Furnace /AC Fan randomly turns ON/OFF - When Outside Compressor is OFF

[NatB]

I have a Rheem 2 Stage HVAC system in my home. System has been reliable for years and I have made no changes to it.

Recently , the system attic furnace fan randomly runs when the outside AC Compressor is OFF. I swapped thermostats, but the issue remains. Even when I turn the system off via the thermostat, the furnace /AC fan in the attic unit runs. It is random, right now the system is operating normally.

I suspect the Furnace Control Circuit Board's fan motor relay is faulty. Tech came to the house and checked out all the safety switches and the wiring, and ruling out everything else and concluded it was the fan motor relay on the circuit board. ON start , we heard a little "chatter" emanating from the fan motor relay box on the circuit board ( circuit board part # 62-102637-01).

We also noticed the CFM light on the circuit board would occasionally flicker very faintly when the system fan was off , as if there was some voltage seepage . Is this normal, or another sign of a failing circuit board ? My Tech's technical support guy seems to think that is not abnormal - but based on the symptoms also thinks it is the circuit board that needs replacing.

Looks like a straightforward job for me to replace the circuit board myself, so I am not concerned about doing the work myself.

Since there was no definitive diagnosis and the issue is random, should I go ahead and have the circuit board replaced or wait for it to fail completely? Seems like the main risk could be the motor relay fails and the fan fails to start ( not turning off in the short term is probably not an issue).

Thanks in advance for any helpful suggestions or ideas.

[BNME8EZ]

Is this an ECM motor? Assuming it is, your green light is not an indication of a problem.

Checking safeties when is it not exhibiting the symptom does not tell a lot. Intermittent problems are the worst, hard to catch in the act. Sometimes the proper knowledge of Sequence of Operation and control logic will lend some insight.

Is this happening in heating or cooling? Heat pump or furnace/AC?

[pacnw]

Many times, as referenced by BNM8EZ, the faint flicker is just a status or check light and means all is normal in standby mode. I would not base any diagnosis off that, unless mfr says otherwise.

[NatB]

Many times, as referenced by BNM8EZ, the faint flicker is just a status or check light and means all is normal in standby mode. I would not base any diagnosis off that, unless mfr says otherwise.

There are 4 light on the Circuit Board. The red like labeled "CFM" is the one that faintly flickers when the system fan is off. The Tech explained that when the system is operating , it blinks to indicate 100CFM per each blink .

So sounds like the faint flickering of the light is not indicative of the circuit board failing.

What about the "chattering" coming from the fan motor relay on the circuit board. The tech did not seem to think that was normal. Thoughts?

My gut is the fan relay is on the way out and the source of this strange fan behavior.

I am about to just throw $ at a new circuit board and just swap it out, or at least have it on hand when it will surely fail over the holidays ( Murphy's Law).

[BNME8EZ]

The relay won't operate on ti's own something has to power it. What ever is turning it on is possibly not feeding full voltage so it could be a short in the wiring or a safety sensing something not there, or something that is there but borderline out of spec. You mentioned the thermostat. It doesn't happen to be on "circulate"?

[NatB]

Thanks. No the thermostat is not set on circulate.

The Technician checked the wiring to the thermostat - it was good.

Yes, seems odd. I hate to spend the national average to have the circuit board replaced by the contractor , then to find out it was something else. That's why I am thinking to just buy the part and keep on hand to install myself when it fails altogether.

What about the "chattering" noise coming from the fan motor relay on the circuit board during fan startup ? Is that normal , or a failing relay ( which is what I believe the problem to be). That said, I see your point that the relay has to receive power from somewhere. If the Circuit Board has live power (not just the 24V transformer feed for Thermostat) - then I suppose the board could be shorting?

Given the HVAC unit works in AC and Heat mode, and the issue is the fan intermittently goes ON/OFF while the system is not running seems like the 3 suspects are:

1) Thermostat ( replaced - no change)
2) Thermostat wiring - Tech checked visually and said good
3) Furnace Control Circuit Board - which the Tech suspects ( but I did too)

Also, as I mentioned, when the fan is running and the outside compressor unit is OFF, it continues to run even when I turn the unit off via the thermostat. Weird. So far , this week-end the system is operating normally .

The circuit board unfortunately is not intelligent enough to provide the tech a diagnosis - so it seems we're left to a process of elimination ( i.e. swapping parts).

[pecmsg]

The circuit board unfortunately is not intelligent enough to provide the tech a diagnosis - so it seems we're left to a process of elimination ( i.e. swapping parts).

Good luck with that. Changing parts is the worst troubleshooting there is

[NatB]

I agree with you. I will talk to the Tech next week, I want to be sure that he fixes the issue and not have to keep coming back . Otherwise, I can swap the board myself and just be out the cost of the part.

[Mr Bill]

Did you see if the tech pull all the wires off the board, to see if it still does the same thing? This would eliminate the low voltage wiring and thermostat being the problem.

[NatB]

Hi Mr. Bill,

The Tech did not pull any wires off ( I was up in the attic with him the whole time). But also , the unit was not acting up of course while he was at my house . It's strange, like so far this week end seem to be operating normally. Are you talking about pulling off the thermostat wires? I could try that at the thermostat at the wall - but what would it tell me?

The fan running seems to only occur when the AC is running and then the outside unit shuts off. randomly , the system fan would run , and turn off and on by itself. This occurred even when I turned OFF the system via the thermostat . The AC turns on normally, no issues until the outside compressor shut off . This is when I notice the fan running off and on intermittently , The thermostat shows the system not running when this happens.

It's still too warm to run the furnace - so not enough time on it to know whether the issue happens in both furnace and Ac modes.

[beenthere]

Probably not the board. More like a safety.

The system off setting on the thermostat does actually turn off anything with the furnace or AC. It just won’t send a heat or cooling call to the furnace. But if while set off you turn the fan switch to on, the fan will still run.

How long does the fan run and then stay off while this is happening.

[pacnw]

What thermostats are you using/swapping?

[NatB]

Probably not the board. More like a safety.

The system off setting on the thermostat does actually turn off anything with the furnace or AC. It just won’t send a heat or cooling call to the furnace. But if while set off you turn the fan switch to on, the fan will still run.

How long does the fan run and then stay off while this is happening.

Thanks for responding !

So far this week end, with the AC running , the fan is behaving and turning on/off with the outside compressor unit. The issue appears to occur randomly ( it happened for several days last week; and then about a week ago).

When it occurs - the furnace/ac fan behaves these ways:

1) The fan runs continuously after the outside compressor shuts off- until I power off the power to the furnace
2) Sometimes the fan shuts down when the outside compressor shuts off; but then the fan start up and runs for say 10 minutes and then off (sometime repeating this cycle).

Note even turning the thermostat off does not shut off the fan when it is in this "ghost" cycle.

The Tech who came out checked the safely switches , but said they were OK. Then again, the unit functioned properly when he was checking it out.

[beenthere]

The thermostat doesn’t have any ability to shut off the fan. It can only turn it on or to automatic.

A safety can work properly intermittently, it could be a safety or the board.

[NatB]

Thank you ! Understand, but confusing as my NEST thermostat has a command for system "OFF". I thought that mean't shutting everything down?

So a failing safety switch can cause the fan to start like I've described? I think there are 3 on my unit ( 2 flame; 1 aft of flame; and the door safely).

Can the safety switches be definitively tested by the tech ?

Also, could a "chattering" relay on the circuit board have anything do do with what I've described?

[NatB]

I have used a NEST for over 5 years; and got a newer version to swap.

[pecmsg]

Nest

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[NatB]

Nest

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What specifically do you not like about the NEST thermostat? I have used them since they first came out over 5 years ago, and never had any issues at all with mine. But I am open to know of any shortfalls.

Do you have a recommendation for another residential thermostat that connects to WIFI , and can be controlled via an smartphone APP ? If so, what does it do better?

I am always interested in better technology solutions.

Thanks

[pecmsg]

NEST Hands down causes more control issues then all other stats on the market!

Nest
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[NatB]

Thanks, interesting to know given the current issue. Although I do not believe in my case Nest is at fault.

Do you have a suggestion for a better alternative that does everything Nest can do (i.e. wifi connectivity and phone app controlled)?