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Ask Dave: Why does my RV’s furnace shut down before reaching the set temperature?

Dear Dave,
My furnace is not working properly. When I turn up the thermostat, it will kick on but then turns off before reaching the desired temperature. I’m thinking that the issue is with the thermostat. Is there anything I can do to fix it? Thank you. —Chiara, 2020 Forest River Wildwood 40FDEN

Dear Chiara,
First a question or, rather, clarification. You state “When I turn up the thermostat….” So, does the furnace work as designed when you leave the thermostat at a set temperature? For example, if you have it set for 68 degrees and it works fine, but you turn it up to 74 because it’s a little cold and then it starts and stops?

The reason for this question is, if the furnace works at the set temperature consistently and shuts off when set to a higher temperature, it would probably be the temperature sensor at the back end of the furnace.

How a thermostat and furnace work

I know we have covered this before, so bear with me, my readers! The thermostat is only an open and closed circuit provider. When you select a temperature and the ambient temperature drops below that setting at +/- 2 degrees, the thermostat closes. That sends the 12-volt power to the module board similar to touching two wires together.

The module board starts the blower fan to exhaust any fumes in the chamber outside and also pulls interior air from the coach through the air returns. The air flow going out the interior air chamber to the vents passes over a sail switch that must be lifted to verify there is enough power and airflow to open the gas valve and light.

The module then opens the gas valve, creates a spark, and starts a flame in the burner assembly. The air passes over it and flows to the vents inside the rig. At the end of the burner assembly is a high limit switch. That is a temperature sensor that regulates the temperature. If it gets too high, it will shut off.

If the thermostat is bad, you can verify by pulling off and connecting the wires directly. I’m not sure which model you have in your rig. However, you should have an owner’s manual with a basic wiring diagram. If you connect the wires and it runs, it is the thermostat. If it continues to shut off, it’s most likely the high limit switch.

One more thing to check. Make sure all your vents are open and not covered. Also, if you have the “elephant trunk” hoses, make sure they are not pinched or kinked. If you have a few vents closed, covered, or obstructed, the airflow will be restricted and might create a high temperature back at the high limit switch.

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.

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Chris Crosswhite
8 days ago

This could be a normal operation.

The room thermostat (#1) closes when the room is cooler than desired, telling the furnace that it should circulate heat into the room.

The furnace ignites (noisily). Once a minimum temperature (#2) is reached, heat is circulated through the room by turning on a ‘room’ or circulating fan.

The furnace continues to heat until it reaches a maximum temperature (#3), then shuts down. The ‘room’ fan continues.

A person may hear this as the furnace ‘shutting down’, but heat is still being sent from the furnace to the room.

Chris Crosswhite
8 days ago

— continued —
After a while, the furnace is cooled down below the maximum temperature and re-ignites. The person may think they’re hearing the furnace starting again, but this is a normal operation.

There are two temperature systems – the furnace burner cycles on and off between the minimum and the maximum setting of the furnace thermostat (#3). The ‘room’ fan stays on continuously until the room is heated.

Three thermostats, one system

Brad Teubner
8 days ago

If not on shore power, could also be the battery going down enough to not maintain the sail switch.

Steve
8 days ago

Also, thermostats are usually made to shut off the heat source a degree or two before the fan shuts off so the fan can get all the heat out of the air box and into the room before the fan does shut off. Rv furnaces aren’t usually that sophisticated in their design.