I have and ADP air handler that serves both my hydroair heat and central AC. We always keep the thermostat switch to the "auto" position for the fan. A couple of times this winter we have come down stairs to a cold house and noticed the temperature was lower than the Tstat was set at. So we noticed the thermostat calling for heat but the air handler doesn't turn on. So we switch the fan switch to the "on" position and the fan blows and everything works. Then we switch it back to the "auto" position and it keeps turning on and off as it is supposed to. But then days later the same thing will happen. So we go through the same procedure. Do you think it could be a relay switch in the air handler?