Furnace on by itself

**Dorwinion** · 07-13-2019, 10:01 PM

Furnace & fan were not working while out of state. Took 303RLS trailer into a RV shop. Technician found the furnace control board was damaged due to moisture (I have no idea how that happened). A week later while traveling set up trailer and when we went into the trailer the furnace was on but no fan. Thermostat Control was off and I cycled through it to try to stop the furnace. Called the RV shop that did the work and they suggested I pull fuse to shut off furnace. Which I did. The technician said it could be bad AC control board which controls the furnace board or a bad Thermostat Control. Currently we are in Wyoming South of Yellowstone. Mornings can be quite chilly even during summer. Any ideas?

**huntr70** · 07-14-2019, 04:17 AM

When you say the fan was not on, do you mean the furnace fan or the a/c fan?

It could just be how you have the fan setting on the thermostat.

**JCR GD** · 07-14-2019, 04:19 AM

Not sure how to fix but the Furnace should not even be able to run unless the furnace fan is going. It requires (or should) the sail switch to be tripped before allowing gas to be introduced.

EDIT: I read as furnace fan but Hunter is on to something if A/C fan is referenced.

**Dorwinion** · 07-16-2019, 09:24 AM

Furnace heater was on but not blowing, fan was not on. I could hear that the heater was on when I walked into the trailer, but the thermostat control was not on. I walked outside the trailer and there was heat coming out of the furnace exhaust.

**Dorwinion** · 07-16-2019, 09:27 AM

Furnace heater was on but not blowing, fan was not on. I could hear that the heater was on when I walked into the trailer, but the thermostat control was not on. I walked outside the trailer and there was heat coming out of the furnace exhaust.

**J&G garage** · 07-16-2019, 12:18 PM

Just trying to cover all the bases here. Are you sure it wasn’t the water heater that was on. There are a couple failsafes in the furnace that would keep the burner off if the fan is not running. Not trying to insult just hit every possibility

**Dorwinion** · 09-20-2019, 08:54 AM

Sorry for the delayed response. While in Wyoming, a RV technician performed work on the furnace problem. He found the furnace metal shield was protruding too far out and contacting the furnace cover preventing a seal to the furnace housing. Water was allowed to seep into the inside of the cover, see photographs. The water oxidized the furnace PC Board which caused a short and turned on the furnace. The RV technician cleaned the oxidation off the furnace board and tested the board with a test instrument to assure the PCB was okay. He cut back at least 3/16” of the metal shield in an attempt to create a proper seal to the housing. The RV technician informed me that the furnace running without the fan running could have destroyed our trailer. This issue is obviously a safety and manufacturing matter which was not caught in quality control when the trailer was being built.