Apple #298: Furnace Noises

The house where I live has an older gas furnace. Recently, I've noticed that after it shuts off and the blower stops, something booms. I want to know what this means, and if this might require extensive repairs or if it's maybe a small issue.

Looking for an answer to my particular question, I discovered that furnaces can make lots of noises, depending on what's going on with them.

Note of caution: this list represents best guesses about what's going on. But sometimes furnace problems can be tricky to diagnose. If you have any doubts that what's going on with your furnace doesn't match what's described here, call a repair person to check it out for you.

Similarly, because natural gas is by nature explosive stuff, it's best to call a professional if you're uncertain about how to fix the problem.

That said, here are some noises that forced-air furnaces typically make and what they probably mean.

A boom, sometimes loud enough to shake things a bit, when the furnace ignites.

• This usually means that the burners are dirty.
  

• The burners are the series of round pipes underneath the flames. They have a wide opening at one end.

• Over time, carbon and ashy stuff accumulates on the burners. The gas builds up behind that debris until it can finally push through, and boom! the gas ignites. Sometimes the burners light one after the other in a series: boom, boom, boom, boom.
  

• If you wait to fix this problem, it will get worse until that booming ignition causes a lot of damage.

• Have a professional come in to clean the burners for you. It won't take long, and they shouldn't charge much to do it.

Forced-air furnace. The burners are those three pipes in a row with big openings at the end. Probably your furnace has more than three burners.
(Diagram from Don Vandervort's Home Tips)


Furnace is working away, and a series of booming or popping or rattling sounds occur, sometimes in or around one room.

• The metal ducts that carry the heat to a room or rooms are expanding, and that's what makes the booming or popping noises.

• If it's a rattling noise, it's possible that something in the ductwork is loose.

• If you can, locate the place in the ductwork where the sounds are happening.

• Usually, this spot will be sort of loose compared to the other ductwork.

• You can tighten up the location by putting a small dent in the duct metal. Or you can attach some sort of metal brace.

• If you can't identify the noise to one particular location and the noises seem to travel throughout the system, you can have a contractor put flexible insulation in the ductwork. That will make everything less rattly.

Furnace runs, then the gas flow shuts off but the blower stays on, and a series of clicking sounds occur before the gas reignites.

• The sensor that tells the furnace whether the gas is lit has got build-up on it and is giving the furnace bad information.

• The flame sensor is often a single wire or rod that sticks up in the center of the flame.

• You can shut off the furnace and the gas and clean the sensor with some sandpaper.

Someone has detached the flame sensor and is cleaning it.
(Photo from The Family Handyman, which has a lot of step-by-step maintenance tips)

• If that still doesn't work, the sensor may need to be replaced.

Squealing or grinding

• If the blower is making a squealing sound, the belt is slipping. It's the same thing that happens with cars.

• The belt is probably worn in one spot and needs to be replaced.

• If the belt is too tight, that can wear out the blower motor's bearings, and then you have a really expensive repair on your hands.

Furnace doesn't stay lit

• My furnace used to have a problem where it would turn on, but all that came up through the vents was cold air. I'd go into the basement to check, and none of the burners would be lit.
  

• So I'd have to take the cover off and re-light the thing. It was a pain because I'd have to light a match, press hard on a knob and turn it, and then light the pilot, which would in turn light all the burners. Except it usually took me several tries to get all that to happen right.

• In some cases, the reason the furnace keeps blowing itself out is because the gas valve is bad. This is an expensive box that sits between the gas line and the furnace, and it essentially regulates the flow of gas. If this is the problem, it will be fairly expensive to replace it.

• But the problem might be much simpler and cheaper to fix. Like the burners, the pilot can also get built up with crud.

• If there's a place sticking up where the gas comes out and you can light it the way you light the burners on your stove, that's the pilot. If there's a bunch of gray, ashy stuff all around the base of that thing, that build-up might be the source of your problem.

Somebody is blowing through a straw to get the crud off the un-lit pilot. But there might be too much crud on the pilot for this to do the trick.
(Photo from The Family Handyman, which has a lot of step-by-step maintenance tips)


Alternatively, you might have an electric igniter, which looks sort of like an elongated, flattened paper clip. If yours is as cruddy-looking as this one, it probably needs to be replaced.
(Photo from Arnold's Service Company, which sells ignitors)

• Call a professional to clean or replace the pilot for you. And hopefully, like mine, your furnace stop blowing itself out.

If you want more detailed tips on these and other furnace issues, see this really helpful guide by Mark Bower and Robert Hardy, Gas heater maintenance and troubleshooting, or The Family Handyman's Fall Furnace Tune-Up page.


I couldn't find much of anything helpful about noises after the furnace shuts off. I think I might have to call for a repair person and find out what's up. I'll keep you posted.


Sources
Bill Brainard, AllExperts, "Older Gas Furnace," January 5, 2008
Don Vandervort's Home Tips, Forced-Air Heating and Furnace Repairs
gas furnace ignition delay...boom, DoItYourself.com, December 28, 2003
furnace noise due to, DIYForums.net, September 13, 2006
Rheem gas furnace clicking incessantly, HVACMechanic.com