Furnace or Thermostat?

[Lmdye]

Had the same issue in our brand new 2018 Whitehawk that we were living in after the Camp Fire. So were a lot of others having the same problem with the Dometic furnaces. The repair guy would take it out get it to run fine on the bench and it would fail again once it was put back in the unit. One dealer had gotten an solution from Dometic, which was to replace the control board in the furnace. So far no more issues since replacing the control board. Unfortunately Dometic will not acknowledge this problem and do a recall. New board cost about $200 from what I remember. Hope this helps.

[jimmoore13]

Originally Posted by RogerR

I think but cannot be 100% that your furnace fan turns on means the thermostat is working. But the culprit here is most likely the safety switch in the furnace that detects that the fan is running properly, vents aren't blocked, etc. This is called the "sail switch". It is located in the furnace and when the fan comes on and is up to speed it is supposed to move the "sail" or wind sensor enough to close a switch and allow the burner to come on. It is a safety precaution to keep the furnace from lighting when there is not enough air flow to prevent it from overheating and causing a fire.
Research "sail switch" along with your model number for location and replacement. If you can locate it and manually push it you may get the furnace to run, which proves that is the problem. Check all your fuses and connections while you are in there and make sure your air inlet and outlet are not blocked or the runs pinched or clogged.

Sail switch failure is not uncommon.

X2. Just did my sail switch AND cycled the power/reset switch on the furnace. I pulled out the sail switch, used canned air to blow it out, and reinstalled. Problem solved.

I'll also ditto the "check your connections" message. My connections were "made," but I touched a crimp connector that ties the thermostat to the furnace wiring, and the connection fell apart! Whoever stripped the thermostat wire got a little too ambitious and left only a couple strands to make the connection. Doh! It's not a really easy job to pull the furnace and check the sail switch, so while you are in there, take a good look around and make sure everything is in good order.

[Koppster]

Originally Posted by jimmoore13

X2. Just did my sail switch AND cycled the power/reset switch on the furnace. I pulled out the sail switch, used canned air to blow it out, and reinstalled. Problem solved.

I'll also ditto the "check your connections" message. My connections were "made," but I touched a crimp connector that ties the thermostat to the furnace wiring, and the connection fell apart! Whoever stripped the thermostat wire got a little too ambitious and left only a couple strands to make the connection. Doh! It's not a really easy job to pull the furnace and check the sail switch, so while you are in there, take a good look around and make sure everything is in good order.

Hope you have better luck than I did. The dealer cleaned my sail switch and then it failed again so I cleaned it. Got about two more nights out of it and it failed again. I called the dealer, they warrantied the switch and let me change it out myself which saved me from having to tow the trailer in.

[TaftCoach]

Originally Posted by RonkoCamper

My RV Tech came by today and took a look at the system. Same as before- thermostat would trigger the furnace to run, fan would spin but not ignite.

Took the furnace out. Checked fit of connectors, inspected circuit borards, looked for dust bunnies, checked the sail switch, etc. Found nothing irregular. He hit the reset button and it fired right up! It works!

He has no idea what he did and felt bad for charging me a service call! Works for me! At least we'll have heat this weekend at Hershey Park.

My tech went ahead and replaced all the connectors to a more robust system after experiencing the same phenomenon. He said eventually the originals would be a problem.

[jimmoore13]

Originally Posted by Koppster

Hope you have better luck than I did. The dealer cleaned my sail switch and then it failed again so I cleaned it. Got about two more nights out of it and it failed again. I called the dealer, they warrantied the switch and let me change it out myself which saved me from having to tow the trailer in.

For brevity, I left out the sail switch test I performed. First, when I cleaned it, I focused on jetting canned air down into the switch to dislodge any grime that might have snuck into the switch components. These are NOT fancy, sealed switches. They are simple and cheap, and they live in an inherently dirty environment. Then I used an "electronics spray cleaner" to wash it. It is just a simple "plunger switch" that is "normally open" [off] when the fan isn't running on the furnace. When you push the plunger [button] the switch "closes" [turns on]. The cleaning efforts gave me a reliable switch.

I used a multi-meter with alligator clips to test the switch function after I cleaned it. With the multi-meter attached in "resistance" [ohms] mode, I tested "continuity." I repeatedly operated the switch to ensure that it was making a good, reliable connection. In my case, I was lucky, and the switch performed flawlessly.

To be honest, the switch seemed fine to begin with, but since I didn't test it prior to cleaning, I don't really know. The real solution may have been to cycle the power/reset switch.

As I understand it, the power/reset switch functions somewhat like the reset switch on a common power strip. If you overload the power strip, you must "cycle" [switch off then on] the switch, and it "resets" the power strip. The "mystery" is why the circulator fan will run, but the rest is shut off. I have not researched that, but many who guided me recommended cycling the power/reset switch, so I did.

So, if you cycle your power/reset switch on the furnace, I recommend that you cycle it about 5 to 10 times. It, too, may be a bit dusty inside, and by cycling it multiple times, you may improve the contacts connection when the switch is closed [on].

This whole process is a bit like "kicking it" or shaking it to make it work. These are not definitive fixes. But if you've ever dealt with a scratchy volume control on an old radio, you know that if you vigorously (even violently ) operate the volume control, it will dramatically improve the contacts on the (likely) rheostat and make the change in volume far less scratchy. That's kind of the principle involved here.

If all else fails, and if you test the switches and discover they are unreliable or failed, you'll need to replace them.

And then there's the matter of all the other things that can go wrong, from the circuit board (which on mine sits out in harm's way), thermostat, and so on. Any one of these can fail. But the smart money is on the sail switch and power/reset switch. Only after checking them would I move on to other possible failures.

Finally, you must pull the furnace (or have an outside access panel - lucky you) to service the sail switch. But make note of the location of the power/reset switch. It's likely that you can reach it and cycle it without removing the furnace. That might come in handy some night at 3 AM in October when your furnace won't start. Just sayin'.