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Ask Dave: Why does the furnace keep blowing the thermostat fuse?

Answers to questions about RV Repair and Maintenance from RV expert Dave Solberg, author of the “RV Handbook” and the managing editor of the RV Repair Club. This column appears Monday through Saturday in the RV Travel and RV Daily Tips newsletters. (Sign up for an email reminder for each new issue if you do not already receive one.) Today Dave discusses a furnace that blows the thermostat fuse.

Dear Dave,
Good morning. I have a 2015 Crossroads Zinger, RB25. The issue I’m having with it is the heater/furnace, Atwood, AFMD 25121_A, 19,000 btu output. The thermostat blows the 2.5-amp fuse when I select heat on the thermostat. When I wrap the 2.5-amp fuse with tin foil, the furnace runs as it should: all safety features, blower, gas for flame, no noise, basically a quiet, smooth operation. With very careful observation of the furnace, it has no damage. At the time of finding this issue, I had a problem with critters/mice, which I finally got rid of. But could the mice have eaten into wiring, thermostat wiring, hence, blowing the thermostat fuse, or causing not a good thermostat connection? —Christos

Dear Christos,
It is obvious there is a high amp draw blowing your 2.5-amp fuse. Wrapping tin foil around the fuse is dangerous – even though we used to do that back in the old days with cars that had wiring issues and we used gum wrappers! I’m not sure gum even has foil wrappers any more.

Verify what size fuse it should be

The first thing I would do is verify what size fuse should be installed, as 2.5 amps seems small. Next, you could use a multimeter to find out what your amp draw actually is. Even if the furnace seems to be working fine, if there is a high amp draw then somewhere you have an issue that could actually start a fire!

If you had mice or other rodents inside the rig, then you possibly have wiring issues as you indicated. They can be very difficult to find – we call them “gremlins.”

I would suggest taking the existing thermostat off the wall and rewiring it direct to the furnace module with a new wire. If it works, then you know it’s the wire somewhere, and I would just run a new wire. If it still blows the fuse, try a new thermostat with dedicated wire.

It may be the furnace wiring

Then, if it still blows the fuse, it’s in the furnace wiring and I would start by running a new dedicated ground wire, then power wire. If it still blows, it’s in the furnace itself and you will need to remove the furnace and bench test all the wiring.

Read more from Dave here

Dave Solberg worked at Winnebago for 15 years developing the dealer training program, as marketing manager, and conducting shows. As the owner of Passport Media Creations, Dave has developed several RV dealer training programs, the RV Safety Training program for The Recreation Vehicle Safety and Education Foundation, and the accredited RV Driving Safety program being conducted at rallies and shows around the country. Dave 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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Thomas D
1 month ago

Dave makes it sound like it’s an easy job to change the thermostat wire. I know in my coach and as an electrician it would be impossible without removing walls. It seems 2.5amp fuse is small, especially for a fan motor and gas valve and fan relay. Check online for your make and model number. 25. Amp sounds better. You know the furnace will kill your battery overnight. Thats a lot of current

Bob p
1 month ago

One thing I would ask are you the original owner if not the fuse may have been replaced by who ever sold it just to make the sale replacing the fuse with whatever size they had. 2.5 amps seems very small for a furnace.

Bill
1 month ago

Another, check the owner’s manual to verify what fuse should be used with the furnace. Then verify what size wire is run to the Furnace.
I did check online, and it states a different fuse size for that unit.
The owner’s manual should have the information.

Safety is the main issue.

I remember the old houses with a single light fixture in the ceiling in each room. If you needed a plug, they would add plug conversions to the light socket.
If the fuse blew, they would put a penny under the fuse in the panel. A Big No. =Fire=

Like Dave indicated, there are many issues that effect mobile units. And a complete check is required.