Bryant model 137-394U Periodically (3 times over the last month) the furnace fan will run but the heat does not come on. Switching the furnace off for a few minutes and back on rectifies the problem. The latest episode required turning the furnace off at the furnace rather than the thermostat. Is it possible that the programmable thermostat is malfunctioning? ...or is it the control board? ...or something else.