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 heater fan.
Dear Dave,
I have a 2005 Jayco Jay Feather. The furnace blower runs 24/7 but if I pull the fuse it shuts off the refrigerator, AC and heater. I do not plan on using the heater due to its age and not knowing its history. I am a retired FF0 so know the dangers. How do I shut off the heater fan and leave others running? —Steve
Dear Steve,
First, let’s break down how each of the appliances you have described are powered and what “fuses” might be involved. If the refrigerator is an absorption-type model, it will run on 120-volt AC or LP with both requiring a 12-volt DC house battery power. On the AC side, there is a circuit breaker in the main distribution panel that is dedicated to the refrigerator only. If you shut this circuit breaker off, it will only disable the 120-volt side of the operation.
Typically there will be 12-volt DC fuses in the distribution panel, as well, and there is an automotive-style fuse for the refrigerator. Again, it is only wired to the refrigerator. On the back of the refrigerator in the module board is an inline fuse for both the 120-volt and 12-volt operation.
The roof air conditioner only operates on 120-volt power and has its circuit breaker in the distribution panel dedicated to the roof AC.
And finally, the heater, which typically operates on LP with power coming from the 12-volt house batteries, has an automotive fuse in the distribution panel. Pulling this fuse will stop 12-volt power to the furnace but cannot have any effect on the 120-volt operation of the AC unit. I wonder what fuse you are pulling that seems to shut off all three?
Back to the main issue regarding your heater fan
Let’s look at the main issue: The furnace blower runs all the time. This could either be a setting on your thermostat or it could be a faulty thermostat. Most have a setting for “Fan Mode,” which would run all the time, or heat.
This thermostat controls both the furnace and the roof AC, and some models do have a fuse inside the case. If you pulled this, it would disable the roof AC and the furnace, but would not affect the refrigerator. The “Fan Mode” setting is controlling the fan only when the furnace comes on. Having the fan on all the time moves the air.
If you could provide a few more details on what fuse you are pulling and verify if the refrigerator is actually shutting off or if it might just be getting to the set temperature and shutting off coincidentally at the same time, I could give you a more precise answer.
However, in the meantime, you should be able to pull the 12-volt automotive fuse in the distribution panel just for the furnace.
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Just another item to check – on our Dometic system, if the fan mode is set to anything other than “Auto”, it will stay on all the time with the AC, Heat Pump or Furnace cycling on & off as needed to reach the set temperature. However, if I set the Fan mode to “Auto”, the fan cycles on and off with the AC, HP or Furnace as they come on and go off….
If it’s just the heater fan running without heat, a limit switch may be tripped. Check for a manual reset one or an open fuse link.
Dave says the thermostat may be defective. The fan mode on thermostat only controls the A/C fan, not the furnace blower
The furnace blower is controlled by the module board for the furnace. In heat mode the furnace fan comes on and the module takes over. If the sail switch sees sufficient air flow the module then calls for the module to ignite the burner. It is possible that the fan relay on the board is stuck on or the board itself is defective. Disconnecting the 12 volt supply on the furnace itself would solve the problem
Typical lack of providing detailed circuit diagram to the final user.
As I was told by a customer service representative there isn’t a wiring diagram as each team may wire the units differently, but all will be wired according to the schematic. So basically in order to trace a wire you need a power source to apply to a wire then go check which wire is powered at the other end. If you can’t find where the power is going that means you have found the broken wire… MAYBE! SIL’s stereo speakers don’t work inside, he hooked up a speaker to the harness and it worked so somewhere there is a bad wire to his inside speakers. He said if they want to listen to the stereo they’ll go outside. Lol