Gas furnace randomly blowing cold air

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Old 10-28-13, 01:14 AM
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Question Gas furnace randomly blowing cold air

I have an American Standard gas furnace with an automatic ignition device that's probably 10-15 years old. A month ago it started having an issue where the furnace would kick on (due to the current temp) but blow cold air (about 10 degrees below room temp) instead of heat regardless of the thermostat settings. I turn the thermostat to 'off' but it continues to blow cold air until I flip the switch to kill power to the furnace. Once I flip the power back on, the furnace kicks back on automatically but blows hot air again. And it'll work until a few days later. The issue is getting more frequent and I'm not sure what's causing it.

I've had two different repairmen look at it, and both said it was working normally. Of course, the problem didn't surface while they were present. I also had the thermostat replaced this past summer, so that shouldn't be the issue but I wouldn't rule it out.

One other possibly related issue: The furnace frequently kicks on, shuts off, and then kicks on again one or two more times before the blower starts up. Both repairmen said that's normal, but color me skeptical.

Any ideas what I'm looking at here?
Thanks!
 
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Old 10-28-13, 02:30 AM
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Welcome to the forums.

Without knowing what model you have there... do you see a control board with a blinking light on it ? May be visible thru blower door before removing it. Most modern furnaces have a control board with a diagnostic LED that will help pinpoint problems.

Otherwise....you will have to sit there and watch the furnace. You will need to tell us everything it does. Like:

1) thermostat calls for heat and small blower (inducer fan starts)
2) after 30 seconds igniter starts glowing
3) 20 seconds later igniter goes out or burner lights and goes out in 8 seconds.
4) etc.
5) etc.
Without this type of information we are only guessing at your problem.
 
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Old 10-29-13, 01:40 AM
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Pulled the model number off the side: CDB080A945D1

It malfunctioned again today. I don't know much about gas furnaces, but here are my observations (with assumptions based on sounds heard):

1.) Small internal blower kicks on w/ small stream of gas for 60s.
3.) 3s burst of gas before returning to previous state.
4.) Small blower & gas stream continues for another 30s.
5.) Blower and gas turn completely off for 2s.
6.) Repeat #1 for about 20s.
7.) Main blower kicks on while the small bower and gas stream seem to stop. No further sound of ignition, as if everything else simply turned off but the main fan.
8.) Temperature of air blown gets progressively colder.

I once let it run 45min in that state, but it never changed and thermostat controls had no effect. Only way to turn the furnace off was throw the power switch. Once turning the power back on, it works fine...for a while.

When it works, the process is exactly the same except for the final two steps:
7.) Main blower kicks on while the gas stream increases and there's an obvious 'whooshing' sound w/ ignition.
8.) Heat actually comes out of the registers and stops once temp is reached.

Regardless of whether it works or not, the only indicator I see is a steadily-flashing red light that's consistent whether the furnace is running or not. It's behind the upper panel and viewable through a small plastic 'window' w/o removing the cover.
 
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Old 10-29-13, 07:48 AM
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Correction when the furnace works:
7.) The gas stream increases and there's an obvious 'whooshing' sound w/ ignition.
8.) About 20s later the main blower kicks on.
9.) Heat actually comes out of the registers and stops once temp is reached.
 
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Old 10-29-13, 08:18 AM
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the only indicator I see is a steadily-flashing red light that's consistent whether the furnace is running or not.
That flashing light is giving you an error code. The number of flashes is what indicates what the error is. So the first step is to count the number of flashes. The cover on the control board should have a list of error codes. In your sequence list you did not indicate the ignitor energizes. The ignitor should glow shortly after the inducer fan starts. If it does glow and ignites but shuts down shortly thereafter it is probably a flame sensor rod issue. You can clean the flame rod with some fine steel wool. The attached photo is the ignitor for your furnace and is what should be glowing.
The second photo is the flame sensor rod. The part that needs to be cleaned is the rod above the ceramic base.
 
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Last edited by firedawgsatx; 10-29-13 at 08:45 AM.
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