Too often, and unpredictably, when the thermostat tells our gas furnace to turn on, all that turns on is the fan that exhausts the moisture from the combustion. It is not followed up with the flow and ignition of the gas.
If I flip off the master switch that controls power to the furnace, and flip it back on after a short time, the fan starts up, and the ignition always occurs.
But if I go to the thermostat and raise the temperature there, by breathing on it, for example, that shuts the furnace off; but when the temperature at the thermostat drops back to the ambient temperature, and the fan starts up again, ignition never occurs.
So it looks like there is a difference in what is shut off by the master switch and by the thermostat, and that the problem has something to do with that.
Any suggestions?