# Gas Furnace Blower/Fan Occasionally Keeps Running



## Dr_Zoidberg57 (Jan 30, 2010)

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


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## Wuzzat? (Jan 30, 2010)

Dr_Zoidberg57 said:


> it seems that I may have a failing fan limit control switch.
> or what it looks like.



The generic answer
fan limit control switch - Google Search

You can probably find an exploded view for your furnace online.  This switch should be downstream of the heat exchanger which, for an upflow furnace, means it's above the heat exchanger. 
Or you could go to a parts place and ask for this part and see if you can peek at the exploded view on his computer screen.

Some furnaces control this fan function with a timer.

Post a schematic in any case; it will be pasted onto the inside of a furnace panel.


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## Dr_Zoidberg57 (Jan 30, 2010)

I've included with the schematic a picture of both the upper and lower sections of the furnace. I checked the part number on the black device withe the two red wires attached to it, located behind the gas control unit and it comes back listed as a Carrier Limit Switch (HH12ZA189 replaced by HH12ZB190). Trying not to appear too dense but is this the part that needs replacing?

Just for information, after I removed the covers to take the pictures, when I put the upper one back, the fan started up and kept running. I had to remove the cover again.

I very much appreciate your patience and assistance.

Gerry


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## Wuzzat? (Jan 30, 2010)

I can't make out some of the details on the schematic.  There is usually a high temperature limit switch that shuts down the burners.

I would look for the switch that turns on the blower when air through the heat exchanger is sufficiently hot.  It seems to be turning on the blower without reason.

On the top half of your diagram, the schematic [not the connection] diagram, there are three relay contacts that control the blower motor.  One looks like "CFR" and it decides the speed the blower will run at, the other looks like HFR [Heating Fan Relay].  
Either of these contacts closing can turn on the fan at either the heating speed or the cooling speed.   One of the CFR contacts is normally closed, I'd say during heating because it goes to the Medium Low speed tap on the motor.  

I'm kind of talking myself into the HFR contacts closing without reason.  I'd say the control board is telling this relay to close.  The coil that controls the HFR relay is in the little box that says HFR, LOGIC, TIME DELAY.  It's likely the problem is in this box or upstream of this box or the control board is getting a sensor signal that is erroneous [see the second sentence above].

To go further I need a legible copy of both the Connection Diagram and the Schematic diagram.  

If you or a tech fixes this you have an even chance of having this furnace make it to 37 years, according to my replacement age samples.  But, check on the price of a control board.

When the fan comes on by itself, does it run at the low heating speed or the high A/C speed?


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## Dr_Zoidberg57 (Jan 30, 2010)

I am trying to get you a link to the manual that has the schematic; however the site from which I got it is currently down.



> I would look for the switch that turns on the blower when air through the heat exchanger is sufficiently hot. It seems to be turning on the blower without reason.



At the risk of sounding ignorant, in looking at the photos I provided, where would I find that particular switch.

I appreciate the patience.

Gerry


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## Wuzzat? (Jan 30, 2010)

Dr_Zoidberg57 said:


> I am trying to get you a link to the manual that has the schematic; however the site from which I got it is currently down.
> 
> 
> 
> ...


I'd need to see an exploded view of the parts to answer that, but if an HVAC guy would chime in he can probably answer that without referring to a diagram.


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## Dr_Zoidberg57 (Jan 30, 2010)

I've spent all afternoon looking for one. Guess it must be rare for a furnace this old to still be functioning.


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## Dr_Zoidberg57 (Jan 31, 2010)

I managed to remove the actual unit's schematic from the cover panal and scanned it. Hopefully this will help as it appears the earlier one was for a different but similar model.


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## Wuzzat? (Jan 31, 2010)

I don't see anything on this second schematic that could cause your symptoms, except the control board.  Probably no company will give out schematics for their control boards.   There's no microprocessor on this simple board so you may be able to trace out the wiring yourself but it is a very tedious process.

A new furnace installed may run $7K if your present ductwork layout is OK for the new furnace.  
Just the parts cost for a new control board may be 5% to 10% of this $7K.  The price is designed to put you on the fence between repair and replace.  
A successful service call will cost . . .?  Successful or not, a service call is probably a minimum of $100.

You can replace parts at random, hoping you'll get lucky, but it is a slippery slope.  The more money you invest in parts, the more you'll want to.
Sunk costs - Wikipedia, the free encyclopedia

BTW, when the blower goes on by itself and it's at the higher speed, it could also be the 'stat, but I don't think that's likely.


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## DDH10756 (Jan 24, 2013)

Replace the limit switch the black square with the 2 red wires, its a wafer switch that can be intermittent when closed it tells the blower there is heat a to turn on.


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