We've had our heat on for a couple of weeks now and it has been working fine. However, two days ago we noticed that the forced-air gas furnace does not engage in the morning when we set the programmable thermostat to raise the temperature -- we use the programmable thermostat to turn the heat down overnight.
Investigating the situation, we find that the pilot light is on and appears normal. Setting the thermostat to engage the system results in no noticeable behavior (completely silent). We also tried shorting the two thermostat wires (RH and W) which had no effect. We were able to restart the furnace by turning the pilot light off (disabling the gas from the line, then turning the furnace gas switch to off) and then relighting the pilot (enabling the gas from the light, turn the furnace gas switch to pilot, light and hold down for a minute, then turn the switch to "full-on"). After this "pilot light reset" the furnance will engage under the thermostat control for the rest of the day (on/off several times). However, the next morning we had the same problem.
Any ideas of what could be wrong? The furnace looks to be about 20 years old. We just moved in so this is our first winter with this furnace.
Thanks for any advice.