Furnace blower will not shut off


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Old 01-05-12, 03:18 PM
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Furnace blower will not shut off

Hi everyone,

The furnace blower does not turn off when the thermostat is turned off. I have to turn off the furnace power switch to turn the blower off. I've read several postings stating that the fan limit switch needs to be replaced but my "fan switch", if it can be called that, looks nothing like anything I've seen.

Any help would be appreciated. Thanks.

Attached are images of my furnace and the insides.



 
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Old 01-05-12, 03:51 PM
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Your furnace turns the fan on as you describe when the low voltage (24 VAC) power supply to the "R" terminal is disrupted for some reason.

The most common reason for that is a burned out transformer due to thermostat wires cut by sheet metal, but a number of other things can cause that.

Review the circuit diagram on the furnace and check out the circuit beginning at the transformer and ending at the R terminal.
 
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Old 01-05-12, 05:04 PM
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Does the furnace operate otherwise? (Do the burners come on?) If yes, the relay on the board is stuck in the closed position. If no, bad transformer, open limit, open fuse link or open rollout switch.

If the transformer is the culprit, investigate the wiring as S/P has stated
 
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Old 01-05-12, 10:39 PM
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The furnace turns on automatically including the burners when the thermostat is set, but when the thermostat is set all the way to 0, off, the blower continues to run.

Is the relay something that I can replace on my own?

Thanks.
 
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Old 01-05-12, 10:43 PM
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Good call there, MBK3. It didn't occur to me that the burners might be operating.

Usually the relay turns the fan on when the relay isn't energized and shuts the fan off when it is energized.

Is this condition implying that the relay is burned out or otherwise not being energized when it should by the circuit board?

You'd need to replace the circuit board. The replacement circuit board would have two relays, one to turn the fan on and the other to switch between a high and low fan speed. If you check the manufacturing date of the circuit board I bet it dates from 1973-1975 or so.
 

Last edited by SeattlePioneer; 01-05-12 at 11:15 PM.
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Old 01-06-12, 07:12 PM
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Thanks everyone.

A couple taps to the relay while the furnace was running fixed the issue for now. I'll have to look into replacing the circuit board when I get a chance.
 
 

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