Furnace Blowing COLD Air - HVAC - DIY Chatroom Home Improvement Forum

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Old 12-05-2011, 08:32 AM   #1
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Furnace blowing COLD air

I am helping a friend diagnose a problem with her gas furnace. I don't have the model number of her furnace yet, but here is the general problem:

The furnace starts up normally and blows hot air for about 6 minutes, then near the end of this cycle the furnace blows ice cold air for about 3 minute and stops.

Is this normal?

Could this be a sign that the fan relay is malfunctioning? If this is the likely culprit, does anyone have a link to picture of how this relay might look like?

Please help me as soon as you can. Thanks.


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Old 12-05-2011, 10:50 AM   #2
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no, this is not normal. Is the tstat satified at the point when the furnace cuts off, or is it a degree or so still below the setting? Normally a cycle would begin when the tstat setting starts up the burner, The burner will run for approx 3 minutes or so then the fan will come on. The burner should run for another 3-4 minutes {approx 7 minutes total in all )then shut off and the fan should run another 2 minutes or so and then shut down. If the fans " cut-off' setting is set too high then the fan will continue to run a bit longer and only deliver cool air Since I do not the model,etc of the furnace I cannot tell you how to adjust the "fan off" setting{if it even has one}. You may have to buy an new fan control.


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Old 12-05-2011, 04:56 PM   #3
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Have her check if the AC is running outside. Even if the furnace fan sticks on the air coming out of the registers should be room temp not "ice cold". If the furnace is in the attic also have her look for leaks in the return air duct which could be bringing in cold attic air.
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