Oil Burner Rumbles at turn off

I have an HB Smith boiler with a Carlin burner- Over the last couple of weeks, the furnace is making a rumbling sound after the burner turns off. It sounds a lot like the rumbling you would get on startup when the burner does not start right away and there is unburned oil in the burn chamber.

The burner is starting fine just not turning off the way it should. Any ideas of what to look for?

Thanks!

 

I suspect the fuel pressure cut-off valve (internal to the burner fuel pump) is not closing properly allowing an after dribble of fuel while the burner motor is coasting to a stop.

 

Another thing which will cause a rumble on shut down is air in the fuel system. This is most common when two copper tubes of the same size are hooked to the fuel pump.

 

I pulled the burner assembly this weekend and found quite a mess- The flame retention ring was loaded with carbon and there was lots of soot-

I cleaned everything up and replaced the nozzle with the same nozzle type. Put things back together and fired it up-

Looked in the inspection hole and holy smoke, looked like it was burning way to rich- So I adjusted the air band and got a nice looking burn. Not sure why the tech closed down the air band so much when it was serviced in March of 2006.

Anyhow, things seem stable now. It still starts good and I have no rumble at turn off.

I guess I probably need to get someone in to clean the heat exchanger. With how rich it was burning and the amount of soot/carbon on the burner assembly, I can only guess what the heat exchanger looks like.

Thanks for all comments, Jim

 

The combustion should be checked at a minimum by measuring the CO2 level in the flue gases, judging the flame by eye won't cut it. You could be on the wrong side of the curve and generating a lot of carbon monoxide, or the unit could be set too close to be reliably stable. Just my opinion, FWIW.

Pete

 

Another thing to watch for; assuming the burner was set right a few years back, the carbon build up could be caused by poor atomization of the oil at the nozzle.. Could be caused by low pump pressure? Clogged strainer? Just a few guesses. You should have a pro check it before the heating season.