Fan runs several times a day while system is OFF

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Old 09-19-20, 01:22 PM
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Question Fan runs several times a day while system is OFF

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 while I set SYSTEM=OFF, FAN=auto. The issue could exist for a while though I might just haven't known it before, because the weather just became cooler in September so no AC or heat is needed then I could notice the fan ran for no reason. 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 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, this morning 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(is that true??). 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.
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 pattern is found yet) running for a long period of time no matter the SYSTEM is set OFF/COOL/HEAT.
Could it be the fan just smart to turn on daily for air circulation? I really doubt a feature like this, and this doesn't make much sense to me.
Please advise. Thank you!

 
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Old 09-19-20, 02:47 PM
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We need the complete model number from the air handler unit.

It could be a delay timer or blower control relay.
Could be a sequencer if electric heat is involved.
Can only guess at this time.
 
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Old 09-19-20, 06:47 PM
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Thanks PJMax. Where can I find the "air handler unit"? Is it within the furnace, so I need to open the furnace cover to look inside the motor for the model number? This HVAC is in the attic, I can climb up to take a look.
Since it's gas furnace not electric heat, so I assume the problem is not the sequencer, but delay timer or blower control relay, is that right?
I don't understand the logic why the fan can start while the system is set OFF, some thing must trigger the fan to start, but where is such signal coming from? If the fan stays running for a while after the cool/heat turns off, that makes sense, but now the cool/heat doesn't run at all…

 
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Old 10-10-20, 07:31 PM
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Hi PJMax,

I found the system is called YORK Diamond 80, Model No: P3HUA12N04801C
I attached 3 pictures of the unit.
Where can I find the delay timer or blower control relay?
Thanks!





 

Last edited by PJmax; 10-13-20 at 10:02 PM. Reason: cropped/labeled 3rd picture
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Old 10-13-20, 07:09 AM
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Hi, is this something that just started after the new thermostats were installed?
Geo🇺🇸
 
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Old 10-13-20, 10:00 PM
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Yes..... that's a York gas furnace. It uses a White Rodgers 50A50-241 control board.
Since you've removed the stat and still have the problem.... it's a furnace problem.

It's one of two problems.....
1) the heat of the attic triggered a heat sensing safety switch to open.
2) the control board is defective.

The control board has a timer that turns the blower on and off in heating mode.
I labeled your 3rd picture. The control board has a fault LED on it. (red arrow)
The chart tells you what fault you have based on the flash count. A safety switch opening will trigger a fault code. If you are getting the normal LED flash.... then the board may be defective.

You need to get the flash code before removing power. The switch in blue circle kills all power to the furnace when blower door is opened. You must read the code before opening the door and erasing it. There may be a sight glass where you can look thru at the code.
 
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Old 10-14-20, 09:20 PM
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Hi PJMax,

Thanks for the suggestion, I didn't noticed the troubleshooting LED is ON or FLASHING, but I will make sure I check next time when I go up to the attic. Currently I am doing a test by removing the Green(G) cable from the control board:

************* Before disconnecting the Green cable ( see the attached picture#1 )*************

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 ( see the attached picture#2 )*************

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 24 hours since I disconnected the Green cable(and kept SYSTEM=OFF), I haven't heard any sound from the attic yet, but I will wait for another two days.

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


When I was at the attic, I noticed another Green cable detached, I am not sure what's that for. I attached the picture below, can you advice?






Also, I want to mention again another test I did before(which I mentioned in my original post by following the Pepco staff's advice): I disconnected the front part of the thermostat(the main part of the thermostat, with circuit board, LED screen and control buttons…) from the wall part(where the color cables are connected), the problem persisted: either the fan ran for 30+ minutes then stopped, or I heard the short(<1 second) buzzing/starting sound from the attic.

So I don't think the problem is because of a bad thermostat control board, but I don't know about if something went bad with the connector part of the thermostat, or something wrong with the green cable(from the thermostat to the control board in the attic…

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)

Thanks!!
 
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Old 10-15-20, 12:27 AM
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That three wire cable you identified comes from the outside condenser.
It only requires two wires..... red and white... the green is a spare.

Ok...just to clarify. You said you removed the green wire from the control module and the blower did not come at all or when it wasn't supposed to. You also said that when the wire is connected to the module and the thermostat is removed from the base.... the blower runs randomly. Is this correct ?

That would indicate a defective thermostat sub base or a shorting wire. Shorting wire is pretty rare.
What model thermostat is that ?
Some.... like nests, ecobees, etc. have electronic sub bases.

If a defective cable is considered..... your thermostat cable has three spare wires.... black, brown and orange. You could use one of those three as a test in place of the green wire. Just connect the spare wire at both ends while leaving the green disconnected.
 
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Old 10-15-20, 10:28 PM
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Hi Pete,

Re: "just to clarify. You said you removed the green wire from the control module and the blower did not come at all or when it wasn't supposed to. "

Right, since I went up to the attic and disconnected the green wire from the control board within the furnace on 10/13(when I also set SYSTEM=OFF), I haven't heard any sounds from the attic or noticed the fan runs. Now it's more than 2.5 days.

Re: "You also said that when the wire is connected to the module and the thermostat is removed from the base.... the blower runs randomly."

Yes, Pepco staff came onsite weeks ago and replaced the thermostat(the main part but kept the base untouched), he told me to remove the main part of the thermostat from the base for testing, if the blower/fan still runs randomly, then it's not a thermostat problem and I should call HVAC company. I followed and I did notice the fan ran even the thermostat was removed(base was still on the wall though).
Now I suspect if something wrong with the base, but the base is just a simple wire connector...

The thermostat is provided by the local utility company called Pepco(provider serves quite a few cities in Maryland), I couldn't find the model with it. I attached the picture of the thermostat below.

Thanks!

 
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Old 10-16-20, 07:36 AM
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So you've eliminated the thermostat. That leaves a problem wire.
Try switching from the green wire to one of the spares at both ends.
 
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Old 10-16-20, 11:06 AM
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Yeah, I think the thermostat control part is good, because I once tested with the main part of the thermostat removed from its base(the base had a few color wires connected), the fan could still run randomly. So I agreed with the Pepco staff that in this case, the problem is not because of their thermostat(well, it could still be the wire connection they did, or a bad wall/base part…).

Before I switch the green wire to one of the spare wire, I got a quick question for you: if I leave the G(green) wire disconnected from the control board in the furnace, and set SYSTEM=HEAT(now it's OFF), to monitor if the problem happens(weird sound from the attic, or fan randomly runs), will it bring myself at risk to cause fire and get my house burned? That is, without the G cable connected, will the fan turns on as expected when I turn HEAT on, and the furnace is running for heat but fan motor stops?

Thanks!

 
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Old 10-16-20, 11:56 AM
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Hi, have you removed the wire from the “G” terminal on the Stat subbase ? Be sure to reconnect it back at the furnace first, you never mentioned if the problem existed before the Stat was replaced.
Geo🇺🇸
 
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Old 10-16-20, 07:53 PM
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You have a York gas furnace. The control board controls the blower in heat mode.
The G/green wire is for the A/C blower and FAN ON mode.
You can leave the G wire off and the furnace will operate in heat mode correctly.
 
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Old 10-17-20, 11:38 PM
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New update for my latest test:

I disconnected the Green cable from the "G" terminal on the furnace control board(50A50-241 White Rodgers), while I 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, considering I learned 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 both 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 have 3 spare cables to replace the Green cable.

I will post another update early next week.

Many thanks everyone!
 
 

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