IM 422 / Page 11 of 28
Programmable Thermostat – The major difference in
connecting the programmable thermostat is the requirement for
the transformer common, the introduction of the thermostats
power source. The BK wire loop, shown in Figure 21, is where
power is supplied to operate the circuitry of the programmable
thermostat. The BK wire loop usually connects to the “trans-
former common” terminal of the thermostat. The position and
designation of the terminal for incoming thermostat power dif-
fers between manufacturers. Refer to the instructions furnished
with the chosen thermostat to locate the proper terminal. Note:
It may be necessary to place a larger VA transformer in the unit
when using certain programmable thermostats. Check the VA
draw of chosen thermostat to be sure it does not exceed 5 VA
power draw.
Figure 21. Programmable Thermostat
Wires From Control Slave Unit
Figure 22. Master/slave with Programmable Thermostat
Wires From Control
Factory Supplied
Low Voltage
Plug & Receptacle
Disconnect
Wires From Control Master Unit
Factory Supplied
Low Voltage
Plug & Receptacle
Disconnect
NOTE: When employing a slave, sever
the BK wire loop of the Slave Unit.
Master/slave Operation With Programmable Thermostat
– Master/slave operation can also be controlled by a program-
mable thermostat. However, the connection of the master unit is
slightly different than that of the Non-programmable thermostat.
As described earlier, the programmable thermostat will need the
common to energize the electronics. This power can be sup-
plied by tapping into the BK wire loop. Note:
It may be neces-
sary to place a larger VA transformer in the unit when using
certain programmable thermostats. Check the VA draw of cho-
sen thermostat, plus the VA draw of the slave unit to be sure it
doesn’t exceed 10 VA power draw of the master transformer.
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