HVAC Experts or Handymen - a question?

[BadgerDave]

How do I stop my AC ducts from "banging" when the blower turns on and off? I solved part of the problem by bracing some foam against the vertical ducts, but I'm stumped on how to brace or insulate the big, thin sheets of steel that form the ceiling ducts. Sounds like a cross between a shotgun and a steel drum.

Any suggestions?

[marcnorth]

Are they in the basement or attic? Do you have access to the ducts? Insulation sounds like the answer to me.

added this link, it might help you:

http://www.doityourself.com/stry/troubleshooting-a-furnace-duct-banging-noises#.Uf8KJG3GCUk

[RCM78]

You need access to the ductwork. Usually screwing some strips of metal to the sides of the duct in an X pattern will work. For really bad cases I've cut the ducts open and braced 3/4" PCV pipe in there...

[velorush]

You need access to the ductwork. Usually screwing some strips of metal to the sides of the duct in an X pattern will work. For really bad cases I've cut the ducts open and braced 3/4" PCV pipe in there...

This. What's going on is flex in the ductwork. An angled piece (an "L," like mini angle iron) screwed across the offending panel will also do the trick. What you're trying to do is instill some structure to stop the panel from flexing under pressure changes.

Careful insulating ducts that provide air conditioning. Condensation can form when the cool air comes through. Cool, damp and dark can become a great environment for mold.

[BadgerDave]

The offending ducts are in my basement, easy access, no ceiling in place (yet). Does anyone make a kind of insulating blanket that has adhesive on one side? If so, is this a promising solution?

My skills are more in line with "peel-and-stick" rather than structural bracing.

[stobro]

You could always switch the fan from "auto" to "on" and then the blower will stay on continuously.

[Armitage]

You can buy some Fatmat (made for car audio enthusiasts). Basically a rubbery tarry tin foil that stops metal from ringing and transferring sound. It works great. There are other more expensive ones, but they're only marginally better, for a lot more money. I wouldn't use it for the first 10-15 after the furnace though, that part of the duct can get hot.

http://compare.ebay.com/like/190688110305?var=lv&ltyp=AllFixedPriceItemTypes&var=sbar

But you could also fix the real problem, though it may take a little detective work. It sounds like you're pushing too much air and not getting enough of it to move. You may not have enough intakes, or enough exhaust vents, or the ducts are undersized or clogged. If it's the cold air return ducts, your air filter may simply be too dirty.

[BadgerDave]

Thanks for the great ideas and advice. I changed the filters and found some adhesive backed rolls of metallic and foam duct insulation at Home Depot. After applying the insulation, the noise is just about completely gone. There seems to be one panel of ductwork that still "pops" when the air flow starts or stops, but it's now muffled and the other panels don't amplify the noise.

[Armitage]

But you still have an air pressure problem. Check to see if it's an air return or out from the furnace. There's a reason for this and will impact your heating and cooling as well as the efficiency.

[BadgerDave]

But you still have an air pressure problem. Check to see if it's an air return or out from the furnace.

Thank you for you help on this. When you say "Check to see if it's an air return or out from the furnace", what am I looking for? Sorry, I know very little about HVAC.

The filters are clean, I had the whole house ductwork cleaned this spring and I'm certain that there's no blockage. I had a furnace igniter replaced in March and a full system check-up by a reliable tech at the same time.

The problem seems to be isolated to a couple of large panels of thin metal in the ducts directly connected to the furnace. These are about 4' x 2 1/2' and they flex when the blower fan starts or stops. My amateur diagnosis is that the panels are mounted in a way that slightly twists them and makes the sheets very susceptable to moving with slight pressure changes. When the air flows, they "pop". Kind of like those old style tin "clickers" we played with when we were kids.

[JGale]

You could always glue those angle pieces to the offending surfaces on the outside. Perpendicular to the 4 ft dimension at the quarter points would be best imho.

[velorush]

You could always glue those angle pieces to the offending surfaces on the outside. Perpendicular to the 4 ft dimension at the quarter points would be best imho.

That is a better way of putting what I was trying to convey in post #4. Sheet metal screws through the angular piece, into the duct surface will do it - worked for me! The flex problem develops when there is a long piece of duct. It just isn't rigid enough to handle the pressure change without flex. The angled piece will provide the necessary rigidity.

[BadgerDave]

Careful insulating ducts that provide air conditioning. Condensation can form when the cool air comes through. Cool, damp and dark can become a great environment for mold.

This explains why I'm noticing significantly more water drainage in the condensation lines. These flow through pvc pipes to a drain in the floor. I started seeing a lot more water immediately after I installed the insullation. Is this a problem?

[Armitage]

Thank you for you help on this. When you say "Check to see if it's an air return or out from the furnace", what am I looking for? Sorry, I know very little about HVAC.

First try to think when the problem started. Did you just change your furnace, just have it cleaned, just move some ducts or was it always like that? Did someone go around closing heater vents or air returns? Can you tell if it’s on the air return side, or the hot air side?

A common problem is when a duct cleaner guy accidently turns a flap IN the duct restricting or redirecting air flow. These are more common in newer homes and can be just inside the duct on the floor, a big round flap. Many times there is one where the air duct leaves the furnace if the duct splits off in two directions.

This can be one of those fun chases if you’re into that kind of thing… or there’s Fatmat.

Just remember with things like Fatmat, You don’t run it down the center, you go along the edge. Great stuff, I use it on a lot of things.

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