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Community and Q&A

HVAC duct size versus grille size

richt1022 | Posted in Mechanicals on

For proper air being returned to the HVAC unit that is the size of the duct have to correlate to the size of the grille for the return air? So for a 12 inch air filter grille what is the largest size return duct that could be attached to that for proper operation?

I’m getting air pressure coming from the attic through the switches ,outlets ,and around the drop ladder in a different zone than the one with what I believe is undersized return.

Thanks Rich

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  1. GBA Editor
    Martin Holladay | | #1

    In general, the size of the grille is the same as the size of the return duct. An 8x14 grille needs an 8x14 duct.

    To make sure that your return air ducts are adequately sized, though, you need to do some airflow calculations. Return airflow is equal to supply airflow.

    If you know the airflow of your system, you can consult the chart below.

    Note that some homes have only one return air grille, but better systems have multiple return air grilles.


  2. Walter Ahlgrim | | #2

    From your description of your air infiltration I think your fault or faults are in the supply ducts.

    I suspect that the duct work is located in unconditioned spaces like the attic or a crawl space and they are very leaky so when the heat or AC blower is running the air leaks into the unconditioned spaces escapes to the outdoors and you have noticed the air entering your home thru every gap to replace the air that leaked out of the supply ducts.

    If at all possible get your duct work into the condition space by eater moving the ducts below the ceiling or moving the thermal boundary insulation against the roof and sealing the attic vents.

    Sealing the duct work is better than doing nothing but not by much.


  3. GBA Editor
    Martin Holladay | | #3

    For a thorough discussion of the type of problems that Walter is talking about, see "An Introduction to Pressure Diagnostics."

  4. richt1022 | | #4

    Martin the only reason I reposted the question was I realized i should have asked in a different topic, thanks for your help though I’ll look into the duct size.

  5. richt1022 | | #5

    Martin the pressure diagnostics did give me the answer I believe, I’ll let you know how it works out. Thanks

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