Obascure Dometic/Atwood Furnace Malfunction

[moby_1851]

If you have the time to read through my saga - can you tell me if the "solution" sounds reasonable. I am 2 months into full timing in a new (Airstream stored it for a year for me) 2018 International Sig / 25RB. So far my water heater caught fire and now: Atwood Furnace Problem:
When I set my furnace to a temperature it blower and furnace come on as they should and warm the trailer to my set temp. Up until a week ago, the furnace functioned normally: temp drops below my setting and the furnace kicks on. A week ago I woke up shivering. The temp dropped below my setting of 62 all the way tp 39 and the furnace failed to start. No blower / no gas. Nothing. II found I needed to turn the Dometic wall controller off and on again, the furnace would start (essentially it became a manual process), bring the trailer to my temp setting, but then fail to start when the temp dropped below the setting. Again, it required a manual restart. I have limited technical experience, so really hoping the Sail Switch is it.

I had the (only) local tech come out. He replaced the Sail Switch with a generic. Failed. He replaced the Dometic wall controller. No dice. He said he'd never encountered the problem.

Well a genius tech at Bay Area Airstream where I purchased the trailer was the only person I encountered who had dealt with the problem. Apparently my Atwood model has an issue that when the trailer is unlevel (I was on a single layer of stackers on the furnace side which I thought had leveled me - but perhaps they did the opposite) the factory Sail Switch can become stuck in the closed position and lock the furnace. Which may explain why it was intermittent. I removed the stackers and got 48 hours of normal furnace function - then the problem recurred. Again, I turned the thermostat off and on and the furnace started, warmed to the desired temp and then let it drop below my setting again. I repeated the process and now have 24 hours of normal operations.

The same genius tech pointed out the a replacement Sail Switch - part 33082 - comes with a bracket that must be installed with the switch. See photo. My guess is this will prevent shifting/sticking when unlevel. It took me several hundred dollars in tech visits to figure out I needed a $25 switch...assuming this is the answer. Anyone experienced this?

[chugchug]

I have somewhat similar problem. In my case the furnace will not start unless I completely turn off the main power switch in the trailer. When I turn it back on, the furnace sometimes starts working for a while, until it stops.

My diagnostic is that the trigger voltage coming from the main controller board in the AC to the furnace is very low, about 100mv. I don't know if it is correct.

My short term solution is a jumper wire connecting 12v to the trigger wire that goes to the sail switch. When I want to get heat and furnace doesn't start, I press the jumper wire 'on' which starts the blower motor, I keep the jumper 'on' until I hear the flame in the furnace comes on. Than I can turn the jumper 'off'. The furnace will keep working on it own for a while, coming on and off a few times as it should, than it stops. The jumper switch is the only way to get it starting again.

So far I replaced the sail switch twice and the controller board in the AC once. I am going to try the controller board in the furnace itself, but I have little hope of it making a difference.

Any ideas will be greatly appreciated.

As a last resort, I am thinking of putting a simple house to thermostat just for the furnace. It will be ugly, but better than waking up freezing in the middle of the night.

[moby_1851]

@Chugchug. Very much appreciate your reply and emphasize with your situation. My problem started when I was at the end of an ill advised experiment to see how long I could run off my batteries - lithium w.solar. I let the batteries run way too low and assume the furnace issue was voltage related. However It continued after I was back on 50AMPs. As I said the local tech replaced the sail switch and thermostat (and has since abandoned me). I ordered and installed the new sail switch referenced above and, although the switch and bracket I removed was clean and new and I had NO expectations that replacing it with another new switch would do a damn thing - but I followed the AS tech's guidance to replace both the switch and the bracket - the furnace has functioned normal for about 5 days now. I hesitate to even write this because I expect it to crap out as a result. At about a week I might call it good. Honestly I suspected my problem was similar to yours and frankly still might. The local tech did replace my new dometic thermostat with another new dometic thermostat which did nothing...honestly it pisses me off that you and I have to deal with this - at least on a new unit. Let me know if you find a solution. At the moment I consider my situation to be luck that will run out.

[Tim A.]

Sticky sail switches seem to remain a problem in some furnaces. We bought our 2002 new and had the sail switch fail about three times. I finally found a way I could bang on the side of the furnace at about the location of the switch and get it to unstick. After a couple years, the switch decided to never stick again. I have no idea why. After two failures of OEM control boards for the same furnace, we installed in the furnace a Dinosaur board and have had no subsequent board-related problems.

Since the two of you are in California, I can recommend consulting with Vinnie Lamica at Vinnie's North Bay Airstream repair located in Wilton, CA (just south of Sacramento).

Tim

[GCinSC2]

Might be time to research what the sail switch actually does.

It confirms to the control board that the blower motor is running by the air flowing against the blade and closing the circuit.

When control board gets this ALL IS WELL air flow signal then it goes thru a time delay period and finally gas valve opens.

You don’t want the gas valve open and burner lit w/o airflow. Not a good thing.

Darren at My RV Works website has extensive videos to troubleshoot these furnaces and docs.

[moby_1851]

Appreciate the responses. Yes I have learned much about the sail switch in the last couple of weeks. The trailer is 3 months into it's first use and I am on my third sails switch. Again - the third seems to have resolved the problem or it was mere coincidence. I will check out the instructional site. Unfortunately I am in North Platte, NE - not California - or I would likely have found a place to diagnose rather than having to figure it out myself. I suppose the latter is better in the long run.

[GCinSC2]

Steve C,

I was on my phone and in a bit of a pickle, sorry I didn't add links.

My RV Works playlist for furnace videos https://www.youtube.com/playlist?lis...0utYK7sVOKwXVY

Service and parts manuals https://myrvworks.com/manuals/

Of course, other sources like Dometic may also have resources.

Gary

[moby_1851]

Awesome. Thanks!