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cold air return whistles

Posted by stvokc (My Page) on
Sun, Mar 6, 11 at 13:23

I just had a HVAC company put new flex duct in the attic as ground duct work was all rooted out (under concrete. They used the 5 year old 5 ton AC Trane & 100K BTU Trane furnace and moved it up to the attic.

The cold air returns whistles a lot with just the furnace on especially on the 12 inch return line. To cold to turn on AC but I know when I turn on AC the blower runs faster and whistle will be louder.

They are going to come out and look but I want to know what to tell them I want done about it and not let them give me a song and dance story.

They put in 2 cold air returns. First run one is a 12 inch flex duct to one return grill. What size return grill should this have?

The second cold air run is a 16 inch flex duct to a junction box that split off into 4 separate runs and return grills. What size do the return grills need to be?

Follow-Up Postings:

RE: cold air return whistles

You are right that cooling will be worse then heating.
your returns are to small( 16" = 1000 cfm flex)12"= 500 cfm
flex duct. 5 tons = 2000 cfm. thats one part of it. The grill size ( 2 cfm per 1sq inch of return area. 2000/2 = 1000 sq " a 20x25= 500 2 pc.
the noise of the grill is due to to much air trying to get by the small grill. they could install commercial style wide faced grill. tell them to check static syste pressure.

RE: cold air return whistles

Last week I got a new furnace. There was a whistle/whine/moan that drove me bonkers. They came back out, went into my attic twice, ran new ductwork, taped things. The noise got worse, accentuated in my hallway. Meanwhile, I researched the harmonics issue online and most people pointed to the return grille. Mine had the gill-type louvers. People suggested turning it 180 degrees, while others suggested an egg-crate grille. The heater people returned today and said, we think it might be your return grille! I resisted saying, I knew it!!! The minute the old one came off...silence. Now I have a nice egg-crate grill and everyone is happy. I'm not sure why they didn't think of that immediately, but all's well that ends well.

RE: cold air return whistles

Your ducts are too small but I find that the whistles are usually the grill or filter. You can change out grill styles and filter if applicable. But your ducts are still too small.

Mostly they are going to suggest changing the filter/grill and it will help. I don't know if this strategy is common place. That is - install everything undersized - create noises - come back with a fix. Then you are happy it is better when it still isn't right. People then feel satisfied and don't want to be too picky. I think it is a good strategy for making customers satisfied while putting in inadequate systems.

Your duct work would not pass code in my area.

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