Domestic furnance DEMD30111

[ThisIsMyUserName]

I installed this furnace in an older RV 12 months ago as the original Atwood furnace is no longer in production. This was dometics direct replacement.

The furnace ran great up until a few days ago, and it was only used during the very short Cali winter.

What it’s doing,
Starting up, fan working, it fires, and stays lit from anywhere from 1 second to several minutes doing this several times over. It eventually turns off and then repeats this process. It is giving no indication of fault via the circuit boards lights so I’ve been left to my own devices as to figuring this out.

What I’ve done/checked
-vents are not restricted and have good air flow
-I replaced the regulator even though I didn’t think that was the problem as all other appliances worked great. Strong blue flames.
-The propane tanks are full and new. And I’ve tried several.
-the sail switch was a little dusty, it got cleaned and I tested continuity.
-the burner was cleaned with a torch and a wire brush
- the spacing on the igniter is still at factory spacing and I do not know specifically what it is. I did take it off when cleaning the burner, was very careful with it so as not to bend it, and put it back on.
-power is good on batteries.

TheRV hasn’t moved in 3 years and has been parked in the same spot under a tent carport with sides and 5 foot clearance from wall. I believe the potential for moisture to have damaged circuit board to be very unlikely but always possible.

Any ideas?

Thank you,

-Cold Electrician

 

[ThisIsMyUserName]

Edit.
When it stops producing heat the fan stays on. Sometimes it fires back up within 10 seconds, and sometimes the fan runs solo for a few minutes before it relights. I don’t know if this is helpful information but thought it worth mentioning

 

[ThisIsMyUserName]

And when it does fire, there is not any clicking as if it’s attempting to fire, seems like it fires right away which I think tells me the spacing on the igniter is fine. Correct?

 

[Gary RV_Wizard]

Your description makes me suspect that the heat exchanger Hi-Limit (over temperature) switch is causing temporary shuts downs. The gas is turned off and the fan keeps running until it cools down, then it reignites and continues. Re-ignition is usually instant when the burner is hot like that. Yes, your igniter is working fine.

A Hi-Limit fault is typically caused by poor air flow over the heat exchanger, such that heat builds up faster than it can be shed into the air circulation system. Common causes are a blocked air intake, clogged or collapsed air distribution tubes, or lots of dust bunnies in the air system.

 

[ThisIsMyUserName]

thank you Gary! Last night after posting this, the heater seemed to be working fine, it did not turn off at all and inside rv reached a temp of 90 degrees ( only for testing purposes) I set the thermostat at 65 and I went to bed.
Woke up cold and having the same issues again today.

The vents all have excellent airflow coming from them so I do not suspect an airflow issue. Although I hadn’t thought to check the air intake yet. Both distribution tubes go into the floor and I am not sure on the RV where the intake even is. it just pulls air in from around the furnace itself correct?

Also, sometimes, not all the time, it doesn’t even attempt to fire and just turns off. It seems to be doing that this morning.

Is there a way to bypass the high limit switch? Only to test if that’s what’s faulty of course. I wouldn’t run it at length with out it.

 

[ThisIsMyUserName]

I pulled the high limit switch out and left it connected sitting to the side (see picture) figuring if it’s no where near the ignition then it can’t be over heating, and am still having the same issue. Would this be a good way to diagnose/troubleshoot that?

And shouldn’t the circuit board be giving me an indication of air flow/limit fault?

 

[Kirk]

it just pulls air in from around the furnace itself correct?

In most cases that is true. There should be a vent for air to get inside of the space where the furnace is located and if it is blocked that might cause the problem.

Would this be a good way to diagnose/troubleshoot that?

That does eliminate actual over heating but it doesn't assure that the thermostat is not bad or the connections to it bad. The best way would be to monitor it and check when the heat fails to see if the thermostat is open.

And shouldn’t the circuit board be giving me an indication of air flow/limit fault?

That has not been the case in the models that I have worked on. Is your furnace a DEM or might it be a DFM which I found a service manual for?

 

[ThisIsMyUserName]

Yes it’s a DFMD30111, I see now that I had the model number wrong in the posting title .

There is no such vent near the furnace itself (see picture). The furnace has 3 vents on the back of it.

Is the air intake located on the top of the RV by chance?

 

[Kirk]

The two round openings on the side of the RV, one(the outside opening) is combustion air intake and the other(the center opening) is exhaust of the burned product. The exhaust is passed through the air intake to transfer some of the exhaust heat to the intake air and thus improve the furnace efficiency. Inside there is a completely separate air system that draws air from inside the RV, pushes it through the heat exchanger and then out into the dusts to the interior. There are two separate blower wheels, one for combustion air on one side of the motor and on the other is one for moving air through the RV to supply heat.

 

[ThisIsMyUserName]

I do not have those two round holes like you described. These are the only vents on the furnace and appear to be all output vents. I just disconnected them and attempted to run the furnace and am experiencing the Same issue. This verifies that it’s not a clogged output vent I believe. There doesn’t seem to be any type of intake vent. I am familiar with the two holes that you speak of because I have a newer RV as well and it has those.

 

[Kirk]

I do not have those two round holes like you described.

You most definitely have them in the picture that you posted.

The round center hole is the exhaust. The slots below the words "HOT" are the air intake. You need to look more closely. The picture that you posted now is of the inside where you have 3 heat ducts for the air exiting the heat exchanger. There is an air intake but it doesn't show in that picture.