Fan randomly runs several times a day while system is OFF

Discussion in 'HVAC Heating & Cooling' started by beeth, Oct 11, 2020.

  1. beeth

    beeth New Member

    Joined:
    Oct 11, 2020
    Location:
    Maryland
    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!!
     

    Attached Files:

  2. fitter30

    fitter30 Active Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    White Rogers module 50A51-242 york # 031-01290-000 controls blower. Couldn't find a wiring diagram on line for your furnace. Some furnaces when the high limit opens the blower comes on, might be fault or the control is going bad.
     
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  4. Bannerman

    Bannerman Well-Known Member

    Joined:
    Mar 19, 2014
    Location:
    Ontario, Canada
    Is there a humidistat connected to the furnace control board?

    While many current thermostats incorporate humidity controls, a separate humidity control may be causing the fan to operate briefly if it senses humidity is higher or lower than a specific humidity setting.
     
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Joined:
    Sep 2, 2004
    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    On many HVAC systems, when set to cooling, the fan comes on when the compressor comes on. When in heating mode, the fan often does not come on immediately, and is based on sensing when the heat exchanger is starting to warm up or from a timed delay so you don't get a blast of cold air when you are trying to heat. Some have a separate sensor for when to start the fan, and a second sensor or logic that will detect when it starts to cool off and shut the fan down when that is reached. The device that detects that may be failing.
     
  6. fitter30

    fitter30 Active Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Disconnect the R wire on the WR board see if the blower starts randomly. This will eliminate any wire or stat problems
     
  7. beeth

    beeth New Member

    Joined:
    Oct 11, 2020
    Location:
    Maryland
    No, there is no humidistat or humidity control with my system. Thanks!
     
  8. beeth

    beeth New Member

    Joined:
    Oct 11, 2020
    Location:
    Maryland
    I found the fan starts randomly(sometimes failed to start, when I heard very short buzz sound from the attic), so I doubt its behavior is usual. I think all the sensor is on the control board.
     
  9. beeth

    beeth New Member

    Joined:
    Oct 11, 2020
    Location:
    Maryland
    What's the R wire for?

    I did a test in the past couple days, by disconnecting the G(green) wire:

    ************* Before disconnecting the Green cable *************

    10/12, SYSTEM=OFF, FAN=AUTO:

    I heard the very short(<1 second) buzzing sound(not sure if "buzzing" is the right word, but the sound was just like a motor tried to start but immediately stop, or like the sound when sump pump starts) from the attic, where the furnace is located, for about 5~7 times a day(mostly in the early morning or late evening as I noticed, but I still couldn't find the pattern), it sounded to me it tried to start the motor for 2~3 times within a half minute period, then stopped trying. This has been an ongoing issue for several weeks.

    10/13 morning, SYSTEM=HEAT, FAN=AUTO:

    My wife turned the HEAT on, after reaching the target temperature, the FAN kept running, while the air blew out no longer warm, so I assumed the furnace turned the fire off. I waited for >40 minutes, the FAN was still running. Then I forced to set SYSTEM=OFF. After about 5~6 minutes, the fan finally stopped.

    ************** Afterdisconnecting the Green cable *************

    10/13 noon, I went up to the attic, disconnected the Green cable from the board, and left SYSTEM=OFF, FAN=AUTO

    It has been more than 2.5 days since I disconnected the Green cable(and kept SYSTEM=OFF), I haven't heard any sound from the attic yet, not sure if I should wait longer…

    ================



    The symptom of the problem sounds to me like this: when SYSTEM=OFF or (SYSTEM=ON but FAN shouldn't be running), the fan tries to start(when I could hear those short buzzing sounds), when it fails to start, it may try again hours later. When it can successfully start, it runs for 30~40 minutes then stops)
     
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