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Old 11-01-2021, 09:57 PM   #1
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Atwood furnace not very hot

I have a Atwood 8535 in a 2000 motorhome and it just doesn't seem to get my rv warm enough, I have checked the ducts and cleaned the burner. I am wondering if I can put in the next size bigger orifice to try and get a little more btu's out of it. Not sure if that is safe or not. I've searched quite a bit and can't seem to find an answer. Is there something else I might be missing that would help?

TIA
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Old 11-01-2021, 11:05 PM   #2
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NO you can not increase orifice for more heat....
8535...8500 Series with a 35K Btu rating (actual 34K btu---highest rating in 8500 Series)

Lack of heating RV
*Dirty orifice obstructing gas flow
*Intake/exhaust ports dirty obstructing combustion air--mud dabbers etc
*Obstructed RETURN air path
*Low LP System Pressure...main LP reg not maintaining correct LP Pressure (low lp press/lower flame temps...less heat generated
*Sooted heat exchanger inhibiting heat transfer

When running how is the exhaust temp
Warm, Hot, Extremely Hot???

Dirty orifice and/or Low LP SYSTEM Pressure most common under firing issue
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Old 11-01-2021, 11:36 PM   #3
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Thanks for the reply OB, the exhaust temp doesn't seem that hot but still more than you can hold your hand over. I will try and clean the orifice as I didn't do that yet and also check on the other suggestions you gave. How would you go about cleaning the heat exchanger? Does that require it be dismantled and cleaned that way?
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Old 11-02-2021, 12:10 AM   #4
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Thanks for the reply OB, the exhaust temp doesn't seem that hot but still more than you can hold your hand over. I will try and clean the orifice as I didn't do that yet and also check on the other suggestions you gave. How would you go about cleaning the heat exchanger? Does that require it be dismantled and cleaned that way?
Orifice...no poking anything thru it or using high air pressure to
blow thru it---that will damage the orifice
Soak in alcohol then air dry

Exhaust should be too hot to hold hand over Right AT the port
2-3 inches away should be warm air
IF VERY Hot....heat is not transferring

Heat Exchanger cleaning'
YES furnace has to be disassembled exchanger pulled from cabinet and then can be washed out
New gaskets required also
**This is LAST Resort!!

Orifice...Low LP Pressure are more likely culprit for under firing
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Old 11-04-2021, 07:54 PM   #5
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Update -

I put a new regulator on and now the burner seems to be cycling a lot then sometimes doesn't come back on, it has a new sail switch so now I'm wondering if the limit switch is bad. I don't know for sure what the limit switch doesn't but I suspect it regulates the burner????

Anyone have any insight on my next step?
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Old 11-04-2021, 10:51 PM   #6
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Limit Switch stops DC to Circuit Board ---shuts down furnace operation due to High Temps IN Heat Exchanger

You have now gone from Furnace not putting out warm air to Furnace shutting down on High Temp Limit switch (190*F Switch)
*New Reg seems to be supplying correct LP Pressure
**Old Reg may have been failing to control/maintain LP Pressure

BUT---

Sounds like Airflow issue....
Fan room air wheel loose/broken vanes
Ducts obstructed/collapsed......compare airflow from each register
Registers Closed....OPEN all registers
Return Air obstructed/blocked...check that nothing is next to/up against furnace compartment/cabinet

Course the above are assuming the Orifice has NOT be damaged when cleaning it
Oversized will cause overfiring
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Old 11-05-2021, 09:04 PM   #7
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After checking all the airflow vents and intake it still would only produce heat for a few minutes, i put a jumper wire in line to bypass the limit switch and everything worked fine, I have one on order and will change it out. It doesn't look as easy to get to as some of the other components, oh well, has too be done.
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Old 11-05-2021, 09:56 PM   #8
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Ne d to measure air temp at discharge with High Temp Limit Switch jumped .....Limit opens at 190*F
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Old 11-05-2021, 09:58 PM   #9
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Ne d to measure air temp at discharge with High Temp Limit Switch jumped .....Limit opens at 190*F
Ok, will do that tomorrow.

Thanks OB!
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Old 11-05-2021, 10:11 PM   #10
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Ne d to measure air temp at discharge with High Temp Limit Switch jumped .....Limit opens at 190*F
Am I going down the right path? It starts and the burner comes up to temp then shuts off and won't re-fire till I turn it all off and let it reset. I'm assuming it's the limit switch but will check exhaust temp too.
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Old 11-05-2021, 10:12 PM   #11
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Ok, will do that tomorrow.

Thanks OB!
That will show whether Limit switch is Opening like it should on High Temp (furnace overheating...LP Pressure, orifice, airflow)

OR that limit switch is faulty
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Old 11-20-2021, 09:01 PM   #12
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Update -
I finally had time to replace the limit switch and now the furnace is working better than it ever has, it puts out good heat and doesn't cycle. My assumption is that both the limit switch and sail switch went bad.

Thanks all who responded and especially Old Biscuit!
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Old 11-20-2021, 09:54 PM   #13
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Thanks for update and resolution

Heat Good!
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