Occasionally my gas, forced air furnace will come on with a very, very loud “bang” that can rattle the walls. It doesn’t do this every time and as a matter of fact I haven’t noticed it happening in the past couple of weeks.
I will be calling a furnace repair place, but before they try to sell me a $500 dupliohectostat (plus labor) to fix the problem, I was hoping people here could give me an idea of what is a reasonable explanation for what might cause something like this. I hate being at the mercy of the repair people.
Anyone have any experience with something like this?
I had a gas forced air furnace that used to do the same thing. A loud metallic bang when it started up. A friend of mine who fixes such things said that it was the metal duct flexing due to the temperature change (either heating up or cooling down). The gas fireplace I have now does the same thing. Given that it seemed benign at the time I never worried about it and sold the house to some unsuspecting soul.
If you are really concerned you should schedule a non-emergency house call by a licensed HVAC guy to take a look to make sure it’s not something more sinister. Expect a $75 charge just to come out to your house…
if it doesn’t happen right at the same time as the furnace turns on it could be the metal duct work flexing as mentioned. the duct could be flexing and popping out or expanding while getting caught at a supporting point and then letting go. to test this have the furnace turn on for the number of minutes needed while you are in the basement, you can pinpoint the location easier.
if this is the case you could leave it alone, it gives the house character. or loosen a screw holding the support so tight or place a washer between the support and wood to give it more space so as to not catch.
As others have said it may just be one of the ducts flexing. If that’s the case it’s just a matter of locating it and bracing it so it can’t do it anymore. A heavy strip of metal and a couple of sheet metal screws should do the trick.
If the bang happens when the burners light, that is “delayed combustion” and your furnace likely needs to be cleaned/ serviced.
Price: $70-139
If the bang happens when the main blower starts, that is “oil canning”; the sudden change in pressure in the duct from the blower starting is causing the duct to [slighlty] implode on your cold air return (as it is under negative pressure), or explode in your supply air (as it is under positive pressure)
That can be solved by screwing almost anything to the duct at the point it is oil canning to add rigidity to the duct at that point. (we use metal, but we’re tinners…)
I was trying to do a little google research today and what others have said about the duct work makes sense. At times the “bang” is violent enough to rattle the walls and has woken me up once or twice.
I called a repair place and they are going to send someone out next week. They suggested a cleaning as it could be carbon buildup on the burners (again, that was reasonable based on my google research) and will run $95, so raindog was very accurate with his prediction.
As I said, it hasn’t happened in a while and if it is duct work expansion the warmer weather could explain that, I think. Unfortunately, that also means I won’t be able to judge if it has been fixed or not until at least the fall.
But I’ll at least sleep better knowing my furnace has been looked at and isn’t about to blow up.
If it is the ductwork (which 3 or 4 of us have suggested) it is a function of pressure in the ductwork, not temperature.
So if it the ductwork you won’t have to wait till fall-----the problem will continue now.
If you have it cleaned and serviced now------a good idea if it hasn’t been done in a while-----then mention that someone suggested it might be delayed combustion.
Ductwork generally isnt loud enough ‘to rattle the walls and wake you up…’ and so I’m leaning towards delayed combustion, which is something you need to have looked at right away.
No, but it can be a pretty loud POW. One thing to listen for is that often times (not every time IME) there’s two POWs. One at the beginning and another one when the fan turns back off. I simple experiment you could do it to just go into the basement and find a spot somewhere on the ductwork or plenum where there’s a cross break (a big X folded into the sheet metal) and push on it/hit it so that it flexes in and out and see if that noise it makes is somewhat similar to what you’re hearing. I should add, I’m not saying it’s going to make the exact same sound but it might be similar enough for you to say “Yup, that’s type of sound I’m hearing” or “Nope, that’s not it at all.”
ETA, if it’s what Raindog is saying about delayed combustion (and I’m understanding it correctly that it happens when the gas comes on) you’ll hear it before the main blower starts up…but after the inducer motor starts. So I guess that’s something to listen for, do you hear it about 15-30 seconds before air starts coming up through the vents or closer to 1-2 seconds before or after you feel the air?
On my oil burner, there is a flap that allows basement air into the exhaust flue. My BIL (the HVAC) guy, says it has something to do with the draft into the chimney needing to be equalized/adjusted/regulate or some such.
If the weight on it isn’t adjusted properly, when it opens it “opens to the stops” and can cause a banging. Could this be it? Do gas systems have this? If not, ignore this post.
