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.
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
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.
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.
Read more from Dave here.
HAVE A QUESTION FOR DAVE?
Ask it here. Please be as brief as possible. Attach a photo or two if it might help Dave with his response.