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Old 01-04-2025, 06:32 PM   #1
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Furnace Issues Help

About 4 months ago our furnace quit working randomly. We were hooked up to a big propane tank from a 3rd party company. That began to mess with our propane regulator automatic changeover, in turn I believed it was messing with the furnace which would quit working. If I drained all the propane lines and rebooked the propane up opening the nozzle slowly the furnace would work for a day or two then stop again. Eventually it stopped working completely, I changed the propane regulator with no luck. Again dug into the furnace and saw a couple loose connections and pushed them on the furnace worked for one night. The blower does not kick on at all. I receive the Limit switch/airflow fault code, I’ve ensured all my intakes and exhausts are clear. I replaced the limit switch and it didn’t work. I dug into it again checking voltage, everything was checking out to my understanding. Couldn’t figure out where exactly I needed to track it to (I was worried about a bad motor). While I was checking voltage it turned on again I’m not sure if I moved something while I had the control board pulled out, I began to put it back in and it tripped the new limit switch. I verified it was open, grabbed the old one and verified it was closed. The furnace began working again. I didn’t run it long because it’s daytime right now and hot in Arizona. I will provide an update if it begins to not work through the night again. I guess I can’t figure out what’s tripping the limit switch and how they return closed after opening. Any help would be great.
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Old 01-04-2025, 07:18 PM   #2
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Brand/Model of furnace?

Limit switch measures the air temp flowing thru the heat exchanger
If air temp exceeds to pre-set temp of limit switch it opens
*could be 130*F, 140*F or even 190*F -- model dependent

When limit switch opens........fan continues to run cooling heat exchanger down which then allows limit switch to cool/close
DC returns to circuit board and ignition cycle resumes


Limit switch/air flow could also mean Sail Switch is not closing
Fan starts and at 75% motor speed should be supplying enough air flow to move the 'paddle' and close Sail switch allowing DC thru Limit/sail switch circuit to the circuit board for ignition
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Old 01-05-2025, 01:06 PM   #3
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So I did replace the sail switch a couple weeks ago. The furnace did not work at all over night. I’m not sure what is opening the limit switch. Could the LP pressure be an issue?
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Old 01-05-2025, 04:10 PM   #4
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Limit switch opens if overheating occurs

Could be due to High LP Pressure (S/B 11"WC ---- 0.39 psi with 50% of appliance in service)
Could be due to restricted airflow...combustion side
Could be due to restricted RETURN airflow
Could be defective limit switch

IF........Limit Switch is opening
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Old 01-06-2025, 07:13 AM   #5
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Quote:
Originally Posted by Old-Biscuit View Post
Limit switch opens if overheating occurs

Could be due to High LP Pressure (S/B 11"WC ---- 0.39 psi with 50% of appliance in service)
Could be due to restricted airflow...combustion side
Could be due to restricted RETURN airflow
Could be defective limit switch

IF........Limit Switch is opening
Okay thank you. I did replace my propane regulator at the source a few months back so that is brand new. It is rated properly or even for less pressure I believe. My furnace has one flex that is an intake and one flex that goes to the channel duct to feed floor vents, where would the return airflow be?
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Old 01-06-2025, 09:17 PM   #6
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Return air to furnace...........

Open passage that results in air coming back to furnace cabinet

What is the Year/Brand/Model of RV????
What is the Brand/Model of furnace??
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Old 01-07-2025, 07:23 AM   #7
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Quote:
Originally Posted by Old-Biscuit View Post
Return air to furnace...........

Open passage that results in air coming back to furnace cabinet

What is the Year/Brand/Model of RV????
What is the Brand/Model of furnace??
I will have to look into that tonight didn’t notice any return when I was under the trailer. It’s a 2023 Keystone Montana High Country 331LR, Furnace is a dometic DFLD40131
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Old 01-07-2025, 05:50 PM   #8
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Return air is INSIDE the RV........open pathway back to furnace

Could be via that center island or even under the steps up to bedroom
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Old 01-15-2025, 06:54 AM   #9
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Okay thank you. Return airflow is clear. What would be the airflow on the combustion side? I’m kinda at a loss and wondering if I need to get into the blower motor. I replaced the circuit board and bled propane lines yesterday, it faulted twice the first two startups and I noticed while it was faulting it was showing some continuity between the sail switch even though the blower was not running and the switch would show open when not faulting. I ensured good connections again and got it running. Sail switch, board and limit switch should be good. It ran from 8pm to 4am and then quit again. I’m thinking I need to pull everything apart but what else could my issue be besides blower motor or maybe the gas orifice switch?? (Not sure if I’m saying that right)
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Old 01-15-2025, 09:01 AM   #10
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Motor is a dual shaft motor

One side is the air wheel for the 'room' air

Other side is the air wheel for ''combustion air
That air is pulled in via the intake/exhaust port on exterior wall of RV

Both air wheels need to be clear/unobstructed and no broken vanes or loose on motor shafts



Exploded Views of Dometic DFLD40131 https://pdxrvwholesale.com/products/...31-tune-up-kit
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