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Old 10-19-2013, 01:51 PM   #1
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Furnace Duct Noise


I live in a 3 bedroom bungalow with a reasonably new high efficiency FAG furnace. When the heat comes on, which it has started to do now, the ducts make a lot of noise... banging and thumping. I know the cause is expansion and shrinking from the heat but I don't know why my duct work does it and other houses do not. It is a completely unfinished basement so I have access to the ducts. Is there a common way to stop this from happening?

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Old 10-19-2013, 02:57 PM   #2
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It is normal for duct work to expand and contract. A lot of reasons, is due to it is rubbing against the wood framing, or no flex joint between the furnace and the trunks. Try and determine where the expansion and contraction is coming from, and then you can work from there.

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Old 10-19-2013, 03:25 PM   #3
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Furnace Duct Noise


go downstairs and get the wife to turn up the tstat. Then you can find where it is banging. May need to reinforce that spot with some sheet metal. houses expand contract with humidity and seasons change so you cannot compare 2 houses.
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Last edited by yuri; 10-19-2013 at 03:31 PM.
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Old 10-21-2013, 06:25 PM   #4
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Furnace Duct Noise


what do you mean when you say the heat comes on? are you talking about when the burner ignites or are you talking about when the blower kicks on? If it bangs when the burners ignite you might want to have it looked at. If it's when the blower kicks on follow Yuri's advice and wait for system to cool down then have somebody turn up heat and wait while you are standing beside it. I would start with the plenum on the top or side of furnace. sometimes they pop or bang when the blower first kicks on.It might just require some type of stiffener to be attached to the side to take out the flex.
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Old 10-21-2013, 07:57 PM   #5
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It may also be that your duct work is oil canning. This is caused by the furnace being to big for the duct work. Is the new furnace the same BTU size as the old one?
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Old 10-21-2013, 09:23 PM   #6
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I wouldn't think an increase in btu would cause furnace to oil can at start up. Now if the blower is much newer and more power????.Maybe the bang isn't an expansion but just the opposite? were there any modifications made to the cold air return(s) which maybe cause a negative pressure inside the blower compartment/plenum when the blower first starts
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Old 10-22-2013, 04:56 AM   #7
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A newer furnace of the same size may need to move 400 plus more CFM to stay in its safe temp rise range compared to what an olde heavy walled heat exchanger furnace did.
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Old 10-23-2013, 02:23 AM   #8
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Never heard the term oil canning.
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Old 10-23-2013, 04:42 AM   #9
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Quote:
Originally Posted by bobelectric View Post
Never heard the term oil canning.
Do you remember when motor oil came in 1 quart cans. You inserted the pour spout. Then turned it ove to pour into the fill line, and as the oil ran out, with no other way for air to get into can. The sides of the can sucked in, until the oil stopped coming out while a slug of air when up through the spout, causing the side to come back out.
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Old 10-23-2013, 06:36 AM   #10
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Quote:
Originally Posted by beenthere View Post
Do you remember when motor oil came in 1 quart cans. You inserted the pour spout. Then turned it ove to pour into the fill line, and as the oil ran out, with no other way for air to get into can. The sides of the can sucked in, until the oil stopped coming out while a slug of air when up through the spout, causing the side to come back out.
Interesting...I was always told it refered to the type of oil can used to drop oil a motor, etc. You used your thumb to "pump" the bottom of the oil can to flow the oil out the spout. May be a regional thing

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