Fan randomly runs several times a day while system is OFF

3 years ago

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!!

Featured Answer

kevin9408

3 years ago

last modified: 3 years ago

HU-373650237, Can you tell what speed the fan is running in when it comes on with the T'stat off? High speed is used for fan on and A/c, and low speed for heat. If you say the fan runs in high speed the problem is in the "Fan on or A/C circuit. The only way the blower can run in high speed is if the Heat/cooling relay on the white rogers board is energized to close the contact supplying high voltage to the blower motor's high speed tap.

So as mike suggested you need to measure the voltage of the "G" wire disconnected from the furnace control board along with pulling the "Y" and "W" wire. The "G" and "Y" are connected together in the thermostat when the fan is in auto so you want to totally isolate the thermostat from everything else. Connect the red to the "G" wire and the black to a ground, basically any metal part of the furnace. The Push switch right above on the frame can now be pushed in and held or taped to turn the furnace on. Any voltage will indicate leakage and be suspect. The Heat/cooling relay holding coil is really a current operating device and doesn't need 24Vac to energize the coil, but only enough voltage with just enough current to energize it. Here's where it gets tricky, Just because there is enough voltage/current to energize the coil doesn't mean there's enough to hold it energized for long. As the coil temperature heats up from current running through it the coil resistance increases, and the voltage/current draw will also increase to a point the relay coil will drop out and the blower stops. When the coil cools the process starts over. Even a low voltage will indicate leakage (short) somewhere in the T'stat or wires to the T'stat.

Stick with me, If the fan runs in high and testing the T'stat circuit showed no voltage on your meter I have another test for you to do. Let the power button on the frame turn off the power and with all the wires "G", "Y" and "W" still disconnected from the main board do the following WITHOUT power on at the furnace. Check the voltage across the "W" terminal on the furnace control board and ground. Any voltage at all will indicate the compressor contactor in the condenser is leaking voltage/current back feeding the Heat/cooling relay. If so go turn off the power to the condenser and recheck the voltage drop across the "W" terminal again, it should be zero. If that's the problem go into the condenser and clean the dirt and cobwebs off the contactor with the power OFF!

What you just checked are the only components in the fan on and A/C circuit that could energize the heat/cool relay to cause the fan to come on in high speed. If it comes on in low speed then it's in the heating electrical/electronics size and another story. I found most of the wiring diagram for your unit and was able to confirm what I just said. If you do find the problem please tell us, I'd love to here back.

For reference the high temp limit is the black plastic plate right of the gas valve but PLEASE do not mess with it at this time. It's a critical safety component and if the fan runs in the off position it is not involved so leave it alone and I'm sorry I mentioned it.

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Comments (15)

mike_home
3 years ago
Is this furnace connected to a humidifier?
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HU-373650237
Original Author
3 years ago
No, it's not connected to a humidifier.
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mike_home
3 years ago
Another possible problem could be the G wire is shorting to 24 V somewhere. If you have a voltmeter take a measurement between the G terminal and ground. With the system set to Off the reading should be zero. Do this when the fan is not running and when the fan is running by itself.
kevin9408
3 years ago
I couldn't find a electrical schematic online for your unit so if the furnace has one glued to the cover take some really good pictures and post them.

You say it's in the attic? How hot does it get up in your attic? The reason I ask is related to the high temp limit switch located to the right of the main gas valve. These switches are set to turn the fan on and off at between 100 -130 degrees and the attic temperature temp may be triggering the switch because it's out of calibration (broken) or crud on it. I need to see a schematic to determine how it's wired in to the control board and also while I was searching for a schematic I notice the replacement OEM control board (031-01266-000) is upgraded and doesn't exactly fit right without drilling some new holes. If it's the control board there is nothing you can repaired unless the same components can be found to solder in.
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HU-373650237
Original Author
3 years ago
@mike_home

Did you mean to measure the G terminal on the board inside the furnace(that is, not the G terminal on the base the thermostat in the room)?

I will have a problem to check the G terminal on the board while the fan is running by itself, because the fan runs randomly, easily to miss. Also, in order to access the board, I need to open the door of the furnace, as soon as I remove the door, the switch on the door is released and there is no more power supply to the whole system...

Thanks!
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HU-373650237
Original Author
3 years ago
@kevin9408

It's fall now so the attic is not hot. Where's the high temp limit switch? Can you please show me in the picture I attached?

You are right, I tried to google "50A50-241 White Rodgers", I couldn't find any new part, but used part on eBay.
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mike_home
3 years ago
You mentioned that when the fan is not running by itself, you are able to turn on and off the fan from the thermostat. That would imply you still have an interrupted connection back to the control board. Therefore making a measurement at the thermostat would give an indication of what is going on. It is easier to measure at the thermostat, but at the control board would be better if you could bypass the safety switch.

You should take a voltage reading when the fan is not running by itself and verify it is zero volts. It could turn out the G connection is at some voltage between 0 and 24 V. That voltage could be intermittently energizing the fan relay on the control board. That could explain why the fan runs with no set pattern.
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HU-373650237
Original Author
3 years ago
@mike_home

When I measure the voltage of the G connection, I think I should touch the red probe of the meter to G, but where should I put the black probe of the meter? (sorry I am new to this).

What may have cause the G connection some voltage between 0~24V? short? If so, should I switch the green cable with one of the spare cables available?

Thanks!
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kevin9408
3 years ago
last modified: 3 years ago
HU-373650237, the fan won't run if you disconnect the three wires but you need to push in the button on the frame to turn on the furnace to get the 24Vac transformer to work. The meter should be set to Vac and not Vdc.
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mike_home
3 years ago
You should make a measurement of the G wire to ground. Kevin9408 makes a good suggestion of disconnecting the wire at both ends so neither the thermostat or the control board will influence the measurement. If the thermostat has a C wire, then that is a ground connection. Otherwise you have to this at the control board.

This troubleshooting is getting a bit involved so if you are not comfortable doing all this then it may be best to hire a competent tech.

If you do have a spare wire in the bundle, then switching the current G wire to another would be a good test. If after switching wires the fan still runs intermittently, then the problem is probably not a short.
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HU-373650237
Original Author
3 years ago
New update for my latest test:

I disconnected the Green cable from the "G" terminal on the furnace control board(50A50-241 White Rodgers, see the attached photo), didn't change any connection from the thermostat end, but set SYSTEM=HEAT on 10/16, since then the system has been working well as expected for more than a day, that is, the fan ran when the furnace started heating, then stopped within one minute after the room temperature reached the target number, repeated for a few times through the day to warm up the house, that's a good sign; also I have no longer heard any weird random/intermittent, short sound from the attic(the sound might indicate the fan tried to start but failed within one second, even I previously set SYSTEM=OFF, FAN=AUTO).

I will leave the Green cable disconnect and observe for another few days. I heard that I don't need the G cable for heating for the winter so no risk to set SYSTEM=HEAT without the Green cable connected. If everything looks back normal after a few days, then I think the furnace control board and the thermostat control board are NOT bad(correct me if I was wrong), the problem could be on the Green cable, or its connection to the thermostat(something wrong with the thermostat wall base, where all the color wires are connected), or its connection to the furnace control board. I will then follow the suggestions above to measure the G wire with a meter, and I have 3 spare cables to replace the Green cable.

Many thanks!
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HU-373650237
Original Author
3 years ago
@kevin9408

Thank you so much for your suggestions in great detail, really appreciate!

I did a test with Green cable disconnected from the control board and I posted the test detail above.

I can't tell the speed of the fan. When the fan previously randomly turned on(even SYSTEM=OFF), I noticed it ran like the heat was on. But frankly speaking, I didn't realize the fan ran faster for A/C. The winter is coming, I don't think the A/C will turn on before next May.

When you said "pulling the "Y" and "W" wire" when I measure the voltage of "G" wire, did you mean I should disconnect ALL 3 wires: G, Y, and W from the furnace control board, before I do: red probe -> G(green) wire(NOT the G terminal on the board), and black probe -> ground(metal of the furnace)?

Great analysis about the leakage(short) and the coil!

Does the condenser only work for A/C? If so, I doubt it's relevant, but I agree no harm to do that test.

Thanks for the reminder about "The meter should be set to Vac and not Vdc". If I disconnect G, Y & W wires from the board, and I believe my thermostat doesn't run on battery but get the power from the control board in the attic. If G wire has voltage, can I assume the thermostat gets power from R or C of the control board?

Thanks again!
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HU-373650237
Original Author
3 years ago
@mike_home

Thanks for your continued support! I did a test with Green cable disconnected from the control board and I posted the test detail above, good sign!

I thought Kevin9408 suggested me to disconnect ALL 3 wires: G, Y & W from the furnace control board, while keep the wire connections to the thermostat untouched. But you mentioned to disconnect the G(Green) wire at both ends(furnace and thermostat?). I am confused here, please clarify. Thanks! Also, I found two wires(blue & white) connecting to the C terminal on the furnace control board, is that the "ground" for the measurement?

I love to do this troubleshooting and I love to learn from you guys, but hopefully I don't have to deal with this every year :)

Yes, I do have another 3 spare wires(orange, brown and black unused) for test, so if the G wire is the root cause, I will swap the wire.
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mike_home
3 years ago
last modified: 3 years ago
I suggested disconnecting the G wire from both ends to isolate it from the control board and thermostat. If you read any voltage then that would indicate a partial short to 24 V. You could then connect it to the control board and repeat the test. Your problem is only the fan coming on intermittently and not the furnace or AC. So I don't see the need to also disconnect the Y (cool) or W (heat wiring). If you do measure any voltage on the G wire you could disconnect the other wires are remeasure to see if it causes any change. Intermittent problems are usually traced back to a strange situation. It is good to trace every possible cause.

Disconnecting the G wire from the control board is a good idea if the furnace control board turns the fan on when the furnace fires. Disconnecting the G wire eliminates the possibility of a wire short as the cause. That was good idea. I wish I had thought of that!

The condenser runs for AC only. Most furnaces have a least one fan speed setting for heat, and another for cooling. This allows the fan speed to be set for various furnace and condenser sizes.

The R connection is the 24 V supply. The C connection, if your thermostat has one, is the ground. Most thermostats have a battery back up so they won't lose their settings during a power outage. Thermostat acts as a switch between the R connection and W, Y, and G connections. During a call for heating or cooling, the thermostat energizes the W, Y, or G connections to 24 V.
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Fan randomly runs several times a day while system is OFF

kevin9408

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Fan randomly runs several times a day while system is OFF

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