I have a zone system and the heat request light goes on fine and stays on, so I know it is not the thermostats. The system starts to heat and then before the system reaches the proper temp, the gas shuts off and it just circulates cold air. Some times this takes 1hr some times longer or shorter.
I do not thing it is a dirty evaporator, I did fairly well in the filter department after I replaced the evaporator this summer. It just started acting up in the last 3 weeks.
How often does the ?temp? limit switch fail? Could this be it?