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watertender3c
Forced hot air problem?

My forced hot air furnace completes a heat cycle, shuts down, and a few minutes later fan comes on for about 30 seconds and then shuts down. When the ambient temperature is about 30 degrees the furnace will run for ten minutes, shut down for twenty minutes and then repeat cycle. Is this a normal cycle? I am concerned about the short fan run after the shut down stressing the system.
Thank you

keith3267
Re: Forced hot air problem?

The short fan run is due to a buildup of heat in the heat exchanger. It indicates that there could be problem with the burner such as a build up of carbon. It could also be due to someone attempting to increase the efficiency by adjusting the shutdown temperature.

When your furnace cycles on, first the combustion fan comes on to put fresh air into the combustion chamber, then the burner comes on. When the heat exchanger has heated up enough, the blower comes on to circulate air through out the house.

When the desired temperature has been achieved inside the house, the burner shuts down but the blower stays on until the combustion chamber gets down to a preset temperature. When the blower comes on for a short cycle, it means that the combustion chamber has some residual heat or the off temperature has been set too low.

watertender3c
Re: Forced hot air problem?

Thanks for your response. Recently have added air conditioning to a existing furnace. The air conditioning does not cool the house properly. The heating has been affected. The AC coil opening is 188 square inches and the furnace duct opening is 285 square inches. The furnace in the heating mode recycles often and after a heat run will shut down and then after 2 minutes just the blower comes on and runs for a minute or so. My heat costs are up about ten percent and I am concerned that the frequent recycling will damage the furnace and also that it may cause a fire.

keith3267
Re: Forced hot air problem?

***, talk about adding some critical information. It sounds like you have a mismatch going on. I'm afraid I won't be of much help on this, you will need to get a real HVAC man on site to see what can be done. I would expect that the solution might lie in reducing the output of your burners to match the restrictions in the duct size now.

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