Thermostat burning out

[ayresdream]

I'm now on my second thermostat. I just replaced the my 30 year old Suburban NT30s with a more modern NT30sp. The furnace came with a thermostat. I hooked everything up, as far as I can tell, according to the directions. When I fired up the thermostat it started to smoke. The insulation burned off the wires. So... I went out today and bought another thermostat. Hooked it up... same problem. It gave quite a light show before it went out.

What am I doing wrong? Should I not be setting the anticipator to Millivolt? (don't laugh at me if I shouldn't... I'm still learning.... and do have a fire extinguisher handy.

[Janets Husband]

Originally Posted by ayresdream

I'm now on my second thermostat. I just replaced the my 30 year old Suburban NT30s with a more modern NT30sp. The furnace came with a thermostat. I hooked everything up, as far as I can tell, according to the directions. When I fired up the thermostat it started to smoke. The insulation burned off the wires. So... I went out today and bought another thermostat. Hooked it up... same problem. It gave quite a light show before it went out.

What am I doing wrong? Should I not be setting the anticipator to Millivolt? (don't laugh at me if I shouldn't... I'm still learning.... and do have a fire extinguisher handy.

With the thermostat unhooked, what is the voltage on the wires?
Check both AC and DC voltages. I suspect that you are having trouble with the control board. Should be 12Vdc with no AC.

[lewster]

Originally Posted by ayresdream

I'm now on my second thermostat. I just replaced the my 30 year old Suburban NT30s with a more modern NT30sp. The furnace came with a thermostat. I hooked everything up, as far as I can tell, according to the directions. When I fired up the thermostat it started to smoke. The insulation burned off the wires. So... I went out today and bought another thermostat. Hooked it up... same problem. It gave quite a light show before it went out.

What am I doing wrong? Should I not be setting the anticipator to Millivolt? (don't laugh at me if I shouldn't... I'm still learning.... and do have a fire extinguisher handy.

Ayre,

I remember seeing a section on anticipator settings when I was getting certified as an RV tech, but I haven't adjusted one since then. PM me if you want me to look up the settings

The thermostat basically works as an on/off switch for the furnace. As mentioned above, check the input voltage at the furnace's PC board. It should be 12.5-13.2 volts DC! There is no AC used in this type RV furnace. Improper voltace might be causing the t/stat to fry.

[ayresdream]

I can't seem to get an accurate reading off of the multimeter.

The furnace has a brand new dinosaur board in it. I isolated the thermostat wires, and also ran brand new ones. There doesn't seem to be any place for the wires to short out.

[Silver Threads]

I recently ran brand new wires to my thermostat and it quit working also. It worked fine before that. If you can figure out whats going on, I'd love to know also. Also, where can I get a new thermostat that just works for heat? I tried Lowes but all they had were heat and air or just air.

[ayresdream]

Something else I noticed that I didn't mention earlier. Whenever I try to start the thermostat now... the lights dim... a lot. And stay dark until I turn the thermostat off.

Signed...

Freezing in New Mexico.

[Excella CM]

You've got a short circuit in the furnace or very close to it. This can be deduced from the fact that both wires are melted from drawing too much current as also evidenced by the diming lights if I have u;nderstood your post correctly. You have a potential fire hazard until you clear this up. A possible culprit is the thermostat relay in the furnace.

[ayresdream]

I'll pull the furnace out and check the relays. I'll also check the nearby wiring and see if anything is out of sort.

[ayresdream]

IT'S FIXED!!! Thank you Excella CM for the advice. First I picked up a less complicated multimeter that I could actually figure out how to work, and checked the thermostat wires. Turns out one of them is grounded somewhere. I ran a new wire, hooked up another thermostat. This time the pyrotechnics were confined to the furnace, where they're supposed to be.)

Thanks all. These forums work!!!