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Old 11-28-2022, 05:57 PM   #1
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Dometic RV heater problem

Before I left to come home from Nebraska, The Dometic heater in my 2020 5th wheel started acting up. It would ignite louder than normal and the exhaust smoke was white and smelled bad, almost like burning rubber and it would not blow warm air only cold air. Today I thought I would go and take a look at it. I crawled through the small space to look at and I didn't see anything abnormal. So I decided to turn it on and see if it was doing the same thing. It did make a louder than normal ignition, Was blowing hot air through the vents, which was great. The exhaust had a very very faint white color and it still smelled bad.

My model # is dfmd35121

Any idea what could be the problem? I would like to get it repaired before my next out of town job.
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Old 11-28-2022, 06:27 PM   #2
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I just had a Dometic furnace fail in our coach. the thing it seems to have in common with yours is noise, ours got quite loud when it failed. We had a mud dauber (wasp) nest in the heat exchanger. It caused the heat exchanger to overheat, crack, and a hole formed in the heat exchanger when cracks came together and allowed a 1/2 inch diameter hole to form. I tried to inspect the heat exchanger twice before I pulled the unit out to remove the covers on the heat exchanger, but I could not see the hole. After removing and disassembling the furnace I was able to see the heat damage and hole. I tried to weld and braze the damage, but it had been too severely overheated and the metal was too thin to weld with the equipment I had available. (I might I add that I cannot weld anymore because of my onboard defibrillator, so I have to depend on other peoples skills in arc welding.) When I tried to braze the heat cracks there were just too many and they just kept running. I ended up having to buy another unit, because the heat exchangers are obsolete. Good news is yours is new enough that heat exchangers should still be available. Bad news is: I think you are going to have to pull the furnace out and disassemble it to inspect the heat exchanger. White smoke usually indicates incomplete combustion, which can be caused by a blockage in the exhaust (heat exchanger, Dometic calls it an element), and the odor can be the nest burning, gaskets burning, and or the coatings on the heat exchanger, burner assembly etc.
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Old 11-28-2022, 06:35 PM   #3
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Welcome to the forum MillwrightD. In regards to the heat exchanger I know an old trick to check for a cracked heat exchanger in a sticks and bricks is to light an oily rag on fire and put it in the burner chamber and check for smoke coming out of the inside vents with the fan running.
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Old 11-28-2022, 06:50 PM   #4
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Ive had several people tell me that I probably have a wasp nest in there and that I need to run it to burn it out.
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Old 11-28-2022, 07:09 PM   #5
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Went out this morning and turned the heater on. It ignited quietly like there was no issues, but when running it smells bad. There is no smoke, just a bad smell. It is heating like it should
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Old 11-29-2022, 02:21 PM   #6
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Took the heater unit out this morning and disassembled it. I didnít see anything that would make me think there was an issue. I took the sail switch out and it looked ok. I did blow out the heater tube and let me tell you there was a lot of black soot come out. Iíll wait until this evening to test it when the cold front comes. But right now itís back in
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Old 11-29-2022, 02:55 PM   #7
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Quote:
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Ive had several people tell me that I probably have a wasp nest in there and that I need to run it to burn it out.

DO NOT RUN IT TO BURN IT OUT!


In our case the wasp nest was made of red clay, and since we have not been anywhere with red clay it was there when we bought the coach. Our running the heater with it in the heat exchanger is what killed the heater. The nest changes the burn pattern and heat distribution in the exchanger, this creates hot spots that overheat to cracking, repeated heating and cooling cycles can allow cracks to join and a hole to form, at that point you will start getting exhaust into the coach, and temps in the exchanger shoot up from the added oxygen in the exchanger further damaging the heat exchanger.
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Old 11-29-2022, 03:00 PM   #8
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Quote:
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Took the heater unit out this morning and disassembled it. I didnít see anything that would make me think there was an issue. I took the sail switch out and it looked ok. I did blow out the heater tube and let me tell you there was a lot of black soot come out. Iíll wait until this evening to test it when the cold front comes. But right now itís back in

We were getting black soot chunks coming out of the exhaust before we started smelling propane exhaust through the ducts.



Wash the heat exchanger out like the service manual states for the earlier models unless someone has a service manual for you heater that says it is not necessary.
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Old 11-29-2022, 06:22 PM   #9
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I used a 24'' long blow gun and inserted all the way to the first curve, There was a lot of black soot that came out. I turned the heater on tonight and it works like it should, No loud ignition but still a little odor

Dont see anything about washing out the heat exchanger

https://www.dometic.com/assets/14/53...1_01_81453.pdf
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Old 11-30-2022, 07:03 PM   #10
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Let the heater run a few hours today and when I went to check on it it smelled horrible inside and the Carbon Monoxide detector was going off.
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Old 11-30-2022, 07:27 PM   #11
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Let the heater run a few hours today and when I went to check on it it smelled horrible inside and the carbon dioxide detecter was going off.

Do you mean Carbon Monoxide? If it was carbon monoxide that will kill you. Unfortunately I think you have confirmed your heat exchanger has broken and must be replaced.


Washing out the heat exchanger is in the 2015 Atwood furnace training manual, pg 36, and 2004 service manual pg21: "COMBUSTION CHAMBER
Check the air intake and flue areas of the furnace for internal obstructions, such as wasp or bird nests. The life of the combustion chamber is a function of the amount of time that the furnace has operated. Therefore, it is essential to inspect the chamber for cracks and holes. Have the chamber replaced if it has any cracks or holes Ė this condition not field repairable. Chamber should be cleaned if obstructions are present, by removing the chamber and flushing the unit out with water."


Good luck Dave
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Old 11-30-2022, 07:38 PM   #12
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Heater has only been used one winter and still looks new on the inside.
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Old 11-30-2022, 11:12 PM   #13
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Heater has only been used one winter and still looks new on the inside.

The age and how it looked even yesterday don't make any difference. The presence of an obstruction in the combustion chamber changes the flame front, burn temperature, and heat distribution in the combustion chamber, This can direct heat and concentrate it in a small area, overheating and hardening the steel, the extreme expansion and contraction causes the steel to crack. Once a crack develops the low pressure behind the flame draws additional air into the combustion process super-heating the flame and this super heated flame can and will burn a hole through the steel. This hole now allows large amounts of carbon monoxide laden exhaust to enter the heating duct-work and living space.
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Old 12-01-2022, 12:03 PM   #14
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Went out this morning and it’s actually the propane detector that was going NOT the Carbon Monoxide
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