Furnace has a mind of its own!

Took the camper out last weekend for an overnight and furnace worked fine all weekend. I turned the thermostat off before leaving camp. Drive an hour back to town, dump the tanks, drive home, unpack, pull out in the yard and open the fresh water drain and pull the water heater plug. As I'm doing this the furnace kicks on. Run inside, thermostat is off. Tanks are done draining so I pull it over to the parking spot and it's still running.  Pull the thermostat off and replace the batteries, cycle the temp a few times till I hear the click, furnace finally shuts off. For about a minute, then cycles back on again. Thermostat has no control over the furnace. I finally turn off the propane tanks until the furnace blows cool and pull the fuse to shut it off.

I've never heard of such a thing happening. I haven't had time to dig into it any further and won't until this weekend, but has anyone experienced this?

Atwood furnace, I'll have to check on the model this weekend, but it is ducted and pulls 4-5 amps when running. Honeywell programmable digital thermostat, no a/c, heat only.

Sounds like a short between the thermostat and the AC unit.

Pull the shroud where the bundle of wires leads into the AC unit control panel. Pull and replace them one at a time to find out which is the offending wire.

Thanks, but no A/C on this rig. Heat only and it's a 2 wire thermostat. Red and white.

Any chance you could have wet the control wire when you dumped the hot water tank?  A 2 wire thermostat gives either an open (off) or closed (run) connection between the red and white wires.

As the others have indicated, the furnace reacts strictly to the presence of +12v on the line from the thermostat (whether or not it passes through an a/c controller).  Anything that puts +12v on that wire, e.g. a short from another wire, will cause the furnace to start & run.  You can look for the wire at the furnace end and check the voltage there.

There should also be a switch on the furnace itself that you can turn off to prevent it from running. If its an Atwood Hydroflame, remove the outside cover and it should be visible. Not sure how to access it on a Suburban furnace.

A 2 wire thermostat is nothing more then a switch.    So you eithor have a bad thermostat or a short in the thermostat wire.    Just disconnect the thermoset.  If the problem goes away.  That's your corporate.    If the problem prosicts.  Theirs a short in the thermostat wire . but you should disconnect that from the furnace to rule that out to

Alright, did some testing today, the saga continues. Furnace is an Atwood 8520-IV.

I first went in the RV, put the fuse back in, turned the thermostat up to temp and the furnace worked no problem. Did two cycles increasing the temp and it turned on and off as it was supposed to. So, I thought last weekend was a fluke, turned the thermostat off and started working on some other projects. A few minutes later the furnace kicked on again. Thermostat is not calling for heat. Removed thermostat from base, removed thermostat wires from base and made sure they weren't touching. Furnace turned off after a minute or so. Ok, bad thermostat.

A few minutes later, furnace kicks on again with the wires just hanging there. Grab multimeter, continuity. Remove outside cover, disconnect thermostat wires from furnace main harness, continuity between leads from furnace, no continuity in the thermostat wires. Jiggle wires in harness and it intermittently turns on and off, but with no consistency when pulling on single wires. I inspected every inch of wire in there, no breaks in the insulation, or shorts of any kind that I can see.

I removed the circuit board and found that when I move the board it consistently turns the furnace on and off. Do you think there is a weak connection in the circuit board where the wires pin in and is causing a short?  Really at a loss, not sure if I need a new wiring harness or a new circuit board? 

Any suggestions?  Here's a video of how it is reacting with movement of the circuit board.

http://s238.photobucket.com/user/Jeepin4fun/media/E627C002-FB56-4CAA-AD25-ADF4630F1802_zpshurhcdxq.mp4.html

Corrosion on the circuit board connectors? Or simply a bad board?  Intermittently failing furnace circuit boards aren't all that common, but they do happen. And they usually aren't expensive to replace from online sources. Either OEM or an aftermarket brand such as Dinasaur.

Just to update this thread I think I have this nailed. Circuit board appears to be fine, after wedging myself in, around, and under cabinets, stoves, refrigerators, etc I think I finally found the culprit. Where the thermostat wire comes out of the wall behind/adjacent the stove and under the fridge the wire is rubbed through the insulation. The hole is pretty rough, so I'm not sure if it is from rubbing on the hole in the splintery wall or mice trying to climb the wire to get in the hole, but either way, it is located in such a place that I can only get one hand to it, but I separated the wires, applied a liberal amount of brush on liquid electrical tape and then filled the hole with expanding foam to keep mice out if that was the cause.

Hopefully this takes care of it and there isn't issues with the wire portion that is I the wall. The way this is routed and where the thermostat is located there is no way I could re-wire it without opening 2 walls and a ceiling, and that isn't going to happen, so fingers crossed!  As of now, A-OK.

Thanks for all the suggestions!

If that does not fix your problem, maybe you could do a cleansing to chase the bad spirits away.



The information is out there, all you have to do is let it in.

Happy Prospector said:

If that does not fix your problem, maybe you could do a cleansing to chase the bad spirits away.



The information is out there, all you have to do is let it in.

Click to expand...

I already poured some whiskey on it to see if it'd light on fire, didn't work.

I hope that was USED whiskey!