Electric furnace fan won't stop sometimes, however it does turn on okay

Question

I have a York Electric Furnace (10 years old) with a outside heat pump. Something weird happened recently as we started to use the furnace more often as it gets cold now.

I usually turn on the heat when I wake up and turn it off before I go to sleep. However almost 95% of the time when I tried to turn off the heat, the heat pump and the heat would stop right away, but the furnace fan would just keep on running for the entire night, unless I turn off the circuit breaker. The weird part is that sometimes I get lucky and it actually turns off the fan as it should.

I don't remember this problem in the summer when I use the AC in the summer, back when the furnace would behave and turn off the fan as instructed.

I have so far replaced the thermostat with the Alexa smart thermo, and the issue persists. I have also tried unplugging the thermostat while the furnace is running, and it doesn't really affect anything and the fan kept on running. So I am assuming the issue isn't at the thermostat.

I also doubt if it was the fan limit switch, as there is nothing wrong with the heat. the heat would turn on / off normally.

I have thought about the possibility of a stuck relay. However whenever I turn off the circuit breaker and turn it back on, the fan would actually be stopped. It would run when I turn on heat again, until after heating for a while it would fail to stop again. Does this sound like a stuck relay?

Any thoughts are appreciated, thank you guys.

Answer 1

After some 4 minutes or so, on the circuit board check pin g and c and see if you are reading 12-24V, if you are it means you have wiring problems from the thermostat to the board. Somewhere the two wires on the thermostate ( typically R and G wires) are touching. The fact that after sometime, the voltage should be cut off.

If you are not reading any voltage between g and c and your blower motor is running, it rules out the thermostat along with its wires and then you move on to the relay issue next.

Tap that relay a few times and see what happens. If the fan turns off you know you got relay problem. You can then unplug one of the power supply leg to the relay, that would shut it off of course. and you would certainly know the relay is the issue.

If everything passes, and the relay is disengaged next check the pins on the sequencer. The relay feeds the sequencer. Make sure it is not stuck.

There may also be a trigger module on the motor itself. If so equipped, if you unplug the signal wire to it the motor should shut off. If not, then you got motor module problem.

The relay is not a matter of shut or open, now that you know.

The supply voltage to the pin g on the circuit board comes from the thermostate. Pin g feeds the 555 timer and the capacitors which in turn after some amount of time feeds the trigger pin on the relay.

You can unplug the trigger pin of the relay or the pin at g and see if that will turn off the fan. If it does , then that points to the thermostate and its associated wires.

There you have it.

Take care.