Dometic thermostat and control box

[wjoef]

Hello;
I have a 2015 Forest River Forester MBS 2401R. My problem, that I have tried to search for, is that the furnace is not running correctly. That is, I turn on the thermostat and the furnace will fire up, but only runs for about a 5 minutes before shutting down. Using different thermometers I have verified that the temperature in the "house" does not reach that indicated on the thermostat. The furnace does not turn on again unless I switch the thermostat off and on. And this only works once, unless I leave it for 20-30 minutes.

Suburban SF-30FQ furnace, Dometic Duotherm AC (Prod #936000107), Dometic thermostat (3313195.012) with control box (3313191.000). Everything appears to be wired correctly at the control board and thermostat (wires are labeled at the control box).

I have verified that the sail switch and hi-limit switch are not malfunctioning, (bypassed each individually and simultaneously) and the furnace still shuts down early. Also connected the two blue wires as I've seen recommended by others, and the furnace still shuts down early.

So... How can I tell whether its the thermostat or control board?

[wjoef]

So I found out what the problem was...

So being a new RV it has an "excess flow valve" as part of the propane regulator assembly. Its part of the POL connection between the tank and the regulator, a check valve in the POL connection. Anyway, I took the POL from my turkey fryer propane burner and exchanged it with the one on the RV.

Voila, the furnace doesn't go out after firing for two minutes, it actually heats up the RV. Not only that, the water heater works on propane now and I can have them both going at the same time.

Now I understand the concept of the excess flow valve, it limits the flow of the propane in case of a leak. However, how does the inanimate object tell the difference between the stove being used and the increased flow required to fire up the furnace at the same time? Let alone if the water heater needs to fire up also?

So, yes, I removed a "safety" device. A safety device that was not working. It was preventing my furnace from working properly. It would engage when the only thing occurring was the furnace heating up the RV. The same with the water heater. I guess the safety device decided that the hot water and warmth from the furnace was bad for me.

[dougrainer]

A quick way to determine LP regulator operation-----Turn on the Water heater then turn on all the stove burners. A GOOD regulator will keep the Burner flames at the correct full flame. A BAD regulator will show the flames to pulsate or diminish or go out completely. From the rest of the comments, the LP regulator needs replaced. Doug

[DrewE]

BFL13 wrote:
Is this 2015 propane? I don't know if that matters but it might.

Shouldn't make a bit of difference. Propane doesn't go bad.

[BFL13]

Is this 2015 propane? I don't know if that matters but it might.

[DrewE]

Do you have the proper pressure at one connection when another appliance is and has been in use for awhile? Does the stove exhibit the same symptoms when it is on for awhile or when the furnace or water heater are used simultaneously? The stove is not a great tester as it has its own secondary regulator (usually), but of it gives similar trouble it's pretty telling of there being problems with the propane system.

My first guess of a culprit would be the regulator. Debris in the lines would sooner cause clogged orifices than impede flow sufficiently to cause trouble. Air in the lines I think ought to have been worked out of them by now. I am not an expert on these things, however.

If the RV uses portable cylinders rather than a permanently installed tank, try shutting the valve on the cylinder and reopening it very slowly.

[wjoef]

Took the furnace apart, tested the coils, and the gas valve, and everything works.

This RV started out as my mom's beach house. When it was new, right off the lot from the factory, she had water heater problems. As in, it would throw a FL fault and not work, but the electric heat worked. She had a RV Tech come out and look at it and he charged $400 (?) to change the control board on the water heater with a pronouncement that everything works now. She never used the propane to heat water after that time. Later in the season she assumed that the propane tank was empty and that's why the furnace wasn't working.

So, if you haven't guessed, I have had an AHA moment with this RV. There is something wrong with the propane distribution system. The water heater doesn't work on propane, the furnace doesn't work... What they both do is slowly flame out from propane starvation...

I have 11"wc propane pressure at the supply line to each appliance. Now I'm wondering if the tank was properly purged when it was initially filled.

Any other thoughts about why the gas flow doesn't keep up with demand? Do I just tear it all apart and blow air through the lines to clear any debris?

[BFL13]

It was a Suburban WH.

GenoB

Posted:01/07/20 04:42pm Link | Quote | Print | Notify Moderator

**** Problem Solved ****

I got to thinking about what could be starving the flame of gas and decided to look harder at the gas valve. I used my digital meter to check the resistance on each coil and found one was 45 ohms and the other had over a Meg ohms of resistance. Looking up the specs I found a good coil is between 30 and 50 ohms. Taking two wires from my battery I attached them to each coil separately and found one worked fine, the other would lock in but after about 30 seconds it would release and then go right back on. A thermal short !!! I suspect the coil would heat up and a wire inside would disconnect, opening the coil and letting the valve close. Then right away contact as it cooled off and open the valve again. Over and over again !

Bought 2 new coils at about $25 and guess what ! WORKS GREAT !

Intermittent problems as ALWAYS the hardest to figure out fine. Thanks for all your help on this, I learned a lot ! Geno B-

[wjoef]

BFL13 thank you, that does seem to more closely match what's going on. The furnace does flame on, and burn for a few minutes, sending hot air into the "house" and hot exhaust out the exhaust port. I just checked and the heat at the exhaust does slowly fade as it runs until it shuts down.

I will check the propane pressure and gas valve.

Thanks again.

[BFL13]

In a recent thread it turned out the gas valve was only half working (one out of two coils ISTR) so propane sort of died away and no flame after a few minutes.

Can't remember if that was a furnace, fridge, or WH. The key was the flame lasted a while and did not shut off right away as if the flame sensor didn't work.

[dougrainer]

Since the 2 blue wires jumped and it does the same malfunction, the Tstat is GOOD. There are only 2 parts left. The Hi limit, which you bypassed. Result, The ignition module is bad. This is not a uncommon fix when you have your symptoms. The thing is to do what you did. Verify the other possible components are good. Doug