Noisy heating ducts on expansion and contraction


We recently bought a 25 year old house. It uses gas heat. When it got cold and the heat came on I noticed a very loud cracking sound when the heat came on and again when it turned off. This is from the expansion and contracting of the galvanized heating ducts in the basement. The ducts are on metal hangers so they are not touching the floor joists.
I have never encountered this problem in any other house I have lived in. How to best fix this? Someone suggested putting a strip of plastic vapor barrier between the hanger and the duct to allow slippage. Couldn't that be a fire hazard?
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Mike E. Fullerton wrote:

I sure hope the ducts don't get that hot. I don't think that would be my first choice but don't know what I would use.

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Joseph Meehan

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Both of my homes have had this issue. Think of it as a burglar frightening device. Anyone entering through a cellar window (a favorite for intruders) will have trouble differentiating between noisy mechanicals and your feet upstairs as you load the shotgun. Perfect.
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On Fri, 02 Feb 2007 10:55:59 GMT, Mike E. Fullerton

Hi Mike
I think the only fix for it, would be to go to a thicker gauge duct. Thats obviously a pricy solution. Sometimes you can reduce the noise by tightening connection not allowing it to slip more as suggested above.
This is a natural occurance though when a system charges itself and cools down.
REgards Dale
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If you can pinpoint the exact location(s) of the sound, a little graphite might work. Plastic wouldn't be a fire hazard, the ducts are only 130 degrees or so.
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"Mike E. Fullerton" < snipped-for-privacy@spam-killer-remove-techie.com> wrote in
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On Fri, 02 Feb 2007 10:55:59 GMT, Mike E. Fullerton

If the ducts are hot enough to be a fire hazard, then something is seriously wrong with your heating system.
Are these round or rectangular ducts?
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Rectangular.
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On Fri, 02 Feb 2007 19:23:20 GMT, Mike E. Fullerton

sometimes creasing the panels in those in a big X will reduce the booming.
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How do you crease the panels?
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Mike E. Fullerton wrote:

..
That is a professional job. Could that "crackling" sound be described as a booming? If so can you pinpoint the source?
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Joseph Meehan wrote:

I think what you're describing is also known as "oilcanning", when a rectangular panel in the ductwork pops in or out like the bottom of an oiler.
We've got one of those in our office HVAC ductwork which does that when we shut the blower down for the night.
Jeff
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On Fri, 2 Feb 2007 15:30:22 -0500, "Joseph Meehan"

The panels aleady have this X crease in them. I would call the sound more of a cracking/creaking not a "booming". The sound seems to come from the metal duct hangers. In fact it seems to come right where the top of the duct touches the hanger. I put some plastic under the noisiest one and it didn't seem to help much. The hanger is bent in slightly at the top as if the installer put them in too tight against the ducts. I wonder if they were looser if there would not be as much noise. I can't experiment without taking out the ducts though.
This is drving me nuts and we're losing sleep. I wonder if insulating the ducts would dampen the noise.
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Mike E. Fullerton wrote:

Actually removing the hanger, adding some duct insulation and replacing the hanger should help, assuming the problem is what you think it is. Hard to tell from here.
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This sounds like the same problem we had and I FIXED IT just last week. REPLACE the metal hangers. (We did all of them.) Buy some rubber tie-down straps from a hardware store. Cut them into strips about the length of the metal hangers. Take down one hanger at a time and replace it with the rubber strips. We screwed them into the duct and nailed them into the joist exactly where each metal hanger used to be. Haven't heard a thing since. The rubber allows slight movement of the ductwork so it is not forcibly rubbing against any solid object. We have also replaced sections of round ductwork with flexible duct in places where it was jammed in too tight between joists or framing. That helped a lot, too. The idea is you have to allow the ductwork to move slightly as it heats and cools. Trying to force it to not move (as one suggestion above) is nuts. It just makes it worse. If your noise is like ours-clicking and cracking from ducts expanding and contracting-allowing them this extra movement will solve the problem.
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Mike E. Fullerton wrote:

Buy the cheapest, thinnest car floor mats you can get that don't have any carpeting on them. Cut them into strips wide enough to do the job. Or, maybe strips of felt. Or, screw the hanger straps to the duct with self-drilling screws.
But before you do that, can you SEE it happen? Maybe it's not what you think.
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On Fri, 02 Feb 2007 10:55:59 GMT, Mike E. Fullerton

I had some ducts that did that. I took a piece of PVC pipe, 3/4 inch I think, and put it inside the duct so it was jammed between two anchor points (bends, ends, elbows) into a bow that pushed on the metal duct in the middle of the run. That stopped almost all the noise.
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