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Intake Register Size

1- Bryant Air Handler Model FB4ANA036 Product# FB4SNS0336AAA
2- Condenser: Model #561AJ036-A Product #561AJX 036000AAAA
3- 3-ton 1200 cfm
4- Air cleaner 600-2000 cfm Pt# 88NA 2506 MB01 Bryant 902B. Replaceable paper accordion-like filter. Hepa filter,similar to space guard.

Intake Register is 20 x 14 and for three ton needs to be 30 x 14. However if the unit is oversized, is it a good idea to have a smaller return register? (Like to add electrostatic filter at inake grille in addition to in-line hepa filter.

AC cooling load only covers kitchen, dining rm and living rm, then upstairs 3 bedrooms and maybe the hall (no supply register there). I get 1058 sq ft—579 for main floor and 479 for second floor. Cubic ft is 8697 (5218 main floor and 3479 2nd floor.) Oversized?

1924 colonial, clapboard, 4" blown-in cellulose insulation, vinyl replacement windows (double paned.)

I have your fine residential construction academy HVAC book but can't find any AC calc procedure.




  • Empire_2
    Empire_2 Member Posts: 2,343

    A quick answer, You R/A grills have to be for the total tonnage even if it is over sized. Starving for return air will surely give you a lot of head aches.....If you need total allowable cfm per rated register Look at Everything as far as ratings and design characteristics is listed here.

    Mike T.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    In-ee equals out-ee

    Mike is right, David. Do not skimp on return air capacity. Depending on free versus face area (normally about a 70% free area ratio) you want a face velocity of about 350 to 450 FPM. This means 500 to 650 through the slots, maximum. Naturally your ductwork and so on can be smaller but where the air enters is critical for if it makes any noise it would be within the space. It pays dividends in other areas too such as not dragging down your overall airflow.

    If I used filter return grilles (and I do!) Keeps return ductwork clean and lessens changing filters at the units. I size them for about 200 FPM to account for filter resistance.
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