Help it’s cold

[RandyH]

We have a 2020 3121 with dometic furnace. I turn the heat on and the fan runs the you can hear the burner light. It blows warm air for about 3-4 min then shuts off. It’s about 30 degrees outside. Why is my heat not working?

[AZ Traveler]

Randy,

Sounds like a sail switch. There is a Files section next to User CP in the brown bar above that should have a furnace guide.

I assume you have good gas flow to your stove and reefer. If you have just changed a propane bottle run the stove long enough to clear any air out of the line.

[sourdough]

The sail switch doesn't allow the furnace to ignite;

"The sail switch works by sensing the airflow. If there is an insufficient amount, the switch will prevent the furnace from igniting and the fan will be running but you will get no heat. It typically operates through the use of a paddle which gets displaced due to the force of fluid moving past it."

Have you put any kind of "restrictors" on either the intake or exhaust side of the furnace...including insect screens? Does your control board give you any indication of the problem? Once heating the high limit switch can cut the furnace off if it gets too hot but not generally in 3 minutes. Does it continue over and over or eventually quit and you have to reset? I've had the exact problem you describe and it was the control board. Neighbor moved in with a new Cougar last week experiencing the same problem; he described it as short cycling; control board - he kept hitting reset daily until it was replaced.

[Daryles]

If ut runs long enough to light the burner and it blows warm air, I don't think it's a sail switch.
1. Quick test, try swapping the black 15A self resetting fuse with a spare blue 15A fuse.
2. Possible bad circuit board. Jim Vititow documented this. Some bad boards fail in below freezing temperatures. Try heating it with a hair dryer. Once it warms up enough close the cover. The warm air through the furnace should be enough to keep it running until you can get it fixed. I Did that trick one time after watching Jim's video. Control board was replaced under warranty. Never had a problem since.

[WeBeFulltime]

I agree it's probably NOT the sail switch which gets wrongly blamed a lot.I also would start with changing the self resetting 15A fuse to a standard blue 15A.

[RandyH]

So I have figured out that when the furnace kicks off, the relay on the control board is buzzing and I can hit the reset button and wait a couple minutes then turn it back on it the furnace fires up and runs for about 5 minutes then kicks off and the relay begins buzzing again. So I believe the control board is bad. Any other ideas?

[Daryles]

Originally Posted by RandyH

So I have figured out that when the furnace kicks off, the relay on the control board is buzzing and I can hit the reset button and wait a couple minutes then turn it back on it the furnace fires up and runs for about 5 minutes then kicks off and the relay begins buzzing again. So I believe the control board is bad. Any other ideas?

Did you swap fuses?
The black 15A self resetting fuse is notoriously unreliable.

[RandyH]

Thanks for all the advice. Swapping the circuit breaker for a regular fuse seemed to fix it.

[RBul31]

Always check for mud dobbers. They will cliff up your furnace operation and can be a challenge finding everywhere they built a nest. Just when you think you cleaned them up you find more.