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Old 09-16-2019, 08:11 PM   #1
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Furnace comes on when turned off

Strange thing on my 2015 Prime Time Crusader 351REQ 5th wheel: even though the furnace is turned off, every now and then it comes on for about 15 seconds. Had the notorious control board replace mid-trip in January, and haven't used it since. I'm sure this can't be good and would like to mitigate the possibility of another mid-trip repair. Any thoughts?

TIA,
phil
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Old 09-16-2019, 11:44 PM   #2
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Could you add some info on your furnace , brand and model # and type of thermostat , manual or digital .

That info will help other members to help .
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Old 09-17-2019, 10:32 AM   #3
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The heat exchanger on a lot of furnaces have a thermostat that keeps the fan running till it gets to around 100 degrees or so. High temperature inside your heater could be triggering this thermostat? That would be my first guess since it shuts off quickly.
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Old 09-17-2019, 12:04 PM   #4
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Quote:
Originally Posted by Skip426 View Post
Could you add some info on your furnace , brand and model # and type of thermostat , manual or digital .

That info will help other members to help .
Thanks, Skip. That's an obvious fact. Apparently too obvious for me.

It's an Atwood 8535-IV DCLP furnace with a claimed 26K BTU/HR output. In searching for that info, the plot began to thicken.

Inside is that control board. On it is a red light that blinks. A label on the motor with the title Ignition Control Diagnosics tells you what it means if there is one blink with a 3 second pause: air flow/limit fault.

Not sure what that problem is. It has not been turned on for seven months. After reading that, I tried turning the furnace on. It will not ignite.

Hopefully someone can direct me for some more diagnostic testing.
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Old 09-17-2019, 12:06 PM   #5
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Quote:
Originally Posted by Roy1 View Post
The heat exchanger on a lot of furnaces have a thermostat that keeps the fan running till it gets to around 100 degrees or so. High temperature inside your heater could be triggering this thermostat? That would be my first guess since it shuts off quickly.
Thanks, Roy. Because I haven't turned the furnace on, and the daytime temps in my part of NY have only been in the 70s, I'm thinking the info I just posted above will be a clue for someone who has some intimate knowledge of this animal.
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Old 09-17-2019, 05:53 PM   #6
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In the " Files " drop box ( blue bar just to the right of your " User CP " go to " Heating and Cooling " for the ; RV furnace Troubleshooting pdf.

I have a Attwood manual and will see if the code is covered.
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Old 09-17-2019, 06:30 PM   #7
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Here's an old post with info about the , air flow / limit fault , that may contain the repair info you need.

Atwood furnace problem
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Old 09-19-2019, 10:46 AM   #8
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Have you washed the RV/have had heavy rains? On mine, after a heavy rain the control board was sitting in a puddle of water, causing it to come on-or at least the fan. One time it was clicking like trying to ignite as well. Just a thought..Good luck
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Old 09-19-2019, 11:06 AM   #9
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Quote:
It's an Atwood 8535-IV DCLP furnace with a claimed 26K BTU/HR output.
According to the Atwood service manual, the rated output on that model is 34,000 btu.

The fan running for 15 seconds is consistent with what is called a purge cycle, meaning the fan runs to clear any residual gases from the burner chamber.

A high limit fault means the burner/heat exchange exceeds it's max temperature limit, which is 190 F. on the 8935 IV. Since the burner isn't on, the best guess is either a defective light limit sensor or a shorted wire. As I recall, the Hi Limit sensor is normally closed and opens at high temperature. When the controller gets a high limit fault, it shuts off the gas valve but runs the fan to cool the burner chamber down.


All this is covered in the Atwood Service Manual available at http://bryantrv.com/docs.html#furnace
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Old 09-19-2019, 11:54 AM   #10
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This board is mounted on the side, so off the bottom. Kinda dry inside. We'll see...
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Old 09-19-2019, 11:58 AM   #11
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Originally Posted by Gary RVRoamer View Post
According to the Atwood service manual, the rated output on that model is 34,000 btu.

The fan running for 15 seconds is consistent with what is called a purge cycle, meaning the fan runs to clear any residual gases from the burner chamber.

A high limit fault means the burner/heat exchange exceeds it's max temperature limit, which is 190 F. on the 8935 IV. Since the burner isn't on, the best guess is either a defective light limit sensor or a shorted wire. As I recall, the Hi Limit sensor is normally closed and opens at high temperature. When the controller gets a high limit fault, it shuts off the gas valve but runs the fan to cool the burner chamber down.


All this is covered in the Atwood Service Manual available at Service Documents and Manuals
Thanks, Gary. I don't actually see any Atwood manuals in the furnace section, but I will be searching for one. Good idea.
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