I just moved into this house and the gas furnace does not always turn on when it should. I am not sure what model the furnace is but there is a Honeywell S8910U Universal Hot Surface Ignition Module manual next to the furnace. The furnace itself says Rheem on it, but I cannot find a model number. It has four wires going into it, R, W, G and C. R, W, and G are connected to the thermostat. I did have the C wire also connected, but the programmable thermostat manual said the C wire was optional so I disconnected it to see if that helped, but the problem remains. Here is the problem.

The thermostat is set at 70 and when the temp drops to 69 I hear a click coming from the furnace, and then a few seconds later the flame starts, then a minute later the fan starts, and then when the temperature hits 70 the furnace stops. When the furnace is not running the resistance between R and W is infinite. Then when the furnace starts the resistance between R and W drops to about 4 or 5 ohms. It remains there until the target temperature is reached and the furnace shuts off. My understanding is that this is normal.

But once every 24 or 48 hours, when the temperature drops it will drop well below 69 and the furnace does not come on, and I do not hear a click. The resistance between R and W is 5 ohms at such times, meaning the thermostat is working and the furnace should be on. But it isn't. If I then flip the circuit breaker open then shut, or if I turn the furnace off at the wall and then back on, or if I move the thermostat from "heat" to "off" and then back to "heat", in all three of those cases the furnace has then started right up like it should.

I had a repairman out to look at it and it was working fine when he was here, and he suggested to me that the problem was probably the batteries in the thermostat. Last month (before I moved in - I am renting from a person who just bought the house), this repairman had put in a new igniter for the new owner. The owner also removed the old thermostat but I do not know why. The current thermostat is a Filtrete 3M-22 that was still in the box when I got here so I hooked it up myself with help from there 1-800 line. It was easy to do.

I don't know what the symptoms were that caused the owner to call the repairman in the first place or to replace the previous thermostat, and the owner is unreachable so I am trying to figure this out on my own for now. Any suggestions are welcome. Thanks