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?
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.
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.
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.
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
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!
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?
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
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.
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
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
it appears to be heat related. May have to wait half an hour
between tests for it to cool down, etc.
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
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
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.
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
If you haven't taken care of this yet, here's something you may want to try
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.
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
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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