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So - our natural gas, forced-air system is about 4 years old. When cranking up the heat in the morning (overnight it may have dropped to 60 inside) if I raise the setting to 70 on occasion we get this strange outcome. The heat will rise about half way - 66 say, then, without the fan shutting off it will begin blowing cold air. This can go on for a while. Not just a cool down phase. If I then turn the thermostat below the current temp - say 65 and wait for the fan to shut off, then immediately turn the thermostat back to 70 - everything then seems to run normally throughout the day. This does not happen consistently, perhaps once or twice a week.
Seems if the thermostat was bad and causing this that the cool down cycle would end normally - but it lingers. Most probably to me is that the furnace burner shuts off unexpectedly, but the thermostat is expecting it to be running. Thus we end up with just the fan running and running. The heat threshold is obviously never met.
Any clues on the source of this kind of problem?
Thanks for any guidance you can offer.
Seems if the thermostat was bad and causing this that the cool down cycle would end normally - but it lingers. Most probably to me is that the furnace burner shuts off unexpectedly, but the thermostat is expecting it to be running. Thus we end up with just the fan running and running. The heat threshold is obviously never met.
Any clues on the source of this kind of problem?
Thanks for any guidance you can offer.
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