Intermittent Furnace Won't Light

[tmakie]

We have a Grand Design 5th wheel with the Dometic furnace. The symptoms of the problem are as follows:

1) Blower starts, followed by clicking of the igniter and after 3-attempt lighting fails and blower shuts down. I have tried turning the stove on before to remove any air in the line but this does not seem to help. Tanks are full and I have switch between tanks without resolving.
2) I may try to start it 15 minutes later and the furnace starts as expected. I have turned it off and on for a series of 3-5 times WITHOUT failure.
3) A little while later I will attempt to start the furnace and item "1)" repeats where the furnace will not start repeatedly.

I called Grand Design tech support and they said it could be any number of issues and stated I would have to take it to a dealer and they would likely pull the unit out of the RV and bench test it to find out the cause.

I would like to obviously avoid the hassle of scheduling an appt, taking it to a dealer for repair given we use it full-time.

Any suggested trouble-shooting ideas you would suggest prior to the undesirable dealer repair visit?

[tmakie]

Forgot to mention with regards to water heater, i will find at times when showering there is plenty of hot water and then suddenly it will go cold for what seems like a minute before hot water returns. Could the combination of these two suggest a faulty regulator or am I grasping at straws here.

[howson]

tmakie

EDIT: The info below, while accurate regarding a sail switch, is NOT the most likely cause of the failure. See post 6. -Howard

Most likely the sail switch is the issue. This is literally a small switch that, through air movement, tells the control board the fan is working.

The pic below may not be exactly your furnace, but the concept is exactly the same. Find the manual for your specific furnace online to find where it is on your Dometic. Then clean it good with forced air (like you'd do with a computer keyboard) and that may solve the problem,

I doubt the hot water issue is connected to the furnace problem.

[Scott'n'Wendy]

Agree with Howard that it is likely the sail switch preventing ignition from time to time. But...I would check that there are no restrictions in the air flow path before I replaced the switch. Sometimes spiders, wasps or whatever start making a nest and restricts air flow which prevents the sail switch from closing - actually the sail switch doing what it is supposed to do

[Jlawles2]

On the HWH, do you notice the issue when you turn off the water at the head then turn it back on???

As for the Furnace, Howard and Scott's suggestions are where I would start. Check that the sail switch is free and clear of the housing allowing the switch arm to move freely, While in there you may want to unplug and replug the connections on the switch looking for signs of corrosion or anything that may hinder the connection. Finally ensure the switch is properly secured and cannot move which could lead to the arm binding.

[colan]

Doesn’t sound like a sail switch issue to me. Why? The fact the furnace is trying to light (clicking noise) says that the sail switch has closed. Best guess is a flaky control board. Or a mud dauber nest inside the burner, these can cause both intermittent and every time failures.

Furnace troubleshooting process/checklist:

All of this assumes 12v to the furnace and working thermostat.

1. Nothing happens, fan doesn’t run.

a. Dometic only - the furnace breaker switch has tripped. This trips from too much heat in the return air causing higher motor amp draw. Look for holes or unattached ducts/hoses, or blocked return air openings.

a. Suburban - switch on lower left near the control board is off. Unlike a Dometic this is not a breaker type switch (doesn’t turn itself off).

b. Sail switch is closed

c. motor failed or wiring issue.

d. Failed control board.

2. Fan starts and blows for 20 to 30 seconds, then stops. This occurs when the control board doesn’t detect the sail switch closing. Most likely issue is the sail switch didn’t close or limit switch is open. The reason the limit switch can look like a sail switch not closing is the two are wired in series, both have to closed for the board to be happy and attempt ignition.

Very infrequently, low voltage or low fan speed will generate insufficient air flow to close the sail switch.

BTW - Dometic furnaces tend to get unhappiy with only a little build up of dust or lint around the sail switch.

3. Furnace attempts to light three times and shuts down. You may or may not hear any clicking from the igniter, if the furnace blower runs for two to three minutes and shuts down it is safe to assume the board has attempted ignition.

Most likely culprit at this point is the control board.

Other possibilities: Gas valve not opening, check wiring, seldom does a gas valve fail on a Dometic or Suburban furnace.

Igniter/flame detector - almost never, resist the urge to remove the igniter and polish, clean or replace. Dometic igniters almost never fail. Suburban sometimes fail on 15 to 20 year old furnaces, a portion of the igniter wire is within the burner, insulation fails from age and heat.

Mud dauber or other insect has a nest in the burner or igniter area.

[Scott'n'Wendy]

1) Blower starts, followed by clicking of the igniter and after 3-attempt lighting fails and blower shuts down.

Yes, Colan is correct, if you're getting the ignitor click, the sail switch is satisfied. Mud dauber nest is back to sail switch though. I don't agree with the MB just yet, because I've never seen a motherboard fail, then fix itself without the actual culprit being connections, wiring, or other flakey component like a limit switch or whatever.
I'd remove and reconnect the connectors on the sail switch, gas valve, and and MB. Then re-try. Sometimes connections get a little corroded. Also..check the 12vdc at the furnace. maybe it's a little low?

[howson]

tmakie -- yep, colan is correct. Sorry for the misinformation.

[Brent1966]

I had a similar issue. The furnace was working all through the evening but of course it decided to fail in the early morning at freezing. The hint for me was the steam coming from my mouth. I thought maybe low propane as it had happened before and that solved the problem (even though it wasn't completely empty). This time though it still wouldn't start. I could crank it to 80 and nothing. I tried the fan...worked...air con...worked, but no furnace. I checked all the breakers...fine. I then removed the actual fuse...plugged it back in, and away it went. Has been working fine since (2 days now). I'm going to look in the back as suggested for dust, nests etc., but maybe this would be a tip that might work for your situation? Also...any other thoughts on my predicament would be appreciated also. 😀

[Scott'n'Wendy]

I then removed the actual fuse...plugged it back in, and away it went.

Removing and reinstalling connectors has fixed more stuff than parts replacement. All this RV stuff is low voltage so a condition called fretting is very common. The connector shuffle fixes it.

[howson]

All this RV stuff is low voltage so a condition called fretting is very common.

I'd never heard that term before, Scott. So of course I had to look it up.

From https://www.corrosionpedia.com/under...devices/2/6589

Fretting corrosion is a type of motion-induced corrosion that results in a build-up of oxidized wear and debris when metallic surfaces in contact with each other are subjected to small (in the order of a few tens of microns) repetitive motions.

[Scott'n'Wendy]

Thanks for posting up the definition Howard.
It's why, when you remove some connectors, look for corrosion and see nothing obvious, put it back on and your device starts to work, magically, again. (need a microscope to see the fretting) You have scrubbed the fretting corrosion from the contacts. So the average Joe walks away wondering why his thingamajig is working again because he didn't do anything. It's also why I'll never replace a motherboard without first removing and reinstalling the connectors and giving it another try. Replacing the MB can get the same result, but you may end up tossing a good MB in the trash.

[howson]

Thanks for posting up the definition Howard.
It's why, when you remove some connectors, look for corrosion and see nothing obvious, put it back on and your device starts to work, magically, again. (need a microscope to see the fretting) You have scrubbed the fretting corrosion from the contacts. So the average Joe walks away wondering why his thingamajig is working again because he didn't do anything. It's also why I'll never replace a motherboard without first removing and reinstalling the connectors and giving it another try. Replacing the MB can get the same result, but you may end up tossing a good MB in the trash.

Absolutely -- couldn't possibly count the number of times reseating connectors on aircraft avionics cleared up an issue. Explains a LOT!

But I didn't know there was a fancy word for it.

[Brent1966]

I found one more thing that worked for me and has every time since. If I run out of propane the furnace eventually turns off of course. If I don't turn the furnace off manually at the thermostat it won't ignite back on at all. If I turn it off it has come on every time pretty much right away.

[Scott'n'Wendy]

I think that is because when you run yourself out of propane and the thermostat is still calling for heat, the tstat faults and needs to be reset by manually turning off, then back on. howson will be able to confirm or deny my theory.