Furnace blower motor failing

[David-and-Cheryl]

The furnace blower on our 2013 Landmark (built in 2012) runs for 10-15 minutes (during which the furnace produces heat), but I can hear it gradually slowing down, and the volume of warm air from the vents decreases. Eventually the blower stops completely, even though the temperature at the thermostat has not reached the set temperature. If I turn the thermostat off and then back on, the furnace starts up again, but the same thing happens.

We've lived in the unit full time for over five years, during which the furnace has seen a lot of use. So I'm guessing that the blower motor is failing and needs to be replaced. But before I order a replacement motor, is there anything else I should check?

12 volt power is fine throughout the coach (we are plugged in to 50 amp shore power and all lights are working normally) and LP gas pressure is also normal at the stove. And as I mentioned, the furnace ignites and runs properly. There are no unusual noises from the blower, other than it slowing down rather than running strongly (and loudly) as it usually does.

 

[wdk450]

This is a 12 volt DC motor, and they are usually a brush commutator type. The brushes could be worn down, or the commutator contacts dirty. I did not see brush holder caps on illustrations of these motors on E-Bay, so I would guess this is all internal. I was going to suggest spraying the brush/commutator area with Caig DeOxIt electrical contact cleaner/restorer, but at the amount of disassembly required to access the motor, and the about $90 cost on E-Bay for the motor, I would just replace it.

BTW, I would look into using small electrical space heaters and electric blankets instead of running the furnace as much as possible. An added benefit is that electricity is included at most RV spaces, where you have to pay separately for propane heating gas.

 

[danemayer]

Make sure nothing is blocking the ducts, or the exterior intake/exhaust, or the air return inside the coach.

 

[David-and-Cheryl]

Make sure nothing is blocking the ducts, or the exterior intake/exhaust, or the air return inside the coach.

Thanks Dan. The interior returns are clear, and nothing blocking the ducts at least that I can see. I'll check the exterior ports.

BTW, I would look into using small electrical space heaters and electric blankets instead of running the furnace as much as possible. An added benefit is that electricity is included at most RV spaces, where you have to pay separately for propane heating gas.

Thanks Bill. Totally agree. We do use our electric fireplace/space heater whenever we're connected to shore power, but sometimes it's not quite enough when it's really cold outside.

 

[David-and-Cheryl]

I took the outside cover off the furnace. It's an SF-42Q. No blockages seen in the exterior intake or exhaust (which are covered by mud dauber screens anyway). The return air grilles are clear.

I did some Googling and couldn't find anyone describing similar symptoms (blower motor runs for a while, gradually slows down, and eventually stops). I found a couple people who said their motor started out slow and gradually increased speed and/or squealed, indicating a bearing problem, but that's the opposite of what I'm seeing. I'm not a pro on DC motors. Does this sound like a motor problem, and if so what could cause it? Just want to be sure before I go spending $$ on a replacement motor.

 

[David-and-Cheryl]

To close the loop on this thread: back in August when the problem first occurred, it mysteriously went away, and the blower was operating normally. We didn't use the furnace much after that though, until the weather turned colder.

It then happened again in November. This time, after slowing down once, the blower finally quit altogether. I ordered a new motor (Suburban part #233043 from Amazon, https://www.amazon.com/dp/B00R6KK8W2), and installed it myself. The volume of air output is much stronger (and louder!) than before, so the old motor had probably been on its way out for some time.

 

[wdk450]

The slowdown after running some maybe sounds like a DC blower motor commutator brush problem, or a motor bearing lubrication problem.