I have a Consolidated Industries USA 054 NH3R furnace from 1997 from when the house was built in 1997. Last week, it started making a boom, boof, combustion sound after the thermostat calls for the furnace to shut off and right after the burners shut off. The blower fan is still running its 180 second cycle when the combustion sound occurs.

When looking through the observation hole, you see the burners shut off and the boom or boof noise happens within a second of them shutting off. There is no combustion noise or boom when turning on the furnace, only when shutting off.

This is not ductwork. It comes from inside the furnace while the blower fan is still running. I have read it might be a bad gas valve, dirty or blocked burners, rust scale or a bad heat exchanger. Any other ideas or experience?

 

This is a common furnace. When consolidated went out of business, many companies starting making this VSA furnace (eg. Texas Furnace, etc.).

Any thoughts on combustion that occurs after the burners shut off?

 

I have heard furnaces do that, but I don't think it's any thing to worry about. Let's see what others think.

 

Thanks Skaggs. Problem is it just started doing this a week ago.

 

Some make a "pop" sound. Noticeable if standing close.

I would think that cleaning the burner may stop it.

 

It's not uncommon in any bar burner appliance to have a "backfire" when the gas valve shuts off. It happens in my brand new range nearly every time the burner shuts off. The principle is the same as when you'd get an old carbureted gasoline engine that would "diesel" (run on) after you shut the key off due to glowing deposits in the cylinder that would be hot enough to ignite vaporized fuel without the ignition spark. So what's happening here is that the gas valve closes near instantly, which quenches the flame on the burner. However there is still residual gas in the piping and burner tube downstream of the valve that continues to expand from the heat and mix with air, then the residual mixture detonates for one last pop due to the burner itself and/or glow bar still being hot.

 

Has someone come out and check out the furnace. They checked/cleaned the burners and put it back together. The furnace is still making the noise. They did not run a combustion test to check oxygen levels for an internal leak.

Again, the thermostat will reach desired temperature. It will call for the burners and gas to shut off. About 3 or 4 seconds later, there will be a boof or boomp sound while the inducer and fan are still running. The only thing that occurs to make this sound is the shutting off of the burners before any fans or inducers shut off.

Repairman heard the noise, but cannot disassemble the entire furnace to inspect. I am at a loss.

 

Did he check the gas pressure ?

 

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