Kaboom in the furnace


Old 01-14-03, 10:27 AM
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Kaboom in the furnace

This past fall I moved into a house with a gas furnace in the attic. I had the gas co. come out and light the pilot light. He said it's an old furnace (about a 1970 model) but was otherwise in o.k. shape. Recently, and intermittently, when the temp drops and the furnace kicks on there is a very loud thump coming from my attic. It sounds like somebody is dropping a 40lb bag of sand on the roof. Obviously for safety reasons we're not using the heater until we can have it checked out. Any thoughts on what this could be? As I said, it's intermittent. It seems to happen when the furnace hasn't been on for a longer period of time.

Thanks for any insights,
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Old 01-14-03, 12:54 PM
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The most common cause of delayed ignition is dirty burners, crossovers, burner qrifices. Removing and thoroughly cleaning the burner assembly should fix it.
Old 01-14-03, 02:03 PM
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bigjohn is right about a dirty burner being a possible cause.

If the repairman checked the furnace and the burners are clean, then a slow opening gas valve could also cause this symptom.

Based on it's age, it could be the likely culprit.
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