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Old 10-16-2021, 10:02 AM   #1
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Atwood 7920-11 Furnace won’t shut off

Hi,
I have a Atwood 7920 that does not turn off. I replaced the analog thermostat with the Dometic analog thermostat which has not solved the problem. I have the temp. all the way to low and it still comes on and stays on until I manually turn the switch to off. It blows hot air and working properly, just won’t turn off automatically.

I adjusted all the thermostat setting possible (just 2) with not luck. Also called Dometic and they say it is something at the furnace.

This is in a truck cab over camper.
Suggestions?
Thank you so much.
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Old 10-16-2021, 11:22 AM   #2
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What year?

Sounds like the control board is sensing heat still in the furnace and won't shut the fan off until it's cool enough, and even thou it is cool enough it's still reporting heat to the circuitry. That's what it sounds like. So, control board maybe? Wouldn't hurt to open it up and clean everything in there. Blow it out with a low pressure air pump.

On my furnace, it has a time out delay. When the thermostat says it's satisfied, it tells the furnace and the furnace control board shuts off the gas so the flame goes out...and it starts a timer that keeps the fan running for 2 minutes. Then shuts off the fan. That's how my '02 Suburban furnace works. Yours could be temp controlled but I don't see why when a timer has been used successfully for decades. So, yeah, probably the control board inside the furnace.

Here's what taking mine out and fixing it looked like...this at my ad free repair blog and though it's a Suburban model, there will be similarities: Furnace repairs
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Old 10-16-2021, 02:24 PM   #3
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It’s a 2000 Elkhorn. I’m sure it is the original furnace....I’ll try cleaning it all our and thanks for the link a d reply.
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Old 10-16-2021, 04:30 PM   #4
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I got it all cleaned out good with the compressed, all the connections look fine. Still having the same issue.
Stopped by Camping World, picked up a new board and installed. Still having the same problem.

It’s not just the fan staying on, the furnace stays lit.
Any other suggestions?
Thanks
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Old 10-16-2021, 05:45 PM   #5
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Aww. Well, it could be the gas valve. Since it still has flame, that's why the fan keeps running because the circuit still senses heat so keeps it on. Works like it's supposed to.

As far as the gas valve goes, you'd need to shut off the gas (big valve right at the tank), purge the line by calling for heat. Measure the coil with an ohmmeter. If it measures ~25 ohm (or what your repair guide says it should measure) than it's okay. If it reads high, then it's bad. If it measure okay, then take the valve apart and check for debris that might be holding the valve open.

Too bad you didn't notice that it's flame stayed on all the time before buying the new control board. Maybe CW will take it back?
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Old 10-16-2021, 09:03 PM   #6
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Wires to/from thermostat are shorted causing DC to remain on Circuit thru Limit switch/sail switch to Circuit Board...as long as that circuit has DC the circuit board will NOT shut down Gas valve or Fan
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Old 10-16-2021, 09:50 PM   #7
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Quote:
Originally Posted by MikeAlaska View Post
Hi,
I have a Atwood 7920 that does not turn off. I replaced the analog thermostat with the Dometic analog thermostat which has not solved the problem. I have the temp. all the way to low and it still comes on and stays on until I manually turn the switch to off. It blows hot air and working properly, just won’t turn off automatically.

I adjusted all the thermostat setting possible (just 2) with not luck. Also called Dometic and they say it is something at the furnace.

This is in a truck cab over camper.
Suggestions?
Thank you so much.
When you say "stays on until I manually turn the switch to off", do you mean a switch on the thermostat or the switch inside the furnace?


EDIT: Which thermostat, this one?
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Old 10-17-2021, 08:43 AM   #8
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Quote:
Originally Posted by Old-Biscuit View Post
Wires to/from thermostat are shorted causing DC to remain on Circuit thru Limit switch/sail switch to Circuit Board...as long as that circuit has DC the circuit board will NOT shut down Gas valve or Fan
Trust Old-Biscuit. He knows what he is talking about.
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