The problem we have usually happens when it gets really cold, about 20 Degrees - We have a Lux electronic thermostat, and the furnace will not kick on when the temperatures goes below the setting. We have to manually do this - then the furnace runs for a little bit, enough to raise the temperature about one degree, then it shuts off and we have to continue to manually re-set the thermostat... so at night, it gets really cold unless we keep manually re-setting it. Also during the day, when we are at work, it gets really cold. We have replaced the thermostat and the same thing happens. We have replaced the wiring from the thermostat to the furnace - as well as adjusting the damper. We were wondering if the having the exhaust go out the side of the house instead of the roof would make a difference. We've had a technician out once, but he couldn't figure it out. The furnace is just a few years old. Thanks for any ideas or suggestions.

Go to the furnace. Connect a wire between the R and W terminals of the thermostat wire. If the furnace then runs until you disconnect the wire, the problem is in the thermostat. The heat anticipator may need adjusted. See the directions that came with the thermostat.

If the furnace does the same thing, check the airflow and temperature sensors. The controls may be getting an over heated signal and shutting it down.

If the thermostat is not your problem like in my case, you need to trouble shoot your furnace. There is a temperature sensor that is typically located next to one of the end burners. This tells the board the burners are successfully lit. over time the temperature sensor can be contaminated with soot and particulates causing as false reading and the board will shut down the burners. The false read can be intermittant (like in my case). The sensor will have porcelain on it and can be removed and cleaned with an abrasive like steel wool. Some systems use the hot surface ingiter as the sensor and replacing the ignitor could fix the problem. If not, the board may be bad.