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  1. #1
    Join Date
    Jun 2008
    Location
    NC
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    6

    Need Advice Regarding Return Duct Layout

    I recently had a section of my return duct replaced/rerouted (in order to install support posts to raise a sagging living room floor). I am concerned that the way the duct was laid out is causing some airflow problems and am hoping to get some advice about how to proceed. I've hopefully attached pdfs with sketches of the original layout (original.pdf), the duct as it was reconfigured (as_built.pdf), and an alternative layout (alternative.pdf).

    Background: The house was built in the early 1950s. There is a return in the dining room, 2 in the living room, and 1 in each of the 3 bedrooms. The DR is closest to the furnace followed by the LR and then the 3 BR. All of these are 14"x4". The ducts are steel and have been sealed. I recently had a 20"x20" return (feeding 12" flex) installed in an addition built in the 60s. (Yes, someone built an addition and added supplys but no returns.)

    The section of duct that was removed was between the DR and the LR returns. This section of duct was 20"x8" and split into a 9"x"8 to the 2 LR returns and a 12"x8" going to the 3 bedrooms.

    As rebuilt, each LR return is connected to a 6" flex duct which is connected to a new section of 12"x8" return. I am concerned that the 12x8 duct is undersized to handle the 2 LR returns in addition to the 3 bedrooms.

    Following this change, the DR return is pulling more air and is whistling to the point that I leave the grille off. Additionally the BRs at the far end of the house don't seem to cool quite as well as they did. The company that did the work has done good work for me in the past (they did air balancing; added the return in the addition to take pressure off of the DR and LR returns; sealed the ducts, etc), and I am meeting with them later this week to discuss the situation. I wanted to get some advice before talking to them. I'm wondering if the configuration shown in alternative.pdf would improve things. I'm also wondering if putting dampers on the LR and DR returns would help regulate the airflow and reduce the whistling through the grilles.

    Sorry for the long post. Let me know if more information would help, and thanks for any help.
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  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,907
    They undersized your return ducts.
    Have them increase them back to their original sizes, at least in sq in.
    Those 6" returns are probably too small also.
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  3. #3
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    18,836
    If they also added the 12" return to the addition,then what you have could be correct,not saying it is.

    Consider the new 12" is going to move "X" amount of air,so the rest of the return system(original) will have less air to move.Could be the new addition return needs to be "dampered " down a little,as well as the DR.

    Was the system size increased with the addition,or since then?If so you may need the original return duct system restored per your sketch.

  4. #4
    Join Date
    Jun 2008
    Location
    NC
    Posts
    6
    Thanks for the quick replies. This is very helpful.

    dash - I wasn't clear about the timeline. The return to the addition was added a couple of years ago. This was done following an air balancing visit to determine why the LR and DR returns were whistling loudly. Adding the return in the addition helped that a great deal.

    The more recent work involved rerouting the original return and seems to have made things worse.

    Unfortunately both the system and the addition predate me so I have no idea whether the system was resized when the addition was added.

    beenthere - there was debate about 6" vs 8". The 6" is a little smaller than what was there and the 8" is much bigger. Will the 8 pull so much air that the whistling is back? I guess dampers could be used.

  5. #5
    Join Date
    Jan 2004
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    Lancaster PA
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    It shouldn't, slow moving air doesn't whistle.
    With the smaller ducts you are making it harder for the system to get return air. So it is drawing a lot more through your DR return and causing the whistle.
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  6. #6
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
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    Quote Originally Posted by beenthere View Post
    It shouldn't, slow moving air doesn't whistle.
    With the smaller ducts you are making it harder for the system to get return air. So it is drawing a lot more through your DR return and causing the whistle.

    I agree.

  7. #7
    Join Date
    Jun 2008
    Location
    NC
    Posts
    6
    Okay, to summarize, it sounds like I want them to:

    1) extend the 20x8 return so that the LR feeds the 20x8 rather than 12x8.
    2) replace the 6" flex with 8" in the LR.
    3) Regarding the DR: should I have them replace the original duct between the trunk and grille with 8" flex? I don't know the size of the duct there, but I would assume that 8" will be larger. Is there any reason to put dampers on the flex?

    Since they just did this work, are the changes in the LR something I should expect them to take care of at no charge? If so, do you have any advice on how to present this to the guy who I will be meeting with?

    Thanks very much.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Bring all returns back up to the sq in they were before they redid them, weather that be equivelant to 8" or 9". And tapped into the same size as they were.

    Did they mess up, yes. Are they going to be receptive to doing it for free, don't know.
    might be able to work out a compromise on the cost.
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  9. #9
    Join Date
    Jun 2008
    Location
    NC
    Posts
    6
    To follow up: the sales guy came out and we sat down and looked at my diagrams. He still liked the way they did it but he made some notes and agreed to take the drawings and talk to the "production manager" and the guy who did the install. Last week he came back with the duct guy. They listened to the return in the dining room for a minute and agreed that a "high volume grille" would be the right fix. He asserted that making the return larger (as in back to its previous size) would make the noise worse and that the only problem was re-routing the duct. I asked about the drawings and consultations "back at the shop" and sales guy said "I lost them". I also asked whether he brought the airflow measurements made before and after my air balancing and "we couldn't find those". He did agree to do what I wanted for the same price as he charged for the original job. I passed on that.

    Because he is the "expert", I want to make sure I have some understanding about airflow before I push harder on this (I'm also going to have someone else come out and give me an opinion). His argument seems to be that the duct layout is more restrictive now and because of that a change in size won't make any difference (or it will make things worse). My understanding is that the rerouted duct now has a longer TEL which will increase the resistance. Thus, it seems to me that if I wanted to achieve the original level of performance with the rerouted duct, I would actually need to make it larger than it originally was. Is this right? (To be clear, I don't want to or expect them to redo the entire return. I just want to make sure I understand.)


    Thanks for any help.

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Yes. You increase the size of the duct to lessen the static pressure.
    If they would have used the same sizes that it was. you wouldn't have this noise.

    They are trying not to have to fix their mistake for free.
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