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Ask Dave: Furnace blower doesn’t work, so where’s the warm air coming from?

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 malfunctioning RV furnace.

Hi Dave,
We fired up our Dometic furnace while out last weekend and, oddly, the blower didn’t come on. The furnace came on because we felt warm air through the lower vents (and heard it), but the big (loud) blower didn’t come on sending turbine-speed warm air through the top vents. When we turned on the AC to troubleshoot, the big blower DID come on, shooting cold air through all the vents. What’s going on? —Jennifer

Hi Jennifer,
This is a very odd situation as the burner for the furnace should never come on without the blower fan running. What is the make, model, and year of your rig and the model of furnace? Typically the blower starts first and cleans out the burner tube as well as lifts the sail switch at the end of the burner assembly. If the sail switch does not lift, it should not open the gas valve and light.

I would suggest looking at the furnace directly as there should be a small site glass. Maybe the heat you are getting in the vents is coming from some other source such as Aqua Hot or in-floor heating or maybe even your water heater close by?

Plus, the roof AC is not connected to anything with the furnace except possibly the thermostat that could control both. The AC vents should be ducted in the ceiling. while the furnace vents are either ducted in the floor or there are long hoses snaking through cabinets and under the sofa.

Also, the roof AC has its own blower fan located in the unit on the roof. If you turned on the roof AC, like you indicated, it would have started that and blown cold air through the ceiling vents.

Sometimes the roof AC has a heat pump

One other issue that has been a source of frustration is when the roof AC has a heat pump. This is confusing as the thermostat on the wall has a button that slides from Cool to Electric Heat. I’ve had this question many times and especially locally during the Winter Dance Party when Winnebago donates units for the bands and they run out of propane and the “Electric Heat” doesn’t work.

The heat pump option basically reverses the operation of the roof AC and draws btu’s or heat from the outside air but is only efficient down to about 45-50 degrees. However, not knowing this is actually a heat pump tied to the roof AC, owners think it’s an electric heater that should work at any temperature when plugged into shoreline power.

Update: I got a response back from the owner when I sent this rough draft to her. She stated that she did not realize the roof AC and the furnace did not use the same blower, like it does in her home. The slight heat felt was in just one area where a floor vent was located, so she was going to do a little more investigation. I’ll keep you posted.

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Bob P
25 days ago

NEWBIE

cee
25 days ago
Reply to  Bob P

We all started somewhere… even you were a NEWBIE once upon a time.

tom
25 days ago

Always a mystery.

Bob Palin
25 days ago

If in fact the furnace is lighting, which does seem very unlikely, I think you need to send a much more powerful message – turn it off and don’t use it again until it has been serviced, this is a very dangerous situation.

The furnace flame usually makes enough noise to know for sure that it is lit and the exhaust outside will be hot even if the fan is not blowing the fumes out.