Gas Furnace Fan Occassionally Keeps Running After Burner Shutdown

Dr_Zoidberg57 · 01-30-2010, 02:40 PM

Please bear with me as I know little if anything about heaters but am patient when it comes to sorting through a home repair problem, handy with tools and can follow directions.

I have a 25 year old central forced air gas furnace. According to the label inside, it is a Carrier model number 396GAW000075. I first noticed that occasionally after the fan would shut down, following its cool down cycle, about 30 seconds later it would come back on for about a minute and then turn off. Then I discovered, over the last two nights, right after the set-back time on the thermostat, if the heater happened to be on, the fan would continue to run long after the burners shut down. I checked the thermostat and confirmed that the fan was set to "automatic". I feared that I would have to unplug the furnace as the only means to shut off the fan. As a last resort, I over-rode the temperature set on the thermostat and got the burners to light again. This time the fan shut off following the cool down cycle. As yet, I have not noticed the continuous running happen at any other time.

In searching this and other sites for a diagnosis of my situation, it seems that I may have a failing fan limit control switch. If that is the case, none of the answers I looked at described where it would be located on the furnace or what it looks like.

Any assistance would be greatly appreciated.

Gerry

**beenthere** · 01-30-2010, 09:07 PM

Have you changed you air filter lately.

Dr_Zoidberg57 · 01-30-2010, 11:23 PM

Yes and both are clean. I do have pictures of the insides of the unit plus the schematic if those will be of any help.

I appreciate your assistance.

Gerry

**beenthere** · 01-31-2010, 01:25 AM

Pics of inside of unit will help.

Dr_Zoidberg57 · 01-31-2010, 02:45 AM

Here they are.

**beenthere** · 01-31-2010, 09:21 AM

You don't have a fan limit control.

You have high temp limits.

The black rectangular device above your gas valve is the high limit.

NHMaster · 01-31-2010, 10:49 AM

There is a fan delay module on the circuit board that is probably going bad. It is labled 3 on the schematic and has a 2A relay.

Alvis · 01-31-2010, 02:21 PM

Hey doc... Assuming your equipment is in good working order, what is likely happening is ambient heat from the furnace is continuing to build up in the duct work after the furnace shuts off. The sensor in your supply plentum is turning on your fan because the ambient heat has built up in the plentum to the pre-set cut-on cut-off temps. My wood/oil furnace does the same thing, and this is the reason. All is in good working order and doing what it's supposed to. Hope that's your problem, if so, you don't need to do anything.

**beenthere** · 01-31-2010, 04:10 PM

Quote:

Originally Posted by Alvis

The sensor in your supply plentum is turning on your fan because the ambient heat has built up in the plentum to the pre-set cut-on cut-off temps. My wood/oil furnace does the same thing, and this is the reason. All is in good working order and doing what it's supposed to. Hope that's your problem, if so, you don't need to do anything.

Unless someone added one.

That furnace has no temp sensor in the supply plenum.

Alvis · 01-31-2010, 05:09 PM

Quote:

Originally Posted by beenthere

Unless someone added one.

That furnace has no temp sensor in the supply plenum.

The way I remember it, because we mostly heat with wood and were getting too much heat buildup in the plentum, we retrofitted and added the temp sensor after the fact.

**beenthere** · 01-31-2010, 05:27 PM

Quote:

Originally Posted by Alvis

The way I remember it, because we mostly heat with wood and were getting too much heat buildup in the plenum, we retrofitted and added the temp sensor after the fact.

Probably a good thing you added it.

He probably didn't add one though.

barryfish · 11-07-2010, 11:30 AM

Yeah,

I noticed that my 20+ YO Carrier was cycling on high limit.
(call for heat, flame on, gets super hot, hitemp limit switche opens, then the gas valve closes and the fan runs until the limit re-closes .)

Even though the indoor temp was being maintained, its not too efficient or safe to operate in that mode.

I removed the board, traced out the circuit and found that the timing capacitor (C1) was bad.

It's a 15uf tantalium type. It failed shorted which caused the fan time to never operate. I replaced it with 2 caps in parallel that total near the 15uf value and tested the timer. When tested, it times to fan on at 2 min 15 sec. that seems a tad long so I might reduce the cap to 10uf.

Note that capacitors C2 and C3 should also be replaced as old electrolytic caps fail with old age. C2 is likely whats causing the (posted) random operation. If possible find caps rated for 105C (some caps are rated for 85C which would be ok, but the 105C caps will last longer)

Schematic and photos attached.

good luck !
Barry
barry a t barryfish.com

**beenthere** · 11-07-2010, 11:52 AM

What were the sizes of each cap that you put in parallel.

barryfish · 11-07-2010, 01:02 PM

4.7uf and 10uf for the timing cap.
The voltage rating needs to be at least 16V. The internal supply of the board is 12V plus 0.7V for the limiting diode and you want you cap voltag rating greater than 12.7V

I replaced the other 2 electrolytic caps and one of the timing caps that I had installed (the total capacity of my origional pair was almost 20 uf (measured),
now with better tolerance caps the timing is about a minute and a half which seems to work well)

The reference voltage where the comparitor trips is about 6.8V but the cap will charge higher.

I am glad to see that I'm not the only guy that fixes 20+ year old stuff rather than having some idiot tell me I have to replace my furnace (which has freshly cleaned and lubed blower and a new high efficeincy A coil with TXV)

**beenthere** · 11-07-2010, 02:39 PM

The A coil is not part of the furnace.