Are my ducts noisy because of heat or too much air?

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I posted before about my noisy ducts -- crash, bang, pow, all night. I'm about to call a home improvement guy and have him remove the dry wall on the ceiling, replace or tighten the duct, then repair the dry wall. I don't think it will be a big job, but someone told me that the problem might be too much air going through the ducts because of the new more powerful furnace (two stage, variable speed, 95K BTU). Can someone advise me on this?
I think it is the heat because: 1) It happens at only two ducts, 2) I get a noise when it cools down and sometimes long after it cools down.
More importantly, I got the furnace from a VERY reputable firm, and I don't think they would make a mistake like that, and I got two other quotes from other reputable firms, all of whom were ready to sell the same furnace. Surely, they couldn't all make the same mistake.
So, any advice?
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call the company that installed your new furnace and tell them of the issues, there may be a simple solution...
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Oh, i did that long ago. They say it is just oil-canning, and not their problem. But what they don't know is that in cold weather it is intolerable. And whether it is oil-canning or not, I want it fixed.
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On 3/25/2013 12:52 PM, Dom wrote:

if it's oilcanning, it could be caused by simply expansion and contraction. tightening them could even make it worse. you want a bit of looseness to allow the expansion to not force the ducts to bend. however, that makes them leak air, which also isn't good.
you'd need to go to flex ducts to get rid of all the noise i would expect.
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. Good point. I'll be sure to tell the home improvement guy about the flexible ducts. Do you think I'm safe in saying it is not just the amount of air flow?
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On 3/25/2013 1:51 PM, Dom wrote:

that would be pretty far down on my list of possibilities. you'd have to have one heck of a blower to bend the metal. the major cause of this is heat/cool cycles causing metal expansion and flexing.
flex ducts aren't as efficient as smooth metal ducts (they're rougher inside), so you'll get lower velocities i would expect.
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chaniarts wrote:

I agree.
To the OP (Dom), how closely are you able to pinpoint where the popping sounds are coming from? Are you able to tell which duct, what part of the duct, etc. And, if so, is that duct accessible or maybe right behind a certain section of sheetrock? If so, I wonder if you could just cut a small hole in the sheetrock where the noise is located, then put something in between the sheetrock and the duct to see if that fixes or changes the noise problem.
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Yes, I can tell the duct and, unless there is a loud echo, I can tell the part of the duct. It is within a foot or so of the vent. In fact I took real heavy tape, and just taped the sides of the duct to the "boot" that ends in the vent. I did that on all four sides, and most of the noise ended. I'd still replace the last part of the duct just to get rid of the last remaining boom!
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Thanks for the info. You really put my ind at ease. I had heard that flex ducts weren't efficient, but it is only one or two ducts that I want changed, so I think I'll go with the flex ducts less the home improvement guy recommends otherwise.
One more question. Do you think it is a good idea to go through a home improvement guy? I assume he build additions, so he must have good subcontractors for duct work. Good idea?
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On 3/25/2013 2:40 PM, Dom wrote:

it's totally dependent upon the guy. you really want 2 people: one to fix the ducts, and one to do the drywall repair. they won't be the same people usually, if you're using subcontractors. if it were me, i'd do the tear out, have an hvac guy repair the duct work, wait a week to make sure the problem was solved, and have a drywall guy come in to fix the holes.
if you do the tear out, you'll be able to pinpoint the problem just by looking at it, or feeling the duct, after turning on the heat. you can see and feel the duct move. you may be able to wrap insulation around it, or do something to muffle the noise, which may be all you need, without having to replace it.
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That's what I would do too. I surely would not get a general contractor who then subs out the work for a small job like this. Now you're paying overhead for the GC to do what?
Actually, this could be within the scope of a competent handyman. I don't think you probably even need an HVAC guy. If it were me, I'd open it up, and see what's found. If it's a section of duct that's flexing, could be as simple as buying a piece of thin angle stock, like you hang garage door openers with at HD and screwing it on. Or nailing a piece of furring strip to support it.

I would focus on eliminating the movement that is causing it, not trying to muffle it. And I agree, that if he can do the tear out himself, he could save a lot of time and money. It coud take a while to find it, test it, make sure it's eliminated. Especially since it appears to be heat related. May have to wait half an hour between tests for it to cool down, etc.
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On 3/25/2013 4:14 PM, chaniarts wrote:

I don't think that is a good idea. If this is a rectangular duct, then flexible ducts will restrict the air flow... Sounds like you need the insulated duct that I have. It is not metal, and it also keeps the heat in the duct. Mine are fiber insulated with metalic foil. They are very easy to make and I am sure any reputable HVAC firm will be glad to do it, if you are opening the wall or cieling.
--
Jeff

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Jeff, I'll keep that in mind when I talk to the home improvement guy. Thanks
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wrote:

Oil canning is most likely the cause, a very common problem with hot air heat. If the furnace guy put in the furnace and not the ducts, he is probably off the hook on this. Did the old furnace make the same noises?
The home improvement guy is not the guy to fix it unless he is familiar with this type of problem. You need a good sheetmetal man that knows how to design and install ducts.
Most likely, you have to cut the duct and install a flexible coupling to take up the expansion when the duct is heated. If you have long straight runs and do not allow for movement, you get oil canning. It should be allowed to float.
http://www.flexicraft.com/Metal_Expansion_Joints/Metal_Duct/
The furnace guy should have been able to recommend a tin knocker, but it sounds like he just wants out. Now it is up to you to find someone. I'd ask around at the local plumbing supply house or a good heating contractor.
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Great info Ed. Between you, Tony, and Trader4 I have a lot to go on and a lot of things to check out before I hire a home improvement guy.
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Dom wrote:

If you haven't taken care of this yet, here's something you may want to try first.
You said the duct is in the ceiling and I assume there is no attic space above it where you can access the duct from above. So, if it is in the ceiling, that probably means that it runs along the ceiling above the sheetrock between two ceiling joists. Ceiling joists are usually 16 inches apart (16 inches from the center of one joist to the center of the next joist). So, if you were taking out a section of the sheetrock to completely access the duct, you would probably take out a section that is 16 inches wide and whatever length you need. Then, a replacement piece of sheetrock would get screwed back in place to the two joists and the edges taped and patched. (NOTE: If the duct is in a soffit, it may be a little different, but I am assuming your duct is in a flat ceiling).
But, here's my suggestion before taking out a big section of sheetrock. First try cutting a small hole out of the sheetrock in the area where you are hearing the sound. The hole could be just 6" x 6" or so. You just need a utility knife or something similar to cut out that section. Then you should be able to see the duct. With the hole there, watch and listen and see what happens. "Maybe" you will actually be able to see the duct move or pop when the big bang happens. That would be good news. But, even if you can't see anything happening during the big bang, maybe you will be able to hear that it is really coming from that location. And, maybe you can try wedging something in between the sheetrock and the duct and see if that changes the bang. If it were me, that's what I would do, even though I know how to take out and replace larger sections of sheetrock etc. You could even do more than one test hole if needed.
The main reason for trying this first is that is costs nothing to do it, and if you are able to solve the problem from there, re-patching the small hole will be very easy. It can be a do-it-yourself job even if you never did that before just by watching a couple of YouTube videos that show the process step by step. And, taking out a small hole or two first won't change anything in the long run if you later need to take out (or have someone else take out) a larger section of sheetrock.
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OP- You've gotten a lot of great advice. Approach this "fix" is small stages, you'll be sure of getting it right & at the minimum cost. I would suggest cutting "joist to joist" center line pieces so the replacement is easy. With a reasonably sized access panel (~16" x 36 to 48") you should able to observe the behavior & do try a modification.
Good luck, let us know how it turns out
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bob haller wrote:

Hi, In the mean time try to run the air handler at lower speed if you can run the air handler at lpower speed and see what happens. Sounds like duct balance is off between exhaust and return side.
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Hmmmm. Interesting. I have the fan at the lowest possible speed now. I'd didn't think about the balance, but then again, it's only happening at one duct.
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Dom wrote:

Hi, Of course, heat can make the duct work somewhat expand and shrink every time heating cycle comes. When air flow is not balanced the pressure build up can make big banging sound when it is coming on or going off. Also it can happen with filter's resistance to air flow, etc. If you did not have the problem before new furnace.......
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