Furnace Fan not shutting off - Heat still works

[Jim 5]

I'm trying to troubleshoot my furnace. All things point to a failed circuit board. Carrier natural gas furnace..about 9 years old.

Symptoms:

A. Furnace fan runs at all times. As soon as the power to the furnace is on, fan switches on. With thermostat "Fan" setting switched to "Auto", "On" or "Off" the result is the same. Fan runs.

B. Furnace circuit board is not flashing any trouble codes.

C. Heat cycles on and off as normal when it gets cold in the house and a call for heat is triggered at the thermostat. Burner ignites and works normally at all times when it should.

D. High limit switch is closed circuit (not failed indicating a tripping of the high limit switch).

E. High Limit switch recently replaced due to tripping from drywall dust and clogged filter during a recent basement renovation - 8 months ago.

F. High temperature ignitor recently cleaned with steel wool to correct recent trouble code. 8 months ago.

G. Thermostat fan circuits are not shorted. All are showing voltage at the circuit board when they should, and no voltage when they shouldn't.

Troubleshooting theory:

- Symptom A above indicates a failed high limit switch, but C, D and E rule this out.

- Absent a failed high limit switch, the fan switch (in old style furnaces) or the fan relay (on circuit board furnaces like this one) is suspect - be fused. Tapping on the fan relay did not correct the condition.

- I have 2 of these furnaces side by side. The malfunctioning furnace behaves exactly the same as the functioning unit behaves when the functioning unit's fan setting is set to "On"/"Always On". When power is disconnected to either unit, there is a 3 second delay, followed by an audible click that can be felt with the fingers on the fan relay on the circuit board, followed by the fan powering up. I do not believe therefore that the fan relay is defective. Relay on malfunctioning furnace circuit board clicks audibly exactly as on the functioning furnace. Relay appears to be fine.

Everything points to a bad circuit board, and that I can't just source a fan relay for replacement on the circuit board by desoldering/resoldering. Am I on the right track here?

Next Steps:

The obvious way to prove a bad circuit board is to move the board from the good furnace to the bad furnace and see if that solves the problem. I'd like to avoid that if I can because there are at least 20 wires connected to each and this could be rather time-consuming and could potentially leave me with no working furnaces.

If I think I'm on the right track here in diagnosing a bad circuit board, I have a chance at a $100 circuit board that I'll just order.

Has anybody got any thoughts?

 

[Schmoe]

dang....heater time already??

 

[Schmoe]

I had similar problems with my gas furnace last year, Lennox. It was the circuit board, but that thing cost about 175. Then add a tech guy, around 250 total. For C note and get a new board, I'd do it anyway and try it. If it don't work, you got a backup and you know what it "ain't".

 

[Jim 5]

Originally Posted By: Schmoe
I had similar problems with my gas furnace last year, Lennox. It was the circuit board, but that thing cost about 175. Then add a tech guy, around 250 total. For C note and get a new board, I'd do it anyway and try it. If it don't work, you got a backup and you know what it "ain't".


That's what I was thinking. For $100, I have a spare if I'm wrong. Given the market on ebay these things die all the time, and with 2 identical furnaces, sooner or later I'm going to need one. Hopefully my diagnosis is right and it's sooner.

 

[Jim 5]

Originally Posted By: Schmoe
dang....heater time already??


Getting close. I don't really need to fix this as the furnace works fine even with this problem. But I'd rather have it work right and deal with this stuff before the cold weather sets in.

 

[Schmoe]

I was also going to say that depending on your unit's age, you may not come across another board at that price.

 

[Jim 5]

Originally Posted By: Schmoe
I was also going to say that depending on your unit's age, you may not come across another board at that price.


Seems to be a very common board for Carrier/Bryant furnaces. The cross reference page for furnace models was a whole page long for that board. The price that I found is for a refurb unit, so I better jump on it. New ones are $279 on ebay.

 

[Kestas]

This problem sounds like a stuck relay. Try tapping the relays. If you know which relay controls the fan and you're ambitious enough, perhaps you could clean the contacts or replace the relay.

 

[Smokescreen]

I guess that is the tradeoff for higher efficiency and electronic intervention. The oldskool furnaces were not that efficient, but pretty reliable...

 

[brianl703]

If you cannot find an exact replacement relay that fits on the board, use a generic 24V AC coil relay and solder wires to the board.

 

[bepperb]

Buy it, resell the part on ebay if it doesn't fix the problem.

 

[Jim 5]

Originally Posted By: Kestas
This problem sounds like a stuck relay. Try tapping the relays. If you know which relay controls the fan and you're ambitious enough, perhaps you could clean the contacts or replace the relay.


I'd agree that would be the best diagnosis, but see my notes in the original post above around the relay. Tapped it, and the relay clicks on and off with the fan just as the working furnace circuitboard relay does. If you put your finger on it when you power up the unit, you can feel/hear the relay clicking on and off with the fan. This is why I don't think the relay is fused closed.

 

[HTSS_TR]

Originally Posted By: Smokescreen
I guess that is the tradeoff for higher efficiency and electronic intervention. The oldskool furnaces were not that efficient, but pretty reliable...

I agree, the original furnace in my 40 years old house is still working as when I bought the house 13 years ago. It's a little noisy but it's working as it should.

 

[Jim 5]

Originally Posted By: HTSS_TR
Originally Posted By: Smokescreen
I guess that is the tradeoff for higher efficiency and electronic intervention. The oldskool furnaces were not that efficient, but pretty reliable...

I agree, the original furnace in my 40 years old house is still working as when I bought the house 13 years ago. It's a little noisy but it's working as it should.


Agreed. On an HVAC forum they described old furnaces as "Strong and Dumb" and new furnaces as "Smart and Weak".

 

[brianl703]

It sounds like the circuit board's CPU thinks that the thermostat is calling for the fan. Where did you check for a short circuit? Did you try disconnecting the thermostat wiring from the furnace? If the fan continues to run without the thermostat wiring connected, it is a problem with the board's CPU, not the fan relay.

 

[firefighter]

Are you sure the furnace filter is not plugged, two years ago a buddy had a problem with his heat pump, where the fan was running all the time, here the filter was plugged. The furnace sensed this and ran the fan to signal a problem. Put in a new filter, shut off power to unit, restarted and hasn't had a problem since. I know yours is NG so I don't know if this pertains to you.

 

[Jim 5]

Originally Posted By: firefighter
Are you sure the furnace filter is not plugged, two years ago a buddy had a problem with his heat pump, where the fan was running all the time, here the filter was plugged. The furnace sensed this and ran the fan to signal a problem. Put in a new filter, shut off power to unit, restarted and hasn't had a problem since. I know yours is NG so I don't know if this pertains to you.


That has happened to me before. A clogged filter (in my case from drywall dust from basement development) causes poor airflow over the heat exchanger, causing the high limit switch to open-circuit and causing the circuit board to throw a code "13" as I recall.

That unfortunately is not happening here.

 

[Jim 5]

Originally Posted By: brianl703
It sounds like the circuit board's CPU thinks that the thermostat is calling for the fan. Where did you check for a short circuit? Did you try disconnecting the thermostat wiring from the furnace? If the fan continues to run without the thermostat wiring connected, it is a problem with the board's CPU, not the fan relay.


You've pointed out a flaw in my process as I disconnected the wiring from the thermostat at the thermostat. I'm going to disconnect at the furnace as suggested before I order the circuit board. I definitely agree that the board is sending the fan relay a 24v signal to energize it and power up the fan. I can definitely feel/hear the relay clicking on and off. As you rightly pointed out, I have to make sure the circuit board is not getting that signal from a short in the thermostat wiring in the walls somewhere.

 

[Boomer]

Was there a lightning strike recently? Was there a major power problem from the supply side? A large enough surge can cause a permanent electrical pathway across a board that is almost impossible to see but it conducts power to a part of the board that it shouldn't.

 

[Jim 5]

Originally Posted By: Boomer
Was there a lightning strike recently? Was there a major power problem from the supply side? A large enough surge can cause a permanent electrical pathway across a board that is almost impossible to see but it conducts power to a part of the board that it shouldn't.


Could have been. It happened some time ago but since it was summer I didn't care much. We've had a few outages overnight in the last 6 months. I can't isolate the day it happened, but one day I just noticed the fan was running constantly out of the blue.

I've often wondered what caused these problems. There are no obviously burned components or swollen capacitors. I've examined the board pretty carefully for that.