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I have an old Day-and-Night gas furnace heater in my house.

Customer Question
It has been working fine...
I have an old Day-and-Night gas furnace heater in my house. It has been working fine for 20 years. Twice within a week, I heard a loud boom when the furnace was in use. I could not tell whether that came from the above the ceiling of below the floor. I did not smell gas.
I had the gas company come in to check it. They check for leaks in the furnace using gas sensor and soapy water. They also check the area below the floor near the furnace. No leak was found. They also say the heat exchanger chamber was intact and all burner heads were OK.
What could be the problem? What kinds of professional service could help? Thanks
Submitted: 6 years ago.Category: HVAC
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Answered in 5 minutes by:
12/17/2011
HVAC Technician: mranswer, HVAC Mechanic replied 6 years ago
mranswer
mranswer, HVAC Mechanic
Category: HVAC
Satisfied Customers: 6,160
Experience: 20 years experience running my own business
Verified
This is an old unit. It would not be unusual for the gas valve to be sluggish to open or close-either action will cause the noise you heard as gas pools and ignites with a loud boom. I would have this serviced by a heating contractor of your choice as any furnace like this should be inspected and serviced annually. Everything on this is steel and steel does corrode over the years and that can lead to issues as well.
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Customer reply replied 6 years ago
In the furnace, the pilot was always on. If the valve is slow to open, it still should ignite. I do not understand why slow-opening causes pooling of gas, could you explain? thanks
HVAC Technician: mranswer, HVAC Mechanic replied 6 years ago
The flow of gas, if weak, will not reach the pilot as quickly as if the flow was strong, so gas can accumulate then ignite with a boom. The same is true when it shuts down.
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Customer reply replied 6 years ago
You said "the same is true when it shuts down". Do you mean slow shut down will also cause a boom?
I think that if the burner is burning, and the valve is slow to shut down when temperature is reached, then the flame should continue to burn until all gas is burn up. This should not cause a boom because there is no pooling of large amount of gas. Could you explain? Thanks.
HVAC Technician: mranswer, HVAC Mechanic replied 6 years ago
When the flow is not abrubtly stopped you basically have the same situation as when it opens slow. The gas burns off from the unit being on but when a small amount is entering the burners it does not have enough force to exit the the burner and burn where it should- it can then ignite inside the burner. If you look at the burners the flame is above the burner- the gas in the burner cannot catch fire because of its flow rate.
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Customer reply replied 6 years ago
I understand that normally the gas in the burner cannot catch fire because there is no oxygen inside the burner. I understand that if there is a mixture of air and gas inside the burner, then it can ignite with a boom. However, the volume of the burner is small. The boom that I heard was a softer, longer "booooom". It was soft but loud, the house jolted a little. So I guess it was probably not caused by explosion inside the burner, but somewhere else, probably outside the combustion chamber. Is this reasonable? Thanks.
HVAC Technician: mranswer, HVAC Mechanic replied 6 years ago
Any gas is within the combustion area only, so what I originally said is true and a definite possibility regardless of when it happens. I can only tell you what is probably happening here. If the house jolted then you had better have this inspected as I did say as well.
mranswer
mranswer, HVAC Mechanic
Category: HVAC
Satisfied Customers: 6,160
Experience: 20 years experience running my own business
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Customer reply replied 6 years ago
Thank you for your help
HVAC Technician: mranswer, HVAC Mechanic replied 6 years ago
Thanks and good luck here- hope there is no major bad news on this old unit.
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