Furnace problems HELP

ChaseM

Member
Joined
May 11, 2024
Messages
19
Location
Rigby
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’m receiving 1 flash every 3 seconds which indicates Air Flow/High limit fault. I’ve ensured all intakes and vents are clear from obstructions. Just replaced high limit switch and sail switch, still no heat. help
 
Welcome aboard! Glad you found us. Furnace issues are quite common this time of year.

There could be a few things causing your issues. But it would help to start with what you've got. What year and model of trailer? What brand of furnace? Which thermostat?

Given what you've said so far I would do a simple check by bypassing the thermostat. There are a couple ways to do that depending on access.

There are typically 2 blue wires that come from the thermostat via the AC unit. On the furnace main board you can jump those 2 connection points to see if the furnace fan spins up.

A lot depends on what you've got and where it is so the info I asked for will help us give you some direction.
 
Guessing by randomly replacing parts most of the time yields an expensive and unrewarding result. A volt/ohm meter will help you find the problem. You tube has many good videos explaining all the vulnerable failure points. Continuity across the limit switch and sail switch when blower is running narrows the search. 12 volts at the gas valve at some point after the blower runs, confirms it’s working. There is an 1/8” prong that forms a gap within the flame that has to sense a micro voltage passing through the flame. A good grounding of flame sensor is crucial for the mother board to sense the micro volt passing through the flame. If the furnace voltage drops below approx 10 volts during operation, it will cause a shut down of the furnace. Propane can contain small amounts of oil, which if it gats in the tubing can block the gas valve or the flame orifice. As suggested in post #2, shorting the 2 blue to each other will send a start request to the mother board. If nothing happens, look for where the 12 volts stops at each connection.

https://youtu.be/JbqT-9xbmBI
 
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Guessing by randomly replacing parts most of the time yields an expensive and unrewarding result. A volt/ohm meter will help you find the problem. You tube has many good videos explaining all the vulnerable failure points. Continuity across the limit switch and sail switch when blower is running narrows the search. 12 volts at the gas valve at some point after the blower runs, confirms it’s working. There is an 1/8” prong that forms a gap within the flame that has to sense a micro voltage passing through the flame. A good grounding of flame sensor is crucial for the mother board to sense the micro volt passing through the flame. If the furnace voltage drops below approx 10 volts during operation, it will cause a shut down of the furnace. Propane can contain small amounts of oil, which if it gats in the tubing can block the gas valve or the flame orifice. As suggested in post #2, shorting the 2 blue to each other will send a start request to the mother board. If nothing happens, look for where the 12 volts stops at each connection.

https://youtu.be/JbqT-9xbmBI

Okay so I followed your advice as I started to dig into it some more. I began checking voltages, I was having a little trouble following the order of operations to where the voltage should lead seemed like a lot of wires that were not made up neatly when installed. I was also checking connections throughout and had the motherboard pulled out, I turned the heat on and it happened to begin working blower kicked on and ignited. I was moving the motherboard back to its position and it cut the heat off and faulted, not sure if moving it was part of the issue. I then checked the limit switch and it was open, I just replaced that. I still had the old one and it was closed (not sure when they re close after opened). I replaced it with the old one and the furnace again worked, didn’t let it run too long because it was day time and warm out but I will know tonight. What could cause constant tripping of the limit switch.I checked the airflow and it looks good.
 
If the flame only continued for a few seconds, I’d focus on the flame sensor gap, a good ground and the wire connection at the flame sensor to the motherboard. What causes the limit switch to open (assuming it is is opening at the correct limit temperature) is poor airflow through the furnace and downstream ductwork. Blocked floor vents or intake grille can cause that.
 
Thank you, I will look into the flame sensor tomorrow. Do I need to make sure it’s clean and not damaged? As far as the airflow I only have an intake which is clear and an export which goes to a maybe foot wide duct channel that spreads across the camper. All the vents inside look good but not sure how else to get inside it.
 
The motherboard must detect a micro volt current flowing from itself through the wire leading to the flame sensor gap, then pass through the flame and finding a grounded metal surface. It doesn’t take much resistance to block the any part of the pathway of motherboard to ground. If no electrical current flow is detected within a few seconds of the gas valve opening, the gas valve is deactivated.
 
kay 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). Do I need to pull that gas valve deal out and check it?
 
Combustion air is drawn in from outside. You either have a two vent setup on the exterior where one is the furnace exhaust and the other is the combustion intake. For efficiency purposes, most RV furnaces use a 2-in-1 setup where there is one port on the exterior, with the center smaller pipe being the exhaust and the larger outer pipe being the combustion air intake. You said your vents were clear, but was that in reference to the exhaust and intake vents? Being an open vent on the outside, all kinds of bugs and critters get in there and can restrict the intake.
 
Combustion air is drawn in from outside. You either have a two vent setup on the exterior where one is the furnace exhaust and the other is the combustion intake. For efficiency purposes, most RV furnaces use a 2-in-1 setup where there is one port on the exterior, with the center smaller pipe being the exhaust and the larger outer pipe being the combustion air intake. You said your vents were clear, but was that in reference to the exhaust and intake vents? Being an open vent on the outside, all kinds of bugs and critters get in there and can restrict the intake.

I have one pipe on the exterior. It looks clear from what I can see but will have to just rip in to the whole furnace at this point. My intake and export vents are clear. Kinda at a loss on why it runs and randomly stops. Only thing I haven’t ruled out is bad blower motor, something with the gas valve? Voltage seems to be good whenever I check all connections look good. Just randomly will work for a few hours then throw airflow/limit switch fault
 
kay 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). Do I need to pull that gas valve deal out and check it?

What is "some continuity"?
 
The heater’s flame creates a conductive pathway through which a tiny milli-volt from the circuit board is able to jump across from the igniter probe to the burner tube’s metal body. If the circuit board ceases to sense this milli-volt current, voltage to the gas valve is cut off. Poor grounding of any metal parts involved in the pathway will cause cessation of gas flow.
 
Your furnace is a Dometic Atwood DFMD35 series...

Use this link to the service manual for further trouble shooting..

** First thing I would do is check the alignment of the furnace to the outside wall vent and make sure it is meshed together straight and not cocked on an angle...

You are throwing parts at a 2023 furnace that should not be having those issues...

If the furnace to duct alignment looks good, AND YOU hear the ignitor making a ticking sound when listening outside and smell LP and it lights off but then later quits ( as you stated ) I would pull the burner section out and check the ignitor to flame sensor gap (1/8 inch) as well as signs of crap inside the burner section

https://techsupport.pdxrvwholesale....twood-Mobile_Service-Training-Manual-2015.pdf
 
Thank you guys I’m going to check the burner. That sounds like a possible problem. I’m just confused as to why the blower motor fails to start, shouldn’t that happen prior to any gas valve opening? Possibly it’s a tripped high limit switch? But when I do get it running it will run for 6 hours then a fault is present again and I have to work on getting the motor to turn on. When I said “some continuity” I would put my multimeter on the sail switch and there would be no continuity ensuring it’s open and not stuck closed. I would then turn the heater on, (blower wouldn’t start and fault light would begin flashing) I checked the sail switch again immediately and the sail switch would intermediately show continuity across the two ends of the switch. Once I turned the furnace off continuity would be open again. Maybe the two wires connect somewhere else in the process of attempting to ignite. Im a little confused on the process of ignition on this model.
 
I would also suspect the propane regulator given the symptoms and what's already been done. If the OP's regulator is a Fairview then I would just replace it with a Marshal Excelsior and give it a try. Without the proper equipment you can't really test the regulator. They are deceptive because reduced flow only impacts the furnace...the other LP appliances work just fine. Worth a look.

The regulator failed early in our ownership and twice in our friends 5er.
 
... 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.

When I said “some continuity” I would put my multimeter on the sail switch and there would be no continuity ensuring it’s open and not stuck closed. I would then turn the heater on, (blower wouldn’t start and fault light would begin flashing) I checked the sail switch again immediately and the sail switch would intermediately show continuity across the two ends of the switch. Once I turned the furnace off continuity would be open again. Maybe the two wires connect somewhere else in the process of attempting to ignite. Im a little confused on the process of ignition on this model.

This is just a hunch, but you may have a problem either with a connection/wire going to/from the sail switch or a problem within the sail switch...

The way the furnace "should work" is that when the thermostat calls for heat, the control board FIRST looks for an open in the sail switch, then after the fan starts running, the control board looks for the sail switch to close. If that sequence is not followed, a faulty sail switch "COULD" create an unsafe situation. So the control board "verifies" the sail switch is operational as a part of every startup sequence...

Then, if the sail switch "flickers" or "opens momentarily" through the furnace heat cycle, the control board "senses that change in continuity" and shuts down the furnace as a precaution....

In other words, the control board "monitors the operation of the sail switch" throughout the heat cycle and if there is any indication of an open/short in the sail switch circuit that is not expected or that does not follow the exact sequence, the control board shuts down the furnace.

The way the two quotes from you above read, is it possible that your sail switch is not "following the expected "open, closed, remains closed and then open" sequence ??? Remember, the continuity from the control board, to the sail switch and back to the control board includes not only the actual sail switch, but the connectors and wires that attach the sail switch to the control board. Also, to complicate things, the printed circuit/components on the control board may be at fault as well as a "spike/drop in voltage" from the trailer to the control board that allows the fan to drop below a specific speed that would keep the sail switch closed....

My "hunch" (and that's all it is, is a hunch) would be to focus on the sail switch and make sure that its ENTIRE sequence of events follows the protocol. There may be nothing out of sequence, but if there is, that could be the source of your unexpected shutdowns...
 
This is just a hunch, but you may have a problem either with a connection/wire going to/from the sail switch or a problem within the sail switch...

The way the furnace "should work" is that when the thermostat calls for heat, the control board FIRST looks for an open in the sail switch, then after the fan starts running, the control board looks for the sail switch to close. If that sequence is not followed, a faulty sail switch "COULD" create an unsafe situation. So the control board "verifies" the sail switch is operational as a part of every startup sequence...

Then, if the sail switch "flickers" or "opens momentarily" through the furnace heat cycle, the control board "senses that change in continuity" and shuts down the furnace as a precaution....

In other words, the control board "monitors the operation of the sail switch" throughout the heat cycle and if there is any indication of an open/short in the sail switch circuit that is not expected or that does not follow the exact sequence, the control board shuts down the furnace.

The way the two quotes from you above read, is it possible that your sail switch is not "following the expected "open, closed, remains closed and then open" sequence ??? Remember, the continuity from the control board, to the sail switch and back to the control board includes not only the actual sail switch, but the connectors and wires that attach the sail switch to the control board. Also, to complicate things, the printed circuit/components on the control board may be at fault as well as a "spike/drop in voltage" from the trailer to the control board that allows the fan to drop below a specific speed that would keep the sail switch closed....

My "hunch" (and that's all it is, is a hunch) would be to focus on the sail switch and make sure that its ENTIRE sequence of events follows the protocol. There may be nothing out of sequence, but if there is, that could be the source of your unexpected shutdowns...

Thanks for the info. I have replaced the sail switch twice, but I’m definitely going to look into it some more. I have checked voltages a couple times but not enough to track it down and see if there are any spikes/drops so I will do that as well. I can never catch the switch being stuck closed which also confused me as to why it won’t just turn the fan on when I re try the heat after unexpected stoppages. I have to go out to the furnace and try digging into things and randomly I can get the fan to kick on. I’m going to check voltages and connections a little deeper. Also the if the limit switch does trip open? How/When does it close itself? Thanks!
 
Your furnace is a Dometic Atwood DFMD35 series...

Use this link to the service manual for further trouble shooting..

** First thing I would do is check the alignment of the furnace to the outside wall vent and make sure it is meshed together straight and not cocked on an angle...

You are throwing parts at a 2023 furnace that should not be having those issues...

If the furnace to duct alignment looks good, AND YOU hear the ignitor making a ticking sound when listening outside and smell LP and it lights off but then later quits ( as you stated ) I would pull the burner section out and check the ignitor to flame sensor gap (1/8 inch) as well as signs of crap inside the burner section

https://techsupport.pdxrvwholesale....twood-Mobile_Service-Training-Manual-2015.pdf

I have a different manual in the trailer with much less detail. I thought my model was 2023 Keystone Montana High Country 331LR, Furnace is a dometic DFLD40131 couldn’t find exact model number in the manual I have
 
Thanks for the info. I have replaced the sail switch twice, but I’m definitely going to look into it some more. I have checked voltages a couple times but not enough to track it down and see if there are any spikes/drops so I will do that as well. I can never catch the switch being stuck closed which also confused me as to why it won’t just turn the fan on when I re try the heat after unexpected stoppages. I have to go out to the furnace and try digging into things and randomly I can get the fan to kick on. I’m going to check voltages and connections a little deeper. Also the if the limit switch does trip open? How/When does it close itself? Thanks!

That sounds to me like a poor electrical connection. Perhaps a wire makes contact, later vibration causes it to loose contact, then your "digging into it" moves the wire enough to make contact again.
 
I have a different manual in the trailer with much less detail. I thought my model was 2023 Keystone Montana High Country 331LR, Furnace is a dometic DFLD40131 couldn’t find exact model number in the manual I have

That model you quoted is for a 40,000 BTU furnace, but I have only known Keystone to use the 35,000 BTU even in the High Country. Have you confirmed the actual model of yours from the tag on the unit?
 

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