Fan randomly runs several times a day while system is OFF

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New Member
Oct 11, 2020
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I recently noticed an issue with the HVAC system on the 2nd floor, the fan runs for a while(around half an hour or a bit longer) then turns off, several times a day even while I set SYSTEM=OFF, FAN=auto. The other system in the basement(covers 1st floor and the basement, two zone) doesn't have the same problem, its fan always keeps off even I set SYSTEM=COOL, FAN=auto, as expected.

Both systems are controlled by two separate thermostats(same model) provided by the local Electric Power Company called Pepco, This model of thermostat doesn't have an option OFF for FAN, only AUTO or ON. I asked Pepco staff to come onsite and they did me a favor to replace the thermostat(still same model), but I found the problem persisted, the fan ran another half an hour(I still don't get the pattern when the fan runs in the day). Pepco staff suggested me when the fan runs, remove the thermostat from its base(that is, disconnect the front part from the connectors), if the fan still runs, then the problem is not the thermostat. I tried it this morning, yes the fan still kept running for another 20+ minutes even I disconnected the main part of the thermostat from the wall. So, the problem may not be related to the main board of the thermostat(but not sure about the wall part of the thermostat where the wires are connected though).

Pepco staff said it could be a "relay" problem. But when we tested forcing to turn on A/C(just set the thermostat to lower temperature), the fan started well and switched off just couple minutes after the A/C unit was off. The couple minutes extra runtime for the fan was expected. So I don't understand why the 2nd floor's system has the fan randomly(again, not time pattern has been found yet) running for a long period of time even the SYSTEM is set OFF/COOL/HEAT.

Also, I noticed sometime in the early morning when I was still in the bed on the 2nd floor, I heard sounds(low frequency) from the attic, it sounds like something tried to start but shut off right away(the sound last for 1~2 seconds), it tried several times, while SYSTEM=OFF.

The furnace unit is YORK Diamond 80, Model No: P3HUA12N04801C, and I attached the pictures below.

Please advise.
Thank you!!



Well-Known Member
Nov 19, 2007
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Given the results of your test with removing the face of the thermostat; it does sound like a relay problem.
However, the relay itself may not be the problem as it may be doing exactly what it's being told.
Check to see if there are any "pencil switches" on the control board and that they match the other unit's setting. Might be looking at a flaky control board.
If the other furnace is identical make/model; I'd de-energize both units and swap the control boards.
Be very careful to keep track of which wire goes where, take pics from all angles before starting.
See if find the problem follows the control board to the other unit and corrects the problem with this unit.


Well-Known Member
Dec 13, 2018
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Vancouver Canada BC
Are you able to turn the fan on immediately following the humming sound that you hear when it’s randomly hums for a few seconds then stops?

How long has this been happening?

Are there any other devices wired into the controls that may be back feeding power to the G terminal on the board?

Is there any wire damage that may have caused a short circuit in the cable? Recent renovations, punch point along the wire?