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Old 04-16-2022, 12:29 PM   #1
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Brand: Thor Motor Coach
Model: Miramar 37.1
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THOR #12698
Coach furnace not working properly

I just bought a 2022 Thor Miramar 37.1. When I turned on the coach furnace, the fan comes on to clear out the ignition chamber, it clicks and ignites the propane and starts blowing hot air through the system. After less than 5 minutes the system shuts down. I thought that the sail switch might have debris blocking it so I removed the outside cover and removed the sail switch. I checked and saw nothing on it but made sure it would move to be sure no debris had gotten stuck in it. I then put it back in place and fired up the furnace again. The fan came on to clear the chamber, the igniter came on and fired up the propane and it started working again. Once again after less than 5 minutes, the system shut back down but did stay on a bit longer than the other times.

My 2 golf cart 6 volt house batteries are up to voltage. I turned on the two gas burners to purge the line of air. I have a full tank of propane. I do have an appointment at the dealership for late May that I can cancel. The other items on my list I am doing myself but I am at a loss on the furnace issue.

Any thoughts or ideas as to where to look next?

Paul

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Old 04-16-2022, 01:15 PM   #2
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Same problem with engine running or connected to shore power? If yes, you need to take it in unless you are familiar with RV furnace repair.
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Old 04-16-2022, 02:15 PM   #3
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I tried it with shore power and engine running and still got the same issue. I figure it was above my skill/knowledge so I made the appointment.


Paul
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Old 04-16-2022, 02:33 PM   #4
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If the sail switch was a problem it never would have lit.

When it shuts down, does the fan keep running? Does the fan shut down after "cool down" or does the furnace fire back up again? If so, it sounds like the furnace overtemp is kicking in, maybe due to a flow blockage.

If everything shuts down how do you get it to fire up again? Maybe a thermostat issue or just a common furnace control board issue which Atwood furnaces are prone to have.
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Old 04-17-2022, 12:15 PM   #5
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The fan shuts down after it cools down. It does not fire back up. I will check the vents for a blockage. I shut everything down at the control panel and then turn it back on after a few minutes. I get the same results each time. I do get air flow out of the vents when it does run for that minute or two.

Paul
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Old 04-17-2022, 12:55 PM   #6
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Originally Posted by paulwadley View Post
The fan shuts down after it cools down. It does not fire back up. I will check the vents for a blockage. I shut everything down at the control panel and then turn it back on after a few minutes. I get the same results each time. I do get air flow out of the vents when it does run for that minute or two.

Paul
Are you saying when you shut the control panel down and then on restart it will go through the same sequence? Then I would think the thermostat signal to the furnace is being aborted in some way.
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Old 04-17-2022, 01:15 PM   #7
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For giggles, I would turn off the propane tank valve, then open it again. I have a thing about valves... I turn them completely open (until it stops), then back it off just a tad. Whether true or not, a plumber once told me that is supposed to help prevent valves from freezing up.

Maybe a tank regulator problem? ...which wouldn't show up on your stove. The furnace is probably pickier about pressure. Does the hot water tank light and stay lit?

Try the furnace with the hot water tank burner going AND a burner on the stove. Does the furnace shut off - but the stove & hot water tank keep working fine? I would crank the thermostat up as high as it will go.

If no joy, then I agree that it's likely electronics - the control board. Does anything look corroded on the board from moisture?
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Old 04-17-2022, 01:40 PM   #8
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Originally Posted by paulwadley View Post
The fan shuts down after it cools down. It does not fire back up. I will check the vents for a blockage. I shut everything down at the control panel and then turn it back on after a few minutes. I get the same results each time. I do get air flow out of the vents when it does run for that minute or two.

Paul
Then not a blockage issue. It's a thermostat or control board issue.
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Old 04-17-2022, 02:13 PM   #9
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Then not a blockage issue. It's a thermostat or control board issue.
Here's a thread about checking the control board.

https://www.thorforums.com/forums/f4...ing-28974.html
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Old 04-18-2022, 12:36 PM   #10
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Thanks for all of your suggestions. I am going to try the control board. I do get hot water from the sinks and showers and the two burner gas range ignites and stays on. It is a brand new coach. I have an endoscope and will look into the ducts even though I am getting air flow through them.

Paul
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Old 04-19-2022, 01:29 AM   #11
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I checked the vents with my endoscope and no obstructions. I turned off my propane and turned it back on. I turned on my two propane burners and they fired right up with a steady blue flame. I left them turned on and started up my generator. My wall panel said I had over 13 volts. I opened up the exterior panel and checked for obstructions and found nothing. I reset the furnace at the switch. I turned on the furnace and it started up as usual and after less than 2 minutes turned off. The control board started a buzzing noise and the red light started flashing on the control board. It continued flashing non stop until I turned the furnace off. It did not give me a code of a certain amount of flashes but continued flashing until I turned it off. I put the exterior panel back on and tried the furnace again with the same results. It is a Dometic DFMD 35141 model furnace. I am thinking it is the control board that has gone bad.

Paul
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Old 04-19-2022, 01:36 AM   #12
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I checked the vents with my endoscope and no obstructions. I turned off my propane and turned it back on. I turned on my two propane burners and they fired right up with a steady blue flame. I left them turned on and started up my generator. My wall panel said I had over 13 volts. I opened up the exterior panel and checked for obstructions and found nothing. I reset the furnace at the switch. I turned on the furnace and it started up as usual and after less than 2 minutes turned off. The control board started a buzzing noise and the red light started flashing on the control board. It continued flashing non stop until I turned the furnace off. It did not give me a code of a certain amount of flashes but continued flashing until I turned it off. I put the exterior panel back on and tried the furnace again with the same results. It is a Dometic DFMD 35141 model furnace. I am thinking it is the control board that has gone bad.

Paul
I think I would agree. Unfortunately, a common issue back to the Atwood days. Should be covered by warranty though (this time).
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Old 04-19-2022, 01:55 AM   #13
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I have an appointment on May 23 at the dealership. Lucky it is under my 1 year warranty.


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Old 04-19-2022, 02:52 AM   #14
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Furnace

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Originally Posted by paulwadley View Post
I have an appointment on May 23 at the dealership. Lucky it is under my 1 year warranty.


Paul
Let us know if they find the problem. I’m having the same issues. Runs for 4 minutes then shuts off for 2 minutes then repeats.
Mine is under Warranty too. 4 times in the shop and having a mobile tech in a couple days. Replaced circuit board, limit switch, gas valve, propane regulator, and thermostat. Checked vents and pressures. Called Dometic 3 times. Really frustrated.
Good Luck
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Old 04-19-2022, 11:28 AM   #15
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My Dometic was giving me very similar problems but erratic. Fan on, no spark. Fan on, spark, no flame. Fan on, spark, flame, flame goes out in a few seconds, fan continues to run, then shuts down. Fan, spark, flame, run for 5+ minutes then flame stops, fan continues to run for cool down then stops. All of these symptoms were eliminated when I finally replaced the sail switch. The original switch was clean and tested good on the bench, but apparently was not reliably staying on during an entire cycle. Each time it quit I was getting the “low flow” code on the board. I replaced it with a cheap knock-off purchased at an RV store and have not had a single fault out in many, many cycles since.
On a related note, after replacing the sail switch the flame will still cycle on and off but the fan will run continuously until the thermostat is satisfied. I presume this flame cycling is caused by the high limit switch, but since I already have the recommended number of outlets with the recommended surface area it would require major system modification to increase air flow. Does anyone know if this cycling could be considered normal, or if not, could it be a long term problem?
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Old 04-19-2022, 02:28 PM   #16
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My Dometic was giving me very similar problems but erratic. Fan on, no spark. Fan on, spark, no flame. Fan on, spark, flame, flame goes out in a few seconds, fan continues to run, then shuts down. Fan, spark, flame, run for 5+ minutes then flame stops, fan continues to run for cool down then stops. All of these symptoms were eliminated when I finally replaced the sail switch. The original switch was clean and tested good on the bench, but apparently was not reliably staying on during an entire cycle. Each time it quit I was getting the “low flow” code on the board. I replaced it with a cheap knock-off purchased at an RV store and have not had a single fault out in many, many cycles since.
On a related note, after replacing the sail switch the flame will still cycle on and off but the fan will run continuously until the thermostat is satisfied. I presume this flame cycling is caused by the high limit switch, but since I already have the recommended number of outlets with the recommended surface area it would require major system modification to increase air flow. Does anyone know if this cycling could be considered normal, or if not, could it be a long term problem?
That cycling is normal in our RV.
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Old 04-21-2022, 04:00 PM   #17
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Same 2022 Miramar 37.1 here, and same issue with my furnace.
Im an HVAC technician by trade and I plan to dig into it a little this weekend. My initial thought is too check gas pressure so Ill probably start with that. Low pressure would definitely explain the symptoms.

Ill update when I get more info...
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Old 04-24-2022, 06:42 AM   #18
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On ours the propane pressure was set too high

I had a similar problem on our furnace in our 2018 Four Winds 24F. The furnace would light for perhaps 20 seconds, then go out. A friend who is a technician found that the gas pressure at the main regulator by the gas tank was set too high. He reduced it to the recommended pressure. The problem persisted, so he removed the furnace and ran it on the bench. He found that the regulator in the furnace was also set too high and the huge flame was overheating the sensors. He reduced it and it has run perfectly for a year now.
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Old 04-26-2022, 08:28 PM   #19
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Today I talked to a service tech where I have an appointment to have them fix the furnace and a couple of other issues under warranty. He told me that there are actually two sail switches in my furnace. One on the outside(the one I already removed and checked) and another one on the inside of the furnace where you access it from inside the coach. I was not aware of a 2nd sail switch. When I drop it off(May 23) they will check the furnace out and get it operating normally. I will update with what they find as the issue with it. I am also having them install a safety plus steering stabilizer at the same time.

Paul
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Old 05-02-2022, 02:25 AM   #20
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I’m taking mine in tomorrow for the 6th try. They just can’t figure it out. I have the extended warranty. Looks like it would be cheaper for the insurance company to put in a new furnace. They have replaced 5 parts and still doesn’t work right. Lites for 4 minutes then shuts for 2 minutes and repeats that cycle.
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