Gas Shuts Off - Blower and Forced Draft Blower Continue

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Old 01-05-10, 09:32 PM
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Gas Shuts Off - Blower and Forced Draft Blower Continue

I have an Amana Air Command 80 SSE Installed in 1994. Last week it started shutting off the gas while the blower and forced draft continued to run. When I shut the unit off for a few seconds and then turn it back on, it fires back up and may operate normally for a couple of hours or a couple of days, but then shut down again until I turn it on and off. This usually happens when I'm away from the house or asleep, go figure. I've checked what I could think of, but I'm in the electrical business and have only cursory knowledge of heating systems. I checked the vacuum switch circuit by jumpering it out, and it made no difference. I took the flame sensor out and cleaned it but it made no difference. These are the two most common problems I'm aware of so I've exhausted my limited knowledge. If I had a manual on the unit I could probably review the control schematic and find more check points but alas they never supplied one. Any thoughts on what else I should check would be greatly appreciated.

John
 
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Old 01-06-10, 12:36 AM
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There's a fairly good chance it's overheating and shutting off on the high temperature limit switch. See if that switch opens and shuts off the burner.
 
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Old 01-06-10, 09:29 AM
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Originally Posted by SeattlePioneer View Post
There's a fairly good chance it's overheating and shutting off on the high temperature limit switch. See if that switch opens and shuts off the burner.
I will check that tonight. If that's the problem, since this appears to be a normally closed switch that opens at the high temp limit, I'm guessing its likely not a faulty switch? But rather something causing a high temperature condition?

John
 
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Old 01-06-10, 09:42 AM
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I have a York Diamond 80 & just had this problem - it was fixed yesterday. I did quite a few searches & I didn't see anyone mention the actual problem I had. The problem was a small plastic tube that connects to the safety switch, it was brittle and cracked. Once that was replaced, the heater went back to working normally (before, it would cycle on, heat for minute or 2 and then the heat would shut off but the blower kept blowing out cold air).
 
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Old 01-06-10, 10:01 AM
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Originally Posted by Iowapack View Post
If that's the problem, since this appears to be a normally closed switch that opens at the high temp limit, I'm guessing its likely not a faulty switch? But rather something causing a high temperature condition?

John

That's right. If a limit switch fails, 95% of the time it's because it's failing to CLOSE properly and prevents the burners from operating.

If a limit switch opens, there's a very high probability that it is opening because the furnace is overheating.

That's certainly what I'd check first.
 
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Old 01-06-10, 07:21 PM
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Originally Posted by SeattlePioneer View Post
That's right. If a limit switch fails, 95% of the time it's because it's failing to CLOSE properly and prevents the burners from operating.

If a limit switch opens, there's a very high probability that it is opening because the furnace is overheating.

That's certainly what I'd check first.
Thanks Will. I checked the limit switch and it appears to be ok, in fact it looks like (no manual to reference) a thermistor. Being a SS device, when these fail to close, they're burnt out and never close again.

So it would appear to be an overheating problem. As it happens, the last time I reset it last night I left the access panel off, figuring I'd have to come down during the night and reset it, and with the panel off, I can see it light up. With the panel off, it ran fine all last night and today. I'm guessing that points to an over heating problem as well. What should I look at first. It does have a new air return filter, but beyond that I have no experience.

John
 
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