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HVAC Return Questions

X_NavyFC | Posted in Mechanicals on

Thanks in advance for taking the time to read my question about some duct installation that I will be doing myself in the attic.

I have an older home (‘68) in Northern Illinois that has one central return in the hallway and bedroom doors undercut probably an inch or more. I’m not the biggest fan of this because doors look stupid and I assume it doesn’t offer good air circulation among the entire floor. I also can’t see how it would be able to circulate air effectively from the family and dining room.

The central return is in the hallway and utilizes panned off floor joists and the wall cavity.

 I was thinking about using this existing return air structure and installing trunkline in my attic with dedicated returns to each bedroom and dining and family room. I figured one return per supply.

My questions…. is my idea of individual returns going to make that much of a difference to be worth the effort, or am I better off installing transfer grills in the bedrooms?

To get the duct sizing right, should I reach out to one of these design companies that will design your duct run? Any recommendations on a company?

Lastly, solid duct or flexible? I know flex will be quicker, easier, and cheaper. However, is not really able to be cleaned a big enough deterrent? Solid duct will also need to be insulated since it will be in unconditioned attic, so that’s added time and money for me.

Thanks again!

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  1. Expert Member
    Akos | | #1

    First off, you never ever go out of your way to install ducts in the attic. This is a good way to loose a lot of your heat and cause condensation issues in your attic.

    Undercuts are ugly, but if sized correctly they do work and work well. Transfer grills are a better option, there are ones that come with a honeycomb type insert that help with noise.

    Individual returns for each room don't help you all that much provided you have the undercut/transfer grill sized correctly (less than 3 Pa pressure drop at the flow rate of your supply register). There is really no energy efficiency benefit by running individual returns.

    Your time and effort is better spend on sealing up your panned return cavity especially near the ceiling as these usually draw in air from the attic.

    Installing a quality filter grill also goes a long way in keeping your ducts and air handler clean.

  2. Expert Member
    BILL WICHERS | | #2

    Individual returns are a good thing, but ducting in the attic is a BAD thing! Ideally you don't want any mechanicals in the attic.

    Flex duct is especially problematic with returns so you really want to stick with rigid ducting.

    An alternative plan for you might be to modify the existing "panned off floor joist" return so that it extends into at least some of the rooms, then use floor vents as returns. Existing returns in walls can have wall vents easily added on the "other side" too.

    Any easy and inexpensive retrofit is going to be suboptimal here. Transfer grilles allow for a lot of sound to leak between rooms, for example. If that's your only option though, you can minimize sound transmission a bit by offsetting the grilles so that they don't go straight through a wall, then line the inside of the "duct" you've created with rigid mineral wool panels. The mineral wool lined wall cavity between the grilles helps to limit sound transmission. It's not perfect, but it's a lot better than two grilles you can see through into the other room.


  3. Jon_R | | #3

    IMO, +3 pascals is high and could cause moisture problems in the walls. I'd measure where you are at now and shoot for 1 pascal. If you meet that with adequate temperature distribution, then don't change anything.

  4. Deleted | | #4


  5. X_NavyFC | | #5

    After thinking over the options, I’ll most likely go the transfer duct option.

    I’ve never seen any or know of anyone that has had them.

    How much sound do they transfer?
    Is there a specific brand or model that you recommend?

    1. Expert Member
      Akos | | #6

      These units have a honeycomb core between the registers. This does a decent job of filtering higher frequency sounds.

      Make sure to size them to the actual flow rate of your registers as they are more restrictive than just two grills on either side of the wall.

      If you want even better, you can go with a high vent on one side of a stud bay and low vent on the other side. Line the inside of the bay with fiberglass duct liner and a couple of horizontal baffles to increase the path length. Lot more work, would only be worth it for something like a media room.

      1. X_NavyFC | | #7

        Regarding the flow rate of the supply, is that based on the cfm rating of the supply ducts to a room (i.e two 6” round metal)?

        Can I also assume that jumper ducts in the attic would pose the same problem as returns in the attic with condensation? I can see how a large jumper duct with decent size grills could “handle” more airflow.

        Thanks again

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