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Why Does My Gas Furnace Turn Off and On Quickly?

A good residential service technician knows what questions to ask the client so we learned that he, in fact did, placed a wire inside the system allowing it to run. And in doing so, unknowingly jeopardized the safety of the occupants of the premises (his family)

Digging deeper and asking additional questions he also informed us that he replaced this safety several times in the past two weeks before eventually by-passing it with a small piece of wire. (Yes, he's an electrical engineer- those clients are the best). He moved in 2 years ago, keeps the central AC high and in the summer and the furnace low in the winter, changes the filter every 2 months and still have filters from the last owner.

He admitted that he "gave up and decided to call a professional".

Now, let's address the furnace turning off shortly after firing up, in this case 3-4 minutes after initial startup. Inside the furnace there is a high temperature limit safety control. It's mounted near the heat exchanger and if the temperature inside the system gets too hot it cuts off the gas valve while allowing the blower motor to run giving the system air to cool off. If there is a restriction in airflow like a dirty filter, this can cause a system to overheat.

On this service call the filter was clean, but too small to properly filter the incoming return air to the furnace. This gap between the filter and the filter return grill slowly allowed years of dirt and dust to by-pass the filter and slowly accumulate on the central air conditioning evaporator coil until it was amount 1/4" thick. Thus restricting airflow, allowing the system to overheat and caused the high temperature limit to cut off power to the gas valve and allow the system to cool off. At least until a genius engineer customer decided to by-pass this vital safety control limit so the system would run continuously while putting the family at risk.

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