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Old 02-14-2020, 09:09 AM   #1
Craigcounty
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Dometic DFMD 35121

RV is a 2019 Fuzion 373
I am having issues with a DFMD35121. The blower will kick on and the burner will light, it will then run for a few seconds or sometimes a few minutes and then shuts down. It seems to have low exhaust pressure when running as compared to previous RV's I have owned. I did notice what I think are install issues with mine and am wondering if contributing to the trouble
1. Install manual says to have furnace tight against wall, it is spaced a half inch away.
2. Supposed to have 3 or 4 four inch ducts, keystone installed 1 four inch and 2 two inch.

Could the furnace have restricted airflow and is shutting down?
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Old 02-14-2020, 10:10 AM   #2
GeekSquadOfUn
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I have the same Dometic furnace.

Looks like a sail switch issue. Very common these days, they have slightly altered the design on a new switch.

Essentially it is a switch that closes when the proper airspeed is achieved, when not working properly it shuts the circuit down.

Could be something else, but....
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Old 02-14-2020, 11:15 AM   #3
Aireman
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I agree with Geek. Just had my sail switch changed out after having the same problem as you did. Exact same model too. Took the techs about half an hour to fix. Runs great now.
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Old 02-14-2020, 12:49 PM   #4
chuckster57
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If the sail switch is faulty the furnace wonít light.
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Old 02-14-2020, 01:02 PM   #5
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Iíd have a look see at the various ducts coming off the furnace and insect for kinks, etc. if air flow is impeded too much it will cause the Hi limit switch to open thus killing furnace operation. I doubt sail switch has anything tondo with this issue.
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Old 02-14-2020, 01:06 PM   #6
Craigcounty
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Thanks a lot for the input everyone. I plan on digging into it tomorrow and I will report back what I find.
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Old 02-18-2020, 12:33 PM   #7
Craigcounty
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It was the sail switch. Tonies RV in Salem VA fixed me up in one day! Thanks Tonies!
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Old 02-18-2020, 12:48 PM   #8
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Quote:
Originally Posted by Craigcounty View Post
It was the sail switch. Tonies RV in Salem VA fixed me up in one day! Thanks Tonies!
Good deal! It must have been intermittent.
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Old 02-18-2020, 04:57 PM   #9
Craigcounty
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I spoke to soon. Tonies said it had the steel sail switch so they replaced it with an aluminum one and bench tested it. Said it ran and ran without issue. I brought it home and reinstalled it and it ran for a couple minutes and then died. I restarted it and it died, I did this three or four times and it dies every dang time. The burner is lighting and it heats up before it dies. Tonies said if it dies after reinstallation it might be the thermostat. It has the In Command system with the thermostat built into it, could it be the in command controller? I was trying to avoid taking the whole rv in because we use it a couple weekends a month going racing and i didnt want to be without it for weeks on end.
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Old 02-18-2020, 05:53 PM   #10
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Can you access the limit switch? When they bench tested it, there would be about zero resistance and all the airflow would allow the limit switch to stay cool enough. I wonder if the limit switch isnít overheating and doing whatís it made to do.
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Old 02-18-2020, 06:08 PM   #11
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If it is firing up, heating for a while then shutting down only to reignite then shut down again.....look into the limit switch/airflow. Just went through a similar situation and it was a minor blockage of the exhaust (an insect screen). Your furnace has to have free airflow in and airflow out or the heat inside the furnace builds and activates the limit switch to keep it from getting too hot.
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Old 02-19-2020, 04:41 AM   #12
Craigcounty
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Thanks again for the input, I solved it at 10pm est last night. It was the 15A black fuse! I swapped it with a standard 15A and it fired up and stayed running. The black 15A is supposedly re-setable, must be weak. Kinda feel like a dumba** for not snapping to it earlier.
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