Thermostat and Furnace Question

[Stonew98]

I've got an 85 Sovereign with the original heater.

Here's the issue...I'm on my third thermostat. When I turn on the heat, it blows hot for a minute then goes cold and won't come back on. It did this before (twice), I replaced the thermostat, all worked fine for awhile, then it started doing this again so I replaced the thermostat, it worked fine for awhile...rinse, wash, repeat.

THis morning I took the thermostat off and just put the two wires together, now it is blowing nice and hot.

Any ideas? Am I just getting bad thermostats? Voltage problem to the thermostat? Problem with the control board on the heater?

Any help would be appreciated...and go ahead and assume operator ignorance on all things electrical and heater related.

THanks!

[dlb435]

The problem is with the furnace, not the thermostat. The thermostat only gives an ON or OFF signal to the furnace. That turns the fan on and the furnace goes into start up mode.
Does your furnace have a pilot light or an igniter?
You probably have an igniter system. If it works sometimes, then you probably have some poor connections in the furnace. Just unplug and re-plug all of the connections in the furnace several times. This will usually clear the problem and get things working again.

[Silver Goose]

By jumping the 2 wires together and the furnace continues to run tells me this is not a furnace problem. The thermostat likely has a device on it often called the anticipator. It is basically an adjustable resistor that warms the thermostat to allow the furnace to shut off prematurely so that the room temperature doesn't overshoot the set point. When the contacts in the thermostat open, the gas valve closes but the furnace blower will continue to run until the heat exchanger cools down, then the blower switches off and the cycle repeats when the thermostat contacts close again. See if you can see the adjustment if you take the cover off the thermostat. If you see it, make a note of where its set and try changing it one way or the other to see if that resolves the problem. Another possibility is another source of heat near the thermostat that causes it to open prematurely: sun shining on it, a lamp below it, an electric heater in the same area.

[idroba]

I am with dlb345 on this one. All you have done by connecting the thermostat wires together is to switch the furnace on again and it will work for a while, like it did with the new thermostat, then fail again.

You most likely have bad connections in the furnace, a overtemp switch going out, a failed circuit board or something of the kind.

The furnace is 29 years old too. Nothing lasts forever and for safety reasons alone you might consider a full replacement.

[Stonew98]

I'll try the trick of disconnecting and reconnecting everything and see if that works. I'll also play around with the adjustor on the thermostat.

FWIW, with the wires connected (without the thermostat) it ran like a champ for 3 hours this morning, before I did that it would cut off...so not sure there's anything wrong with the heater...unless its something on the control board that talks to the thermostat, and is not working right once the thermostat tells it to shut down (e.g. it never gets the message or can't process the message to restart the fire).

Maybe I'll just hot-wire it when its cold, then use the fantastic fan above my bed to keep me from getting fried ;-)

[dlb435]

If the furnace continues to run after the thermostat has told it to shut off it is because of the fire box temp switch. This lets the furnace know it is still hot and will continue to run, it will also stop the furnace from re-starting. (it thinks it's still hot)
You probably do not have a 29 year old furnace. It was replaced at some time in the past. The 29 year old heaters were mostly pilot light models. The thermo-switches do wear out and need to be replaced about every 5 years or so. There are usually two of them:
The fire box heat switch and a heater over-temp switch.
The fire box heat switch keeps the fan running until the box has cooled down.
The over-temp switch locks out the gas valve and will not allow the furnace to heat. It will allow the fan to run.
Unless you are completely familiar with how a furnace works, you should get a professional to look at it. Older furnaces can burn through the fire box and introduce CO2 into the cabin.
Put plainly, it can kill you!
Stop fooling around. Either don't use the furnace or get it fixed by someone that knows how. Please don't just jump it out and take the risk.