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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!!

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1) remove the G wire on the circuit board ( board ) on the G terminal and put some tape on the end of it. Do not use the AC with it off. Wait a day or two and if the fan starts you may have a problem with the board.

2) If it runs with the G wire off then tap the board with the butt end of a big screwdriver in several places and if the fan stops then the fan relay in the board is faulty and shorting on. That is a older furnace ( 15-20 yrs old ) and the board may be dying from old age/have a problem.
 

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You don't indicate what your t-stat is AFAICT, but some of them (like my Ecobees) have a function where they will run just fan (no heat or AC unless called) periodically to keep air moving around the house.
 

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I would think the power company Pepco if they are installing these thermostats/tstats in lots of houses would be aware of that ventilation function but anything is possible that they don't mention it. If it has one then it should have a set period like once every 4 hours ( on the hour ) it turns on and the OP should notice that pattern. The second one they installed is not doing the same thing.

We had a poster with some power co tstat that had a additional relay on the g wire for ventilation but I don't see that in the pic. Usually they give you a cheap basic programmable tstat.
 

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If your furnace has a pilot light, I think it's possible the temperature high limit switch is faulty and telling the control system that it's too hot and needs to cool off the heat exchanger. When mine went out, the switch/sensor died completely and the fan ran continuously, even with the thermostat set to fan=off. It could be something to check. I was able to check mine by simply bypassing it, which I only did temporarily, while I was monitoring it, then I shut the furnace down until I could replace the switch.
 

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If your furnace has a pilot light, I think it's possible the temperature high limit switch is faulty and telling the control system that it's too hot and needs to cool off the heat exchanger. When mine went out, the switch/sensor died completely and the fan ran continuously, even with the thermostat set to fan=off. It could be something to check. I was able to check mine by simply bypassing it, which I only did temporarily, while I was monitoring it, then I shut the furnace down until I could replace the switch.
No pilot light on that one. You can see the HSI , hot surface igniter in the pic.

Good point though and that can happen in the Summer in horizontal furnaces in attics in very hot areas.
 

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That furnace needs some maintenance.
Flame sensor looks dirty and the ignitor might be cracked or have a crack developing.
 

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That furnace needs some maintenance.
Flame sensor looks dirty and the ignitor might be cracked or have a crack developing.
The HSI has that classic hot spot and may be kaput soon.

If anyone out there has this furnace you need to clean the flame sensor with clean steel wool or #400 emory paper PLUS the face of the burners PLUS the metal on the heat exchanger directly in front of the sensor.

The AC current jumps thru the burner flame to the closest piece of metal and the heat exchanger is close on that one. Then it goes back to the board rectified into DC microamps by metal grounding.
 

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Discussion Starter #9
1) remove the G wire on the circuit board ( board ) on the G terminal and put some tape on the end of it. Do not use the AC with it off. Wait a day or two and if the fan starts you may have a problem with the board.

2) If it runs with the G wire off then tap the board with the butt end of a big screwdriver in several places and if the fan stops then the fan relay in the board is faulty and shorting on. That is a older furnace ( 15-20 yrs old ) and the board may be dying from old age/have a problem.
Thank you Yuri!

I will follow your instruction to remove the G wire. Before that, let me wait until the next time the problem happens(fan turns on while system is set off), so that I can do apple to apple comparison.
I can't easily reproduce the problem, which makes this troubleshooting difficult.
About 2), if the fan runs with the G wire off, I doubt I could have a chance to test with the "tapping" the board, because as soon as I open the door of the furnace in order to access the board, the power supply will be shut off due to the protection switch touching the door.

Yes, the furnace is about 20 years old as the house(built year 2000), which I bought it last year.
 

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No pilot light on that one. You can see the HSI , hot surface igniter in the pic.

Good point though and that can happen in the Summer in horizontal furnaces in attics in very hot areas.
Yes, you are right, the furnace has HSI(hot surface igniter). I have a brand new HSI, as well as new flame sensor, I can replace the old ones when necessary, but I don't think they are the cause of the fan issue I am having now.

Also, you are right about: the thermostats/tstats from Pepco doesn't have the auto routine ventilation feature, confirmed.
 

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If your thermostat uses rechargeable batteries, or has a power stealing function then make sure there is a common wire connected at both ends of the thermostat cable. (That would be the C terminal connected at both the board and the stat)
 

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It's not the thermostat because it was pulled off the base and the same thing happened.

With the g wire disconnected, if it still happens either the board is bad or the limit circuit is intermittently opening due to a bad connection or limit. Furnace runs fan continuously when limit circuit is open. more likely to be a bad board.
 

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Thank you Yuri!

I will follow your instruction to remove the G wire. Before that, let me wait until the next time the problem happens(fan turns on while system is set off), so that I can do apple to apple comparison.
I can't easily reproduce the problem, which makes this troubleshooting difficult.
About 2), if the fan runs with the G wire off, I doubt I could have a chance to test with the "tapping" the board, because as soon as I open the door of the furnace in order to access the board, the power supply will be shut off due to the protection switch touching the door.

Yes, the furnace is about 20 years old as the house(built year 2000), which I bought it last year.
Take some silver foil duct tape and tape it in the closed position.
 

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Discussion Starter #14
If your thermostat uses rechargeable batteries, or has a power stealing function then make sure there is a common wire connected at both ends of the thermostat cable. (That would be the C terminal connected at both the board and the stat)
The thermostat doesn't use any batteries, but uses the power from the attic furnace through the wire(not sure which one, what color?) Did you mean it uses the Common(blue color) wire to get the power, and you want me to check the blue wire connects to both the board(C) and the stat(C)?
Thanks!!
 

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The thermostat doesn't use any batteries, but uses the power from the attic furnace through the wire(not sure which one, what color?) Did you mean it uses the Common(blue color) wire to get the power, and you want me to check the blue wire connects to both the board(C) and the stat(C)?
Thanks!!
Colour code is standard but has exceptions. I can’t say if it’s the blue wire or not without more information. Some thermostats will take power from R and utilize a different terminal to operate the clock, charging circuit, backlight, etc, through the C terminal. If the C terminal is not available then it will send that current through another terminal available. This has caused problems by activating that utilized terminal at a seemingly random interval. To resolve this you would need to install a C terminal wire at the tstat and unit.
 

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It's not the thermostat because it was pulled off the base and the same thing happened.

With the g wire disconnected, if it still happens either the board is bad or the limit circuit is intermittently opening due to a bad connection or limit. Furnace runs fan continuously when limit circuit is open. more likely to be a bad board.
Thanks! Based on this video(
, at 5:42), it could be the problem of the terminal block(the wall part of the stats) as well.
The fan randomly runs, but it will stay stop eventually after 20 mins or half an hour(or a bit more) as I noticed.

Where's the "limit circuit"? Thanks again!
 

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Just disconnect the wire connected to G at the furnace and tape it off to rule everything external out.
 

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Discussion Starter #18
Take some silver foil duct tape and tape it in the closed position.
Did you mean to tape the green wire after I disconnect it from the board for testing, in order to make sure it won't accidentally touch other mental parts? just want to make sure I didn't misunderstand. Thanks!
 

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You can press the door switch IN and then tape it IN so when you remove the fan door the power stays on.

Yeah you need to tape the green wire off and not use the AC as some of them use G to start the fan with the AC. W/O the fan your AC will freeze up.
 
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