Dometic furnace gremlin

Lone-Star · 06-09-2022, 08:35 AM

Hello all — this is my first post but I’ve been lurking and absorbing info for years from all of the expertise here. I have an odd issue with our Dometic DFMD35111 furnace that I hope someone can shed some light on.

When turned on, the furnace will run the blower as normal, trigger the igniter, and light for 2-3 seconds then go out. Twenty seconds later, it does the same thing — light and burn for 2-3 seconds, then repeat. This will go on as long as I allow it to before turning off. Because it lights, sail switch is good, never hits the high limit for a limit switch issue, and the igniter is obviously working. No blinking light on the control board. The furnace probably as 2-3 hours of run time on it and has always gone through this cycle of “light, go out” once or twice before lighting and staying lit.

I just had the regulator replaced by Jayco under the recall. Thought this might take care of the issue. It didn’t fix the issue but it did change something. I’ve now discovered that if I leave the gas off and wait to here the igniter sparking before gently opening the tank valve, the furnace will light and run as designed until it reaches the desired thermostat setting. However, the next time it tries to kick on, back to the intermittent cycle. If I leave the gas on from the tank, the intermittent cycle is all that it will do.

I’m wondering if this is still a regulator issue or perhaps something in the gas valve of the furnace. Jayco replaced the Winntec 6020 regulator with a Marshall unit.

I’m frustrated as I’ve not yet found anyone at the dealership that can’t say anything other than “sail switch”. I’ve pulled the sail and it is spotless — besides, the unit is getting to the point of ignition which should rule out the sail switch as a problem anyway.

Hoping someone here might be able to share a similar experience.

RoadrunnerII · 06-09-2022, 08:55 AM

You can't tell by looking if a switch has a bad connection. I have would grab the sail switch and try it. Jayco's going to cover under warranty I assume. I know pain in the butt getting to it but...
If the sail has a bad contact inside and makes only intermittently could be your issue
Just my 1/2cent thought
RoadrunnerII

Nature Ist · 06-09-2022, 09:40 AM

I assume you do not have any anomalies with any of the other gas appliances. Have you checked the refrigerator flame when operating on gas, and have you also checked the water heater for any different sound or unusual flame characteristics? Do the flames on the oven and the stove burners all appear "normal" and not pulsing significantly?
You didn't mention if this was a problem from the very beginning (though you did say "always,) but I would assume it wasn't because you would have had the dealer repair it by now. So I assume this developed at some point after it had been working properly when new.
I'm wondering if you are getting too much gas pressure at the burner on your heater, which could cause the flame to be too far from the burner - specifically - too far from the thermocouple sensor. The thermocouple will shut down the gas valve in the heater if it thinks there is no flame to prevent unburned gas from building up. If it doesn't get hot enough, it will shut off the gas.
Your experience with the difference after the recalled regulator was replaced is a clue. I assume you never tried the same technique of turning on the gas slowly only after the heater tried to ignite before the regulator was replaced.
If all of your other gas appliances seem to have even a minor irregular flame, especially if the flame seems large or higher, then you should use a manometer to check that you have 11" of water column on your gas line after the two stage regulator on the tank and before any appliance regulators (such as on the range.) You should probably have a propane service tech do that test for you, as I am not so sure your dealer knows how to do it correctly.
I have a manometer and connect it at the gas line connection at the refrigerator (before the third stage regulator that is part of the fridge system.)
Your slow opening technique certainly points to the tank regulator, rather than a third stage regulator at the heater itself (Which I think is part of the heater's gas valve) but, you could have an intermittent problem with the heater's gas valve.
I recommend you take a look at this MyRVWorks youtube video:

https://www.youtube.com/watch?v=elpPdk_MSbU

You can also search his other videos to see you find an example of your exact experience.

Lone-Star · 06-09-2022, 12:16 PM

Thanks for the response. You are correct in that there are no other anomalies with the gas appliances. All work well and as designed.

We bought the 5th wheel about 90 days after the original purchaser had bought it. Got to love how Jayco warranties magically disappear with ownership changes. I’m not opposed to having it repaired at the dealer — if the dealer is competent and isn’t going to cost me weeks of delays especially this time of year. I’m a DIY guy and I’ve not yet waived the surrender flag on this issue.

You mentioned the flame not hitting the sensor and then the corresponding shut off if that happens. The one thing I have not yet checked is the distance from the igniter/sensor to the burner. It’s possible that could be out of spec. I’ve seen a lot of the RVWorks videos and greatly appreciate the effort and knowledge he puts into those. He mentioned the 1/8” spacing between the burner and sensor — that is next on my troubleshooting efforts list.

I accidentally discovered the “clue” when I had my wife turn on the furnace while I was outside at the furnace itself. I had failed to turn on the gas at the tank and remembered that when I heard the igniter clicking — turned on the gas and the furnace lit and remained lit. I was able to repeat the procedure multiple times all with successful operation. But — with the gas system under pressure, the furnace does the cycling every time.

Marcm157 · 06-09-2022, 12:44 PM

I admittedly did not read the entire post as I am at work on a break. Did you check the flame sensor?

Nature Ist · 06-09-2022, 01:17 PM

Quote:

Originally Posted by Lone-Star

Thanks for the response. You are correct in that there are no other anomalies with the gas appliances. All work well and as designed.

We bought the 5th wheel about 90 days after the original purchaser had bought it. Got to love how Jayco warranties magically disappear with ownership changes. I’m not opposed to having it repaired at the dealer — if the dealer is competent and isn’t going to cost me weeks of delays especially this time of year. I’m a DIY guy and I’ve not yet waived the surrender flag on this issue.

You mentioned the flame not hitting the sensor and then the corresponding shut off if that happens. The one thing I have not yet checked is the distance from the igniter/sensor to the burner. It’s possible that could be out of spec. I’ve seen a lot of the RVWorks videos and greatly appreciate the effort and knowledge he puts into those. He mentioned the 1/8” spacing between the burner and sensor — that is next on my troubleshooting efforts list.

I accidentally discovered the “clue” when I had my wife turn on the furnace while I was outside at the furnace itself. I had failed to turn on the gas at the tank and remembered that when I heard the igniter clicking — turned on the gas and the furnace lit and remained lit. I was able to repeat the procedure multiple times all with successful operation. But — with the gas system under pressure, the furnace does the cycling every time.

It is possible the valve is defective; it could be able to open while under less line pressure, function normally, and then not be able to open again the next time under normal line pressure. I would reach out to Dometic tech service - you could get lucky and get an answer and possibly a replacement part directly from them.

Nature Ist · 06-10-2022, 05:32 PM

Your heater issue got me thinking, especially because I could easily find myself in the same situation some day.

https://www.youtube.com/watch?v=z0mTh-cASUc&t=18s

I had trouble loading this link, if it doesn't take to you a single video, look on YouTube for the following:
Dometic Gas Heater burner issue. DFMD30111. by Brandon Griffin

Lone-Star · 06-11-2022, 07:13 AM

Great video. I would never have expected the burner being twisted due to the gas attachment tightening. I’ll try to give his theory a test this weekend. I did pull the assembly and verify proper distance between the igniter and flame sensor from the burner, so no luck there.

Nature Ist · 06-11-2022, 07:26 AM

Just on a whim: have you tried tapping on the gas valve (gently) while it is about to, or has just gone into shutdown?
I'm still suspicious of the fact that it works if you start with the propane tank valve off! Have you ever tried it with your refer, water heater and stove/oven using gas all at the same time? There has to be a connection with the start-up working with low gas pressure

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