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TimVZ26, HVAC Technician
Category: HVAC
Satisfied Customers: 157
Experience:  6 Years exp. Nate Cert. in Air-to-Air Heat Pumps, Lots of Bryant/Carrier exp.
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My hot air furnace duct is making banging sounds prior to ignition.

Customer Question

My hot air furnace duct work is making banging sounds just prior to ignition, what is causing this?
Submitted: 7 years ago.
Category: HVAC
Expert:  TimVZ26 replied 7 years ago.
Are the burners lit before the noise happens or is it banging as the burners lite?
Customer: replied 7 years ago.
I'm not 100% sure, but it seems like the banging sound happens a bit before the actual ignition. It seems to move around. The furnace has recently been cleaned and the ignition tranformer was replaced because it wasn't igniting after it got warm. After this was done the furnace ran beautifully for about a week and a half, now we are in a cold snap and this noise has begun....
Expert:  TimVZ26 replied 7 years ago.
It sounds to me like your burners are having problems lighting causing your noise issue, I would advise watching the furnace and seeing if it is having ignition problems or call the company and tell them that the problem is still present.
Customer: replied 7 years ago.
There is no problem with igniting, as I already stated the transformer was replaced. the banging sound seems to move around to various places in the hot air duct work. It generally will bang one or two times a few minutes before the furnace starts.
Expert:  TimVZ26 replied 7 years ago.

Ok, is the banging located close to the furnace or is it somewhere else in the house. If it is by the furnace and there is no problem with ignition, the heat exchangers may be banging due to expansion/contraction, however they shouldn't be that loud. Have you watched the furnace and seen the lighting process/banging noise? Or do you just hear it in the house every now and then?

Customer: replied 7 years ago.
It seems to be fairly close to the furnace, I have been thinking that it might be the expansion/contraction thing, but it never happened before. The sound has been happening consistently since the cold snap began.
Expert:  TimVZ26 replied 7 years ago.
It is hard to say, if it happens right after all the burners light and before the blower turns on, I would say it is due to expansion and contraction, sometimes that can indicate crack in the heat exchanger, but not always. Not sure why it is happening just during the cold snap either. What brand of furnace and is it a new or old furnace?
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Customer: replied 7 years ago.
Okay, I went down into my gross and grubby basement and waited for it to start, while I was down there I got the name of the furnace, Carlin. The noise is actually happening just after ignition. The sound seems to be coming close to the ignition sight in the furnace. So I'm guessing it could be the expansion/combustion thing, but I suppose I should call the guy who installed the transformer just in case. Thank you,
Expert:  TimVZ26 replied 7 years ago.
Your Welcome, glad to help