Dometic Furnace

[Tbugaren]

2021 Keystone Montana 376FL. Dometic DFMD35131 Furnace won’t stay lit.

Replaced Sail switch and Limit switch.
No lights on the control board
Furnace doesn’t lock itself out
Propane full
Shore power

Furnace will light for about 5 seconds then turn off. 10-15 seconds later reignites. Process will repeat.

[jsb5717]

Welcome aboard! A couple things to check. There are 4 possibilities.

1. Propane regulator - The furnace has a high demand for the full OEM flow from the regulator. An underperforming regulator can cause your symptoms. If your original regulator is a Fairview this is a good possibility. Replace it with a Marshal Excelsior

2. Rusty diffuser/burner - You’ll need to pull the propane valve/burner assembly to check but if it’s starting to rust over then propane flow is hindered.

3. Propane valve - not as likely but also may be underperforming

4. Control board - also not as likely but possible

You’ve already replaced the limit switch. We know it’s not the sail switch since the furnace is igniting.

[Tbugaren]

Copy, i will check the brand regulator here shortly. I got the furnace to stay lit and i ran it for about 2 hours. Not sure if that changes prognosis. Haven’t tried to start it back up

[Carl n Susan]

I seem to remember members having a problem with the 15 amp self-resetting fuse in the power panel. Change it to a normal 15 amp fuse and the problem goes away.

[Daryles]

While you have the burner out, clean it and the flame sensor/ignighter.
The manual is in the files section.

[Tbugaren]

I’ve got a new propane regulator coming tomorrow, I’m at 10000 foot elevation. I’m sure that plays some effect on the system?

[Bourbon County]

Originally Posted by Tbugaren

I’ve got a new propane regulator coming tomorrow, I’m at 10000 foot elevation. I’m sure that plays some effect on the system?

The elevation standard for rating gas appliances is 2000 feet. At 10,000 feet that will absolutely affect the furnace. Don't try to compensate by adjusting the gas pressure up. At high altitudes, the oxygen content in the air is lower, oxygen is required along with fuel for combustion. Normal derating standards are to derate by 4% for every 1K feet above sea level. At 10K feet that would be a 40% derating if you could even get it to burn. I've seen threads on this forum about folks camping in the Rockies at half that elevation having issues with the furnace.

[jsb5717]

Yes, your elevation could easily be a major contributing factor. Did you check your 15a fuse as Carl suggested? If it's black then it's resettable. They are glitchy. Replace it with a standard blue 15a fuse and see if that helps.

[Tbugaren]

SOLVED! Got a Mamometer tested the oem regulator with fridge, stover burners, water heater and furnace trying to light. 11 water column. Lowered the pressure to 8 water column via regulator and furnace works perfectly. Fired up first try. Elevation is definitely a huge factor. Not enough oxygen up here

[Bourbon County]

Originally Posted by Tbugaren

SOLVED! Got a Mamometer tested the oem regulator with fridge, stover burners, water heater and furnace trying to light. 11 water column. Lowered the pressure to 8 water column via regulator and furnace works perfectly. Fired up first try. Elevation is definitely a huge factor. Not enough oxygen up here

Glad to hear you solved the issue.

Now there's good news and bad news. The good news is that you will be burning less fuel per hour. The bad news is the furnace and water heater will be running much longer cycle times. With the gas pressure that low, your furnace is probably only putting out about 28-30K BTUs now.

The fuel to O2 mix is absolutely critical for a gas burner. The only way to dial it in is to increase the air supply or lower fuel pressure. Other than having some obstruction in the intake grille or air inlet tube that can be cleaned, there's no real good way to force more air into these furnaces. You absolutely did the right thing. The natural tendency is to increase gas pressure which only makes it worse and potentially creates safety issues.

I don't know if you're going to be staying in the Monty when the weather gets much colder or not. If you are, you might start looking at some electric space heaters to augment the furnace.

[Tbugaren]

Would it be better to turn the regulator back up to 11 water columns and try to adjust the gas valve on the furnace itself? Yeah I’m going to be in Leadville until the end of October most likely. Worried about pipes freezing so I want to be able to run the furnace to not freeze up.

[Bourbon County]

I'm not sure I can give you a definitive answer, but maybe some pointers. There are 2 basic types of gas burners; an induced draft like your furnace that uses a fan to force air into the burner, tankless water heaters are induced also. The other type is naturally aspirated where the burner depends on how much air the atmosphere can provide; a tank type water heater and your stove is this type. I'm not familiar with gas fridges, but suspect they are natural also. The burner in your furnace is the largest burner on the rig by >2X unless you have every burner and the oven going on the stove.

The induced draft style burners allow the manufacturers to put a bigger burner in a smaller package since they don't have to devote space for a large air supply vent. They also do a good job of venting the exhaust.

If the stove and water heater are working satisfactorily on 8"WC, I don't see the need to adjust the main back up and the furnace regulator down alone. The water heater will take longer to heat but use less fuel. It would also be a good idea to put a thermometer in the fridge to check it's operation.

I had just assumed you had a 40K BTU furnace, seems like that's a pretty standard size for the 40 foot range of RVs. Turns out your Dometic model is only rated at 35K BTU, so it's now somewhere in the 25K BTU range. My biggest concern would be running out of LP. The furnace is going to be running some long cycle times.