Furnace Issues

[NFranzen]

My camper has a SF-20 furnace in it. It seems it does not recognize the thermostat. The thermostat is only a couple years old and has worked fine, but whenever I power up the camper the furnace comes on and runs even if I have it set to off on the thermostat. When I turn the gas on then it starts to heat even when set to off. At the thermostat there is just a white and red wire, and I can see where those are hooked up to 2 wires. How would I go about troubleshooting this or can anyone point me into a direction to start looking? Any help would be appreciated, we are going out in a couple weeks and I would like to have this working instead of just adding in a on/off kill switch to band aid it, I'd like to fix it right.

Thanks,
Neal

 

[Kirk]

Here is a copy of the SF-20 service manual that you can download. The furnace has 2 blue leads that connect to the thermostat and if they are shorted together it turns the furnace on and if open the furnace stops, or doesn't start. It would help if we knew more about the thermostat that you have. Does it control only the furnace, or does it control both the furnace and the air conditioner? If it is shared by the air conditioner, the furnace leads will go to a control circuit board in the air conditioner and from there the signal is sent either to the furnace or to the air conditioner.

If your thermostat is a simple one for the furnace only I would lift the leads from the thermostat and see if anything changes. You can also check the simple thermostats with an ohm meter. Perhaps some pictures of what you have mght help us to help you.

 

[NFranzen]

Kirk thanks for the response. It is a basic HoneyWell thermostat that only controls the heat. So is there a way I can test by unhooking the leads from the thermostat to the 2 blue wires? As in if I disconnect those wires the furnace should turn off? I have a red and blue wire hooked together that control the power, I have a blue wire hooked to a small white wire for the thermostat and then another blue wire hooked to a small red wire that is for the thermostat also. There is then a white and yellow wire hooked together, but no clue what they do because when I unhooked them to test things the system still ran.

 

[NFranzen]

UPDATE:

Last night I went back out again and played with the small white and red thermostat wires that go to the blue wires. I unhooked them both and hooked them back up again, which I have already done and now this time everything is working perfectly. Blower turned off when I hooked them up. Went inside, turned on heat and set temp and it heated to that temp and then shut off as it should. Waited awhile and as the temp came back down it turned on again, heated up and then went off. Any ideas what the problem may be? Something with the t-stat red and white wires perhaps?

 

[Krazeehorse33]

Dirty connections that you corrected when you took them apart and reconnected? Or was it disconnected long enough that a control module reset (cold boot)?

 

[Kirk]

I have a blue wire hooked to a small white wire for the thermostat and then another blue wire hooked to a small red wire that is for the thermostat also.

The blue wires are from the furnace and it really dosen't matter which wire goes to each of the thermostat wires. If you were to disconnect the blue wires and the put your ohm meter between the red & the white wires you can then test the thermostat to see if the contacts are good. It closes to call for heat and should read nearly 0 ohms then and it should read full scale ohms when the thermostat is not calling for heat.

There is then a white and yellow wire hooked together, but no clue what they do because when I unhooked them to test things the system still ran.

My best guess is that those wires do nothing at all.

Any ideas what the problem may be? Something with the t-stat red and white wires perhaps?

Depending on the thermostat that you have, it have been the contacts but it actually sounds like the wires were somhow shorted together. The contacts of the thermostat close to call for heat and open to stop. They actually interrupt the 12V power to the circuit board of the furnace to turn it off and supply the 12V to call for heat. I would be very sure that the place where the blue wires connect to the red & white wires is well insulated as that seems like a possible place where they were shorted together.

 

[Old_Crow]

UPDATE:

Last night I went back out again and played with the small white and red thermostat wires that go to the blue wires. I unhooked them both and hooked them back up again, which I have already done and now this time everything is working perfectly. Blower turned off when I hooked them up. Went inside, turned on heat and set temp and it heated to that temp and then shut off as it should. Waited awhile and as the temp came back down it turned on again, heated up and then went off. Any ideas what the problem may be? Something with the t-stat red and white wires perhaps?

My first response to any furnace problem is to remove the outside cover and take apart any electrical connection I can find and put it back together. Those blade connectors they use often get corroded and loose connection. You can take a pair of pliers and squeeze the female side a bit to make a tighter connection. If the furnace still malfunctions, then it's time to start testing stuff.