We have a propane fueled top vent furnace that out of the blue yesterday morning started making a quite loud pop every time the burner shuts down. It does this if the burner has only been on for a few seconds or for over an hour. I replaced the gas valve a couple of years ago because the original valve was sticking open on shutdown. Other than the pop, the furnace is operating normally.
We've had Williams and prior to that Coleman wall furnaces in our house for 50 years and this is the first time we've ever had this popping noise. Does anyone have any suggestions of things we could check that might be causing this?
Thanks
That is called "Extinction pop". It happens when unburned propane is ignited in the burner after shutdown. I am not familiar with your furnace, but in direct vent fireplaces it is usually caused by improper air/fuel mix.
"Extinction pop is a little different from flashback, but sometimes the two symptoms occurs on the same gas burner.
* Extinction pop most often occurs on propane appliances. The primary cause is reduced gas velocity. The lower velocity causes a lower flame speed atop the burner allowing the flame to drop back inside the burner as the valve shuts down.The rich fuel then expands back to a combustible mixture and the hot burner reignites than small amount of trapped gas. It is not a safety issue.
In a new appliance this can be caused by too low a gas pressure, or excessive primary air, rectify by correct adjustments. But in an older appliance, it is often due to enlargement of burner ports over time, requiring the burner be replaced.
* Flashback generally occurs on a cold burner, when the gas pressure is out of specification required to obtain the correct velocity to propel the gas upward from the burner at the instant of ignition. Thus the gas/air mix in the burner is ignited and the force blows the incoming gas back to the orifice and the flame burns at the air shutter accompanied by a fluttering undersized flame atop the burner, a yellow flame at the air shutter and a loud fluttering noise. The poor combustion also produces a strong aldehyde odor.
If this occurs often, this is generally solved by bringing the gas pressure and air shutter setting into specifications, it is important to address, because burning at the orifice will fill the burner with soot and can cause safety issues to develop. However, if only an occasional occurrence, there is an issue with cold air pressure I the vent of a heating appliance that occasionally causes extreme pressure on the burner which is overcome as the exhaust draft is established."
