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6.
This de-energizes the heater relays opening the
contacts to the heaters.
7.
240 VAC is removed from the relay contacts, the
limit switch(s) and the heater element(s).
8.
At the same time, the 24 VAC is removed from the
low speed tap on the blower motor. The indoor fan
motor low speed tap is de-energized after a 5 sec off
delay.
The heating cycle is complete, and the furnace is ready for the
start of the next heating cycle.
The Cooling Cycle
When the thermostat calls for cooling
1.
The thermostat closes the circuit between the “R”, “Y
and “G”” terminals.
2.
24 VAC is sent from the “Y” terminal through the
Yellow thermostat wire to the 24 VAC coil terminal
on the condenser contactor.
3.
At the same time the “Y” terminal has 24 VAC
placed on it, the “G” terminal is sending 24 VAC
through the green pigtail wire to the 24 VAC coil
terminal on the indoor blower sequencer.
4.
The sequencer goes through a 28 second on delay,
and then closes the contacts sending 24 VAC to the
black wire connected to the selected indoor fan motor
speed tap.
5.
The 24 VAC signal on the “Y” terminal energizes the
compressor contactor closing the contacts and
sending 240 VAC to the compressor and the outdoor
fan motor.
When the thermostat call for cooling has been satisfied
1.
The thermostat opens the circuit between the “R”, “Y
and “G”” terminals.
2.
24 VAC is removed from the thermostat “Y”
terminal, the Yellow thermostat wire and the 24 VAC
coil terminal on the condenser contactor.
3.
At the same time the “Y” terminal has 24 VAC
removed, the “G” terminal is removing 24 VAC from
the green pigtail wire and the 24 VAC coil terminal
on the indoor blower sequencer.
4.
The sequencer goes through a 14 second off delay,
and then opens the contacts removing 24 VAC to the
black wire connected to the selected indoor fan motor
speed tap.
5.
The 24 VAC signal on the “Y” terminal de-energizes
the compressor contactor opening the contacts and
removing 240 VAC from the compressor and the
outdoor fan motor
The cooling cycle is complete, and the cooling unit is ready
for the start of the next cooling cycle.
The PSC Motor Isolation Relay
This relay is used in conjunction with the motors to prevent
the possibility of the motors being energized on two speed
taps at the same time. This condition would burn out the motor
windings.
A black wire is connected from the circuit breaker to the
supply side of the normally open (N/O) terminals on the relay.
The blower motor high speed tap wire is connected to the load
side normally open terminal on relay.
A red wire is connected from the heat relay to the normally
closed (N/C) terminal on the relay. The load side of the
normally closed terminal is connected to the low speed tap
blower motor.
Figure 11: PSC Motor Speed Tap Isolation Relay
Section V: TROUBLESHOOTING
The following checks should be made before troubleshooting
the furnace controls for a no heat issue.
1.
Check all of the circuit breakers. Make sure they are
turned to the “ON” position and have not tripped.
2.
Check all fuses, especially the 3 amp fuse on the right
side of the control box. If the fuse is blown, check the
wiring with an OHM meter for a short to ground.
Repair the short, and then replace the fuse.
3.
Check any switches that are external to the furnace to
make sure they are turned on.
4.
Make sure all wiring connections, especially on any
of the components, to ensure they are securely
fastened.
If you have 240 VAC coming out of the circuit breakers in the
control box and you have 24 VAC between the load side if the
3 amp fuse and ground, then continue on through the rest of
the checks.
5.
Remove the furnace door. Place your hand near the
blower motor. If you feel any heat rising up from the
blower wheel but the motor is not running, check for
240 VAC and 24 VAC at the motor terminals. If the
For personal safety be sure to turn the electrical power
“OFF” at the main entrance (Circuit Breaker Box) and at the
control box circuit breakers before attempting any service or
maintenance operations. Homeowners should never attempt
to perform any maintenance which requires opening the
furnace control box door.
