Note 3
Note 1
Note 2
Note 3
Note 1
Note 2

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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 water temperature
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 4Zone 3Zone 2Zone 1
INPUT120 VOLT CIRCULATORS
END
SWITCH
ZR
XX
ZC
Mixing
Demand
PC705-2
Start 10 30 50 70 90
%Out
Item
C US
158033
OUTDR
°F
VIEW

5 of 12
140°F (60°C)
Boiler Supply
Supply
Sensor
CAUTION
Improper installation and operation of this control could result in damage to the equipment and possibly even personal injury.
It is your responsibility to ensure that this control is safely installed according to all applicable codes and standards. This
electronic control is not intended for use as a primary limit control. Other controls that are intended and certified as safety limits
must be placed into the control circuit.
STEP ONE
Check the contents of this package. If any of the contents listed are missing or damaged, please contact your sales representative for
assistance.
Type PC705-2 includes: • PC705-2 Variable Speed Mixing Control • One Outdoor Sensor
• Two Strap on Sensors • Instruction Sheet
Replacement sensors are available from Taco, Inc. Strap on Sensor – 9300-2044RP Outdoor Sensor – 9300-2052RP
Note: Carefully read the details of the Sequence of Operation sections in all applicable brochures to ensure that you have chosen the
proper control for your application.
STEP TWO
Installing the Control
• Grasp the front cover by the fingertip grips on the
top and bottom of the enclosure and pull the front
cover off. Remove the wiring cover screw.
• The enclosure is mounted onto a 2” x 4” electrical
box near the Taco Zone Control. Ensure that the
depth of the electrical box is enough to house all
of the wiring that will be in it.
• The mounting holes in the enclosure accept #6
screws.
• Wiring to the control enters the wiring chamber
through the back or bottom of the enclosure.
• To reassemble the enclosure, first replace the
wiring chamber cover and then push the front
cover onto the enclosure until it snaps into place.
The variable speed injection pump and system pump
wiring terminates in the electrical box. All other electri-
cal wiring terminates in the two wiring chambers on the
control. The wiring is roughed-in to the electrical box
prior to installation of the control.
BOILER OPERATION
Boiler Sensor Installed
While the PC705-2 has a mixing demand, the boiler supply water temperature is controlled
by turning the boiler on and off. The PC705-2 calculates the required boiler supply water
temperature based on the load on the mixing system. In order to allow the PC 705-2 to
control the boiler, the Mode switch on the Zone Control must be set to Reset.
The boiler operates around a differential that is automatically controlled by the PC705-2.
The control includes a minimum on and off time for the heat source, in order to minimize
short cycling. The automatic boiler differential increases system efficiency by adjusting to
changing loads.
Boiler Sensor Not Installed
When the PC705-2 has a mixing demand, the boiler turns on and operates off the boiler
operating aquastat. The Mode switch on the Zone Control must be set to Normal in this
application. To improve boiler operation efficiency, the boiler design temperature should
be selected as low as possible, and the injection pump, must then be sized accordingly.
Boiler Protection
When the boiler sensor is installed on the supply side of the boiler the PC705-2 can protect the boiler against flue gas condensation.
When this sensor is installed, the PC705-2 ensures a boiler supply water temperature of at least the Boiler Minimum setting. If the
water temperature is less than the Boiler Minimum setting, the PC705-2 turns on the MIN segment in the LCD and decreases the speed
of the injection pump.
Installation
140°F (60°C)
Boiler Supply
Supply
Sensor

6 of 12
Reference Applications PC 705-2
Drawing 1/705-2: Radiant Floor Heating
Drawing 2/705-2: Radiant Floor Heating with DHW generation
Outdoor
Sensor
Boiler
Sensor
Supply
Sensor
Priority Zone Switch
set to OFF
120 V (ac)
PC
705-2 Expansion Switch
set to Master
Mode Switch set to
Reset
M
MMM
TTT
Zone 4Zone 3Zone 2Zone 1
END
SWITCH
A
Zone 4Zone 3Zone 2Zone 1
Taco Zone Control ZVC404-EXP
END
SWITCH
B
T
Boiler
Pump
Injection
Pump
Boiler
Pump
System
Pump
Injection
Pump
120 V (ac)
Outdoor
Sensor
Boiler
Sensor
Supply
Sensor
Priority Zone
Switch set to ON
DHW
Tank
TT TT
POWER
Zone 4Zone 3Zone 2Zone 1 INPUT
END
SWITCH ZR
XX
ZC
Zone 4Zone 3Zone 2Zone 1
Taco Zone Control SR 506-EXP
Zone 5 Zone 6
Zone 5 Zone 6
T
PC
705-2
A
Expansion Switch
set to Master
Mode Switch set to
Reset

7 of 12
Reference Applications PC 705-2
Drawing 3/705-2: Radiant Floor Heating, High Temperature heating with DHW generation Drawing 4/705-2: Radiant Floor Heating, High Temperature heating with 2 boilers and DHW generation
120 V (ac)
T
Outdoor
Sensor
TT
Outdoor
Sensor
Boiler
Sensor
Supply
Sensor
Priority Zone
Switch set to ON
Priority Zone
Switch set to OFF
Boiler
Sensor
(optional)
120 V (ac)
T
Outdoor
Sensor
TTTTTTT
Outdoor
Sensor
Boiler
Sensor
Supply
Sensor
Priority Zone
Switch set to ON
Priority Zone
Switch set to OFF
Boiler
Sensor
(optional)
TT
POWER
Zone 3
Zone 2
Zone 1
INPUT
END
SWITCH
ZR
XX
ZC
Zone 3
Zone 2
Zone 1
Taco Zone Control SR 503-EXP
Boiler
Pump
Boiler
Pump
Injection
Pump
Injection
Pump
DHW
Tank
POWER
Zone 3
Zone 2
Zone 1
INPUT
END
SWITCH
ZR
XX
ZC
Zone 3
Zone 2
Zone 1
Taco Zone Control SR 503-EXP
A
Expansion Switch
set to Master
Mode Switch set to
Normal
Expansion Switch
set to Slave
Mode Switch set to
Normal
PC
705-2 PC
702-1
PC
700-2
DHW
Tank
POWER
Zone 3
Zone 2
Zone 1
INPUT
END
SWITCH
ZR
XX
ZC
Zone 3
Zone 2
Zone 1
Taco Zone Control SR 503-EXP
POWER
Zone 4
Zone 3
Zone 2
Zone 1
INPUT
END
SWITCH
ZR
XX
ZC
Zone 4
Zone 3
Zone 2
Zone 1
Taco Zone Control SR 506-EXP
Zone 5 Zone 6
Zone 5 Zone 6
A
Expansion Switch
set to Master
Mode Switch set to
Reset
Expansion Switch
set to Slave
Mode Switch set to
Normal
PC
705-2
*
*
* Same power supply circuit must be used
* Same power supply circuit must be used

8 of 12
Installing the Outdoor Sensor
Note The temperature sensor (thermistor) is built into the enclosure.
• Take the sensor cover off by sliding it upward relative to the sensor base.
• Use one round or pan head screw to attach the base of the sensor to the wall.
• The sensor is wall mounted and the wiring enters through the bottom of the enclosure. The
hole for the cable entry must face downward in order to prevent water from entering and filling
the enclosure.
• The sensor should be mounted on a wall which best represents the heat load on the
building (i.e. a northern wall for most buildings and a southern facing wall for buildings with
large south facing glass areas). The sensor should not be installed near heat sources
such as ventilation or window openings.
• The sensor should be installed at an elevation above the ground that will prevent
accidental damage or tampering.
• Install the Outdoor Sensor and run the wiring back to the control mounting location.
Installing the Supply Sensor and Boiler Sensor
Note These sensors are designed to mount on a pipe or in a temperature
immersion well.
• The sensors can be strapped directly to the pipe using the cable tie
provided. Insulation should be placed around the sensors to reduce
the effect of air currents on the sensor measurement.
• The Boiler Sensor and the Supply Sensor should be placed down-
stream of a pump or after an elbow or similar fitting. This is especially
important if large diameter pipes are used because the thermal
stratification within the pipe can result in erroneous sensor readings.
Proper sensor location requires that the fluid is thoroughly mixed
within the pipe before it reaches the sensor.
STEP THREE
Wiring to the Zone Control
Connect the PC705-2 cable to the Add-On interface on the Taco Zone Control.
Ensure the Mode switch on the Zone Control is set to Reset if boiler control is
desired, or Normal if boiler enable is desired (refer to the Boiler Operation section).
Wiring the Sensors
Do not apply power to these terminals as this will damage the control.
Outdoor Sensor
Connect the two wires from the Outdoor Sensor directly to the Com — Out (1 and
3) terminals. The Outdoor Sensor measures the outdoor air temperature.
Supply Sensor
Connect the two wires from the Supply Sensor directly to the Com — Sup (1 and
2) terminals. The Supply Sensor measures the water temperature supplied to the
system.
Boiler Sensor (Optional)
Connect the two wires from the Boiler Sensor directly to the Com— Boil (1 and 4)
terminals. The Boiler Sensor measures the boiler supply water temperature.
Variable Speed Injection Pump
The PC705-2 can vary the speed of a permanent capacitor impedance protected
or equivalent pump motor that has a locked rotor current of less than 2.4 A. The
Taco "00" family of wet rotor circulators are suitable for the PC705-2. The variable
speed output must not be used on pumps which have a centrifugal switch. The
PC705-2 has an internal overload protection fuse which is rated at 2.5 A 240 V (ac).
This fuse is not field replaceable. Contact your sales representative for details on
the return and repair procedures if this fuse is blown.
Connect the black wire from the back of the control to the live (L) side of the 120
V (ac) power source. Connect one of the wires from the variable speed injection
pump to the blue wire from the back of the control. The other wire on the variable
speed injection pump must be connected to the neutral (N) side of the 120 V (ac)
power supply. Connect the green wire on the back of the control to ground.
Ground
(green)
Injection Pump
(blue)
Live Power (L)
(black)
System Pump
(Red)
42
BoilSup
1
Com
3
Out
32
Sup Out
1
Com
4
Boil
32
Sup Out
1
Com
4
Boil
Cable Entry
Hole for
mounting screw

9 of 12
Before adjusting the dial settings, read through the sequence of operation to ensure that you understand how the control operates.
STEP FOUR
Mixing Starting Temperature (MIX Start)
The MIX Start setting is the starting supply water temperature of the reset ratio, and can be adjusted from 35 to 150°F (2 to 66°C).
This setting is typically set to the desired building temperature. In applications where fan coils are used, the MIX Start may need to
be set higher to prevent cold drafts during mild outdoor conditions. If the building feels cool during mild outdoor conditions, the MIX
Start setting should be increased.
Outdoor Starting Temperature (OUTDR Start)
The OUTDR Start setting is the outdoor temperature at which the mixing starting temperature is supplied. The adjustment range is
from 35 to 85 °F (2 to 29°C). This setting is typically set to the desired building temperature.
Mixing Design Temperature (MIX DSGN)
The MIX DSGN setting is the water temperature required to satisfy the building heat loss during the coldest outdoor temperature. This
adjustment is typically dependent on the type of heating terminal used. The following are suggested settings for different terminal units:
• Inslab Radiant ..................... 100 to 140°F (38 to 60°C) • Radiant Baseboard ................. 130 to 160°F (54 to 71°C)
• Staple-up Radiant .......... 130 to 160°F (54 to 71°C) • Convective Baseboard ....... 160 to 190°F (71 to 88°C)
• Radiators ............................ 140 to 160°F (60 to 71°C) • Fan Coil .............................. 180 to 200°F (82 to 93°C)
Outdoor Design Temperature (OUTDR DSGN)
The OUTDR DSGN setting is the outdoor temperature used in the heat loss calculation. It is set to the typical coldest outdoor
temperature.
Mixing Maximum Temperature (MIX MAX)
The PC705-2 prevents the mixed supply water temperature from rising above the MIX MAX setting. The adjustment range is from
80 to 225°F (27 to 107°C). The MIX MAX should be set to the maximum temperature allowed in the system loop. There are many
factors which may limit the allowable supply water temperature in a radiant floor heating slab; a few are provided below.
• Nowhere in the concrete should the temperature be maintained above 170°F (77°C).
• The surface temperature of a radiant floor heating slab should normally not exceed 85°F (29°C). The slab surface temperature is
affected by the slab thermal resistance, the heating load, and the supply water temperature to the slab.
Boiler Minimum Supply Temperature (BOIL MIN)
Most boilers require a minimum operating temperature to prevent corrosion from flue gas condensation. The minimum boiler setting
should be programmed to the lowest supply water temperature at which the boiler can operate without causing the boiler flue gases
to condense. Consult the boiler manufacturer for recommended minimum boiler supply temperatures. Some typical settings are as
follows:
• Steel fire tube boiler ….......... 140 to 160°F (60 to 71°C) • Condensing boiler ................. Off
• Cast iron boiler ….................. 135 to 160°F (57 to 71°C) • Electric boiler ........................ Off
• Copper tube boiler …............. 125 to 150°F (52 to 66°C)
Note: This setting is only available if a boiler sensor is connected to the PC705-2.
Warm Weather Shut Down (WWSD)
The WWSD can be adjusted from 35 to 100°F (2 to 38°C). The system will be shut down when the outdoor temperature is warmer
than this setting.
Settings
Control Adjustments
Mixing Start:
Outdoor Start:
Mixing Design:
Outdoor Design:
Mixing Maximum:
Boiler Minimum:
WWSD:
System Pump
The PC705-2 has an internal relay rated for 5 Amps to operate a system pump. Connect the black wire from the back of the control
to the live (L) side of the 120 V (ac) power source. Connect one of the wires from the system pump to the red wire from the back
of the control. The other wire on the system pump must be connected to the neutral (N) side of the 120 V (ac) power supply. Connect
the green wire on the back of the control to ground.

10 of 12
Display Operation
Start
TARGET
DSGN
BOIL
MIX
OUTDR
WWSD
° F
° C
MAXMIN
Mixing
Demand
% Out
10 30 50 70 90
VIEW ADJUST
Start
OUTDR
° F
Mixing
Demand
% Out
10 30 50 70 90
VIEW
Start
TARGET
MIX
° F
Mixing
Demand
% Out
10 30 50 70 90
VIEW
Start
OUTDR
° F
Mixing
Demand
% Out
10 30 50 70 90
VIEW
Start
MIX
° F
Mixing
Demand
% Out
10 30 50 70 90
VIEW
Start
BOIL
° F
Mixing
Demand
% Out
10 30 50 70 90
VIEW
Start
MIX
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
DSGN
OUTDR
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
WWSD
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
OUTDR
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
MIX
° F
MAX
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
DSGN
MIX
° F
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
Start
BOIL
° F
MIN
Mixing
Demand
% Out
10 30 50 70 90
ADJUST
On power up, the
control displays all
segments for 2
seconds, followed
by the control
version number.
The control then
automatically goes
to operating mode,
and displays the
outdoor
temperature.
Outdoor
Temperature
The outdoor
temperature is
displayed when the
OUTDR element is
turned on
Mixing
Temperature
Press and release
the Item button until
the MIX element is
displayed.
Mixing Target
Temperature
Press and release
the Item button until
the MIX TARGET
elements are
displayed.
Boiler
Temperature
Press and release
the Item button until
the BOIL element is
displayed.
To view all control settings, the Item button may be pressed and held while in the View Menu. The contol will then scroll through all
the adjustable items.
MIX Start
Use the and
buttons to set the
desired mixing
starting
temperature.
OUTDR Start
Press and release
the Item button.
Use the and
buttons to set the
desired outdoor
starting
temperature.
MIX DSGN
Press and release
the Item button.
Use the and
buttons to set the
mixing design
temperature.
OUTDR DSGN
Press and release
the Item button.
Use the and
buttons to set the
outdoor design
temperature.
BOIL MIN
Press and release
the Item button.
Use the and
buttons to set the
boiler minimum
temperature.
MIX MAX
Press and release
the Item button.
Use the and
buttons to set the
mixing maximum
temperature.
WWSD
Press and release
the Item button.
Use the and
buttons to set the
desired WWSD
temperature.
Units
Press and release
the Item button.
Use the and
buttons to select
between °F and
°C
POWER UP VIEW MENU
ADJUST MENU
To enter the adjustment mode, Press and Hold simultaneously, the Item , and Buttons. The ADJUST element will turn on.
The control automatically goes back to viewing when the buttons are left alone for 20 seconds
All settings will be saved even during power down of the control

11 of 12
STEP FIVE
The main control functions on the PC705-2 can be tested by pressing and holding the UP button. While the UP button is pressed, the
boiler and the system pump are turned on, and the variable speed injection pump is turned to 100% of its output speed. Once the UP
button is released, the control returns to normal operation.
STEP SIX
As in any troubleshooting procedure, it is important to isolate a problem as much as possible before proceeding. The Error Messages
greatly simplify troubleshooting of the PC705-2. When the control displays an error message, identify the fault from the look-up table
on page 12 and follow standard testing procedures to conrm the problem. If you suspect a wiring fault, return to step three and
carefully check all external wiring and wiring connections.
Sensor and Internal Faults
• If an outdoor sensor fault occurs, the PC705-2 will assume a xed outdoor temperature of 32°F (0°C) and will target the appropriate
supply water temperature. An error message is displayed.
• If a supply sensor fault occurs, and a boiler sensor is installed, the PC705-2 operates the variable speed at an output of 15% and
displays an error message. If a supply sensor fault occurs, and a boiler sensor is not installed, the PC705-2 operates the variable
speed at an output of 30% and displays an error message.
• If a boiler sensor fault occurs, the PC705-2 operates the boiler as if the sensor was not installed and displays an error message.
If the sensor was deliberately removed, remove power from the control to clear the error message.
• If an EEPROM fault occurs, the PC705-2 stops operation until all the settings are veried. An error message is displayed.
Adjustment of Settings
• If the outdoor temperature is cold and the rooms are cold, increase the MIX DSGN setting by 5°F (3°C) per day.
• If the outdoor temperature is near the WWSD temperature and the rooms are cold, increase the MIX Start setting.
Testing the Sensors
A good quality test meter capable of measuring up to 5,000 kΩ (1 kΩ = 1000 Ω) is
required to measure the sensor resistance. In addition to this, the actual temperature
must be measured with either a good quality digital thermometer, or if a thermometer
is not available, a second sensor can be placed alongside the one to be tested and the
readings compared.
First measure the temperature using the thermometer and then measure the resistance of
the sensor at the control. The wires from the sensor must not be connected to the control
while the test is performed. Using the chart below, estimate the temperature measured
by the sensor. The sensor and thermometer readings should be close. If the test meter
reads a very high resistance, there may be a broken wire, a poor wiring connection or
a defective sensor. If the resistance is very low, the wiring may be shorted, there may
be moisture in the sensor or the sensor may be defective. To test for a defective sensor,
measure the resistance directly at the sensor location.
Do not apply voltage to a sensor at any time as damage to the sensor may result.
Troubleshooting
Temperature Resistance Temperature Resistance Temperature Resistance Temperature Resistance
°F °C Ω°F °C Ω°F °C Ω°F °C Ω
-30
-20
-10
0
10
20
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
-34
-29
-23
-18
-12
-7
-1
4
10
16
21
27
32
38
43
49
54
60
66
71
77
82
88
93
234,196
165,180
118,018
85,362
62,465
46,218
34,558
26,099
19,900
15,311
11,883
9,299
7,334
5,828
4,665
3,760
3,050
2,490
2,045
1,689
1,403
1,172
983
829
Ω
Ω
STEP SEVEN
• Install the wiring cover over the wiring chamber and secure it with the screw provided.
• Place the front cover on the control and snap it into place.
• It is important to explain the operation of this control within the system to the end user, and to anyone else who may be operating
the system.

12 of 12
PC705-2 Variable Speed Injection Mixing Control
Control — Microprocessor PID control; This is not a safety (limit) control.
Packaged weight — 1.5 lb. (670 g), Enclosure D, Taco green PVC plastic
Dimensions — 4-3/4” H x 2-7/8” W x 1-7/8” D (120 x 74 x 48 mm)
Approvals — CSA C US, meets ICES & FCC regulations for EMI/RFI.
Ambient conditions — Indoor use only, 32 to 104°F (0 to 40°C), < 90% RH non-condensing.
Power supply — 24 V (ac) ±10% 50/60 Hz 3 VA
Variable Pump — 120 V 50/60 Hz 2.4 A 1/6 hp
Pump Relay — 120 V (ac) 5 A , 1/3 hp, pilot duty 240 VA
Boiler Relay — 24 V (ac) 5 A 1/6 hp, pilot duty 240 VA
Sensors — NTC thermistor, 10 kW @77°F (25°C ±0.2°C) ß=3892
included: Outdoor Sensor M 2036 and 2 of Universal Sensor 071
Factory
MIX Start — 35 to 150°F (2 to 66°C) 70°F (21°C)
OUTDR Start — 35 to 85°F (2 to 29°C) 70°F (21°C)
MIX DSGN — 70 to 220°F (21 to 104°C) 120°F (49°C)
OUTDR DSGN — -60 to 32°F (-51 to 0°C) 10°F (-12°C)
MIX MAX — 80 to 225°F (27 to 107°C) 180°F (82°C)
BOIL MIN — OFF, 80 to 180°F (OFF, 27 to 82°C) 140°F (60°C)
WWSD — 35 to 100°F, OFF (2 to 38°C, OFF) 70°F (21°C)
Units — °F, °C °F
The installer must ensure that this control and its wiring are isolated and/or shielded from strong sources of electromagnetic noise. Conversely, this
Class B digital apparatus complies with Part 15 of the FCC Rules and meets all requirements of the Canadian Interference-Causing Equipment
Regulations. However, if this control does cause harmful interference to radio or television reception, which can be determined by turning the control
off and on, the user is encouraged to try to correct the interference by reorienting or relocating the receiving antenna, relocating the receiver with
respect to this control, and/or connecting the control to a different circuit from that to which the receiver is connected.
Do It Once. Do It Right.™
TACO, INC., 1160 Cranston Street, Cranston, RI 02920 Telephone: [phone] Fax: [phone]
TACO (Canada), Ltd., 6180 Ordan Drive, Mississauga, Ontario L5T 2B3. Telephone: [phone]
Visit our web site at: http://www.taco-hvac.com
Copyright 2004 TACO, Inc.
Portions Copyright 2000
tekmar Control Systems Ltd.
tekmar Control Systems Inc.
Error Messages
Whenever a fault is detected, an error message will be displayed to indicate the location of the problem.
Technical Data
EEPROM
Read Error
Mixing Supply
Sensor Short
Circuit
Boiler Sensor
Short Circuit
Outdoor
Sensor Short
Circuit
Mixing Supply
Sensor Open
Circuit
Boiler Sensor
Open Circuit
Outdoor
Sensor Open
Circuit
Refer to the troubleshooting section for operation details.
Start
MIX
Mixing
Demand
% Out
10 30 50 70 90 Start
BOIL
Mixing
Demand
% Out
10 30 50 70 90
Start
OUTDR
Mixing
Demand
% Out
10 30 50 70 90 Start
MIX
Mixing
Demand
% Out
10 30 50 70 90 Start
BOIL
Mixing
Demand
% Out
10 30 50 70 90
Start
OUTDR
Mixing
Demand
% Out
10 30 50 70 90
Start Mixing
Demand
% Out
10 30 50 70 90
Item
1
H1194A
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)
5 A 1/3 hp
pilot 240 VA
23
Out
Com Sup
Signal wiring
must be rated
at least 300V
PC705-2
C US
Press & Hold:
Item, to view settings
, to test
Press & Release:
Item + + to adjust menu
Boil
4
Do not apply power
Mixing
Demand
Start
% Out
10 30 50 70 90
Made in Canada
VIEW
158033
OUTDR
°F