I have had the fiberboard/flex duct system replaced with standard metal duct for our central HVAC system. The improvement in airflow has been very good, partly because the previous install had the flex loose with lots of tight bends.

The only drawback is the increased (I assume blower) noise from the vents that seems to be resonating up through the duct; it is not caused by the increased flow at the registers. While not totally obnoxious, it is quite noticeable, and if I could knock it down by half easily that would be great.

I would much prefer putting something on the outside of the duct rather than the inside, even it is not as effective. I tried wrapping 6" supplies with fiberglass, but it only helped a little. In one place the final connection to the boot was made with a 4' length of flex pulled quite tight, and those vents seem much quieter but retain decent flow.

So the big question is, what are some accepted methods for reducing blower noise from in metal ducts? Cover duct with thin foam? Replace a short length of duct in each run with flex? Cover the first several feet of main supply plenum with foam board?

I expect (hope anyway) to get a variety of solutions, all of which are appreciated.

FYI I have a Coleman 3.5 ton heat pump, 120,000 BTU propane auxiliary, and 13 (so far) 6" supplies lines. System is centrally located, split into two 8x20" supply plenums and two 8x16" return plenums.

Thank you!

The only true way to get rid of metal duct noise is to insulate the inside of the duct. There are special materials available for that purpose and usually the duct liner is spot welded pin supported. You could possibly glue on but you will have to check the various mfg. requirements.

On advantage of fiberboard duct is noise reduction. I never cared for the stuff and would never use it in my home. Now flex works well if installed to Smacna specs.

The only true way to get rid of metal duct noise is to insulate the inside of the duct. There are special materials available for that purpose and usually the duct liner is spot welded pin supported. You could possibly glue on but you will have to check the various mfg. requirements.

On advantage of fiberboard duct is noise reduction. I never cared for the stuff and would never use it in my home. Now flex works well if installed to Smacna specs.

That is my understanding from previous forum posts, that inside is superior by far. Still, if I could easily reduce the noise a fair bit from the outside I would be satisfied. Or by putting in some flex.

Any chance you could describe the proper way to install a simple run of flex duct? My understanding is that you want it pulled taught, and I have seen it attached always with a zip-tie (professional grade) and sometimes in with duct tape as well.

Any thoughts on using some flex on the system to cut noise?

Thanks!

p.s. I just found this article (http://www.ashireporter.org/articles/articles.aspx?id=41) online. I think the previous installer did ALL these no-no's. What a Bozo.

Is the noise a product of vibration? A dirty blower wheel will be imbalanced and can create a lot of undesirable noise. If this is the case, remove motor from assembly and spray out the wheel and housing to restore balance.

Is the noise a product of vibration? A dirty blower wheel will be imbalanced and can create a lot of undesirable noise. If this is the case, remove motor from assembly and spray out the wheel and housing to restore balance.

I don't think so. Its only 3 years old. It just sounds like fan noise being efficiently funneled

You are on-site and would know better, but I have seen less than 3 years do a lot to a system if the air filtration is inadequate.