We have a Lux gas furnace that isn't working right. We woke up to a much colder house than we had planned. We've reset the thermostat several times and tried several configurations to no avail. We finally heard the heater kick on when we switched the fan button from "auto" to "on". This made the fan run continuously and the gas kicked in. Any ideas about what the problem is and how to fix it? Thanks.

Sounds like a bad fan relay. ''On'' and A/C use different fan windings controlled by a different relay for a higher speed than heating. It could also be a bad connection or even the fan motor partly bad. The gas will shut off again if the fan doesn't come on soon after it does.

Unfortunately, you may be stuck with a big bill to replace the circuit board.

Yeah, Christmas morning and a cold house. Years ago, the pulley on the fan came loose over night. The furnace was out in the garage. I about froze before I got it fixed.