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  1. #1
    Join Date
    Mar 2005
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    37
    Quick question regarding return duct for High velocity system. Can you have to much return air. Or is more return better to service as many rooms as possible with return duct work. Thanks

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,367
    If you have the room to run return to every room, you don't need a HV system.

    HV systems don't need returns to every room, the cool by asperation.
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  3. #3
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Originally posted by beenthere
    HV systems don't need returns to every room, the cool by asperation.
    If a bedroom door is normally closed then it would be best to have a return path of some kind, would it not? I realize that most homes (at least locally) only have one or two returns. But that's a bad situation be it HV or not.

    I'm not a HV expert so please show me what I'm missing!

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    An HV system only moves about 240 CFM per ton, or 40 CFM per outlet. The door needs to have a slight under cut, but not much.

    Asperation is the mixing of the room air and the supply air.(over simplification)

    Its not a total exchange of the room air as in conventional systems.
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  5. #5
    Join Date
    Oct 2005
    Location
    Canada
    Posts
    256
    in Canada with ac present we used to figure roughy 360-400(safe)cfm per ton.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,367
    Originally posted by tyreman
    in Canada with ac present we used to figure roughy 360-400(safe)cfm per ton.
    Your talking abouot a conventional a/c? I hope. Not a HV.
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  7. #7
    Join Date
    Mar 2002
    Location
    Concord, CA
    Posts
    2,635
    Thank you beenthere. I do understand aspiration and the fact that an HV system pushes less volume. I guess the part I was missing was that a door undercut is enough to handle the return. In a small bedroom with just one outlet that would be true, as long as there is indeed a decent undercut. A large master bedroom with a good sized heat load on the other hand-

  8. #8
    Join Date
    Oct 2005
    Location
    Canada
    Posts
    256
    Yes.

  9. #9
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,367
    Originally posted by Irascible
    Thank you beenthere. I do understand aspiration and the fact that an HV system pushes less volume. I guess the part I was missing was that a door undercut is enough to handle the return. In a small bedroom with just one outlet that would be true, as long as there is indeed a decent undercut. A large master bedroom with a good sized heat load on the other hand-
    When you go out to estimate a job, this is one of the many things you have to look at.

    Less then a 1/2" under cut could be a problem for 3 outlets, so in that case you either increase the under cut, or use a tranfer grille, cost alot less then an entire return duct system.
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  10. #10
    Originally posted by beenthere
    An HV system only moves about 240 CFM per ton, or 40 CFM per outlet. The door needs to have a slight under cut, but not much.
    What does that do to the fire rating? A customer of mine lives in a very old home and he has inadequate ductwork, and especially inadequate return capacity. We discussed undercutting the doors a little, but I let him know it was very important that his kids knew how to climb out the window if there is a fire.

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    The homes around here, Lanc PA, that we do HV systems in all have under cuts already. The reason some are less then 1/2" is because of replacement carpet, or new carpet over old finnished hard wood floors. So they don't have a fire rating per say.

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  12. #12
    Join Date
    Jul 2005
    Posts
    717
    Yes, I would think that "less than a 1/2" gap under the door would be a problem. .5 x 30 = 15 sq inches.... A gap of perhaps 1.5 inches (45 sq in) for a bedroom may just help .Any more gap than 1.5 (ugly)and either leave the door open or install a proper sized door grille.

  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Don't need more then 3/4" for an HV system up to 3 outlets in a bedroom. And that a fairly large bedroom.

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