Furnace blows warm at first, after that strictly the fan stays on.

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Old 10-24-04, 12:47 PM
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Furnace blows warm at first, after that strictly the fan stays on.

Great forum you guys have here, I'm looking for an opinion.

This started at the end of last winter and I think it's about time we actually get it fixed . What's happening is when you first turn the furnace on it blows nice and warm. Then once it shuts off the next time it starts up again just the fan blows. If you were to turn the thermostat to the "off" position it still stays running. I have to go down to the basement and flick the switch on the furnace in order for it to actually shut off.

Now, last year a heating repair guy came out and said it's the circuit board that's the problem. I'd like to know if a) that sounds like it's the right problem b) can I buy and replace the circuit board myself c) How much should something like that cost.

I'm pretty proficient when it comes to computers (building them etc) so in my mind the circuit board sounds about the same as the motherboard in a computer . If that's the case it should be a 1-2-3 thing. Any ideas would be greatly appreciated. Thanks in advance.
 
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Old 10-24-04, 05:06 PM
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Sound to me like it could be either the board or the thermostat. Easy way to check is to hook up a multimeter to the control wires and see if on the second attempt the fan wire is being energized instead of the heat wire. If the correct control line is being energized then the fault is likely with the control board.

And yes, the control board is nothing more than a simple microcontroller along wo\ith some peripherals. If you could fault find at the board level my guess is that the repair would costs under a $. Unfortunately most manufacturers do not provide schematics and so you need to buy the board - unfortunately the mark up is several times the real cost of the components.

For someone who builds computer board this should be a simple task -the control board is just a regular PCB with some components on it.
 
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Old 10-25-04, 10:00 AM
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Also could be a safety. Has this furnace ever been cleaned?
 
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Old 10-25-04, 01:50 PM
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Originally Posted by mattison
Also could be a safety. Has this furnace ever been cleaned?
I don't have a multimeter unfortunately. Wouldn't know how to use it either . By cleaned do you mean the filter? Thanks for the responses guys.
 
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Old 10-25-04, 06:59 PM
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No multimeter!! I guess maybe your computer boards work first time I think it is worth having one around the house - very useful instrument. To be honest trouble shooting a real computer board is almost impossible if any of the chips develop a fault - you may stand some chance if you did have access to a logic analyzer and the timing diagrams. However, you can get away with a multimeter and maybe a scope for the furnace electonics...

Anyway- back to your furnace - I would suggest at least maybe trying to borrow a meter and isloating the fault before changing anything.

I think mattison is referring to possibly an overheat condition due to a restricted airflow which is also a possibility.
 
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Old 10-25-04, 07:30 PM
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Thanks for the responses guys. You were right!

A guy came out today and fixed it in 30 minutes. The sensor (?) was dirty. He cleaned it and now the furnace works great. He used a multimeter to test it. Something was .2 (I think) that was supposed to be over 2.

I appreciate all your help.

And in my 4-5 years of building PC's I've never once need a multimeter . I'm quite glad too. I had to test an electric golf cart at work over the summer and didn't have a clue what I was doing.
 
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Old 10-25-04, 07:37 PM
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Possibly a dirty flame sensor perhaps. Coming to think of it this is also a possibility!

Glad you got it fixed
 
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Old 10-27-04, 05:32 PM
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blower not shutting off

Originally Posted by rav12
Possibly a dirty flame sensor perhaps. Coming to think of it this is also a possibility!

Glad you got it fixed
My furnace was doing the same thing. My son shut off the main switch, then turned it back on & it's working again. I'm wondering, though, if we should try to clean the flame sensor so it doesn't happen again. Where would we find it and how would we clean it? We have a 9 year old Dependable 92, made by Goodman Corp. Any help would be very appreciated.

Sharon
 
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Old 10-27-04, 05:58 PM
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dak, I'm glad you got it going.

Sharon, If you're looking at the furnace with the cover off the flame sensor will be located on the left side of the furnace in the burner compartment and it's about the size of a q-tip sticking into the flame. Just rub it slightly with steal wool.
 
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