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Jeremy0911 · 11-04-2017, 11:43 AM

I have a 8531 that keeps shutting off. I am getting the air flow/ limit fault. I have checked the gas lines, re routed the duct work and it still keeps happening. The unit will turn on and run for maybe 10-15 minutes and shut off. It takes about 20 minutes before I can restart it. The limit switch is what I am getting from here. Has anyone had similar issues? Thanks in advance.

Old-Biscuit · 11-04-2017, 12:40 PM

Air flow/limit switch error

Air flow.......sail switch not making up or stuck
Limit Switch error........high temp limit switch opening

So with furnace turning ON and actually running for 10-15 minutes rules out the sail switch
And with it taking 20 minutes or so to restart does point to high temp limit switch

Doubt replacing limit switch would fix the issue.

Limit switch is OPENING due to high temp (10-15 minute run time) and staying open until heat exchanger cools down (20 minute restart delay)

Furnace heat exchanger is overheating:
*Exhaust flow obstructed...mud dabbers/nests etc
*Return air flow obstructed......items stored next to furnace or blocking the return air flow path (65 sq/inch)
*Discharge air obstructed......too few ducts (36 sq/ft---3 ducts min), registers closed off/blocked, collapsed duct (low air flow thru duct)
*Over-firing.....propane pressure too high (S/B 11" WC with 50% of propane appliances in service)
*Defective Limit switch (typically they just fail...not allow running for 10-15 minutes of furnace then shutting down)

You didn't ID which 8531 furnace (II, III, IV)
85-II uses a 130*F Limit Switch (L54) part number 35132
85-III & IV use a 190*F Limit Switch (L190) part number 37022 (old # 36176)

IF you have an 'air temp' thermometer (not a infrared gun) you could measure the hot discharge air temp AT the furnace

I think your issue is obstructed exhaust or airflow (discharge/return)

Jeremy0911 · 11-04-2017, 01:21 PM

Thank you for the advice. i changed the duct work today because it was a mess. It was crossed over and crushing each other. We have 4 exhaust ports venting into the camper. We have not checked the inside of the lines or the exhaust for debris. Today i will check that and let you know what happens

Jeremy0911 · 11-05-2017, 04:54 PM

I shortened three of the vent hoses and cleaned out the others including the exhaust. It has been working just fine since! Thank you so much for the help.