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Ask Dave: Why does furnace function irregularly but works fine outside of its housing?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses a furnace functioning irregularly except when it is outside of the housing.

Dear Dave,
The Dometic heater goes on and off irregularly while mounted inside the housing under the refrigerator. I pulled it out to examine it and possibly take it in for repair. However, it runs fine outside of the housing when on the floor. I repeated this several times because it makes no sense. —Jim

Dear Jim,
One of two things comes to mind if the furnace works outside the housing, such as bench testing. To track it down, let’s review how the heater operates and some of its safety “sensors.” When the thermostat calls for heat, the blower motor starts up and the fan moves air through the burner chamber to exhaust any residual LP or exhaust vapors outside.

At the same time, it pulls interior air from the return air grate and blows across the burner assembly through the vents inside. The air volume must be high enough to lift the sail switch. That tells the module board that there is enough 12-volt power to provide the appropriate air flow and to open the LP valve and start the unit.

When you took the unit out, did you plug into the existing 12-volt house battery plugs on the unit, or use a separate 12-volt supply? If you used the existing plug, that would eliminate low voltage from the RV’s battery supply. I would suggest looking at all the vents throughout the rig. If you have a rug over any of the vents, or have a hose kinked, it could reduce airflow and cause the switch to drop.

Or it could be a weak switch. You should be able to test this with an ohm meter, if you can get the model and serial number and find a service manual.

It could be the high limit switch on the furnace

The more likely component is the high limit switch on the back of the burner assembly. This is a temperature sensor. As the warm air flows over the switch, it measures the temperature and will shut off if the temperature gets too high. This also can be caused by poor air flow through the vents, as it will build up pressure as well as heat.

High-limit switch

If the unit works outside the enclosure, it could also be there is too much heat building up in the compartment and should have some type of venting. Possibly, the airflow is being restricted, or the high limit switch is weak and shutting off prematurely. You should also be able to check the resistance of this, as well.

Since you have the unit out, try installing it again and not hooking up the vent lines. Let it just blow out freely. If it works fine, it most likely is a restricted hose somewhere. Sometimes it’s as simple as a rug over a couple of floor vents.

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Jesse Crouse
20 days ago

My function in life is to drive the MH and fix whatever and whenever it breaks. Many times it involves moving some decoration the boss has put somewhere inappropriate to proper functioning of essential equipment. I do not point out this point. Saves on difficult discussions that I rarely win.

tom
21 days ago

“if you can get the model and serial number and find a service manual.” covers a lot of the problem areas.

Bob p
21 days ago

Excellent answer when we had our Mountain Aire motorhome my wife had put some throw rugs down for decorations and I didn’t pay any attention until the first cool night in FL where we needed a little heat to take the chill off. She had gotten up before me and tried to turn the furnace on but it wouldn’t lite, coming to me to FIX IT, sleepily I fumbled around doing basic checks before going outside to get my test equipment my toe caught one of the throw rugs she had put out. I removed that and one more covering a floor register in the bathroom, the furnace lit and she got warm. The rugs got repositioned away from heat ducts. She didn’t like where they were relocated but she decided her comfort was more important. Lol