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Return air on gas furnace & a/c

James Anderson | Posted in Mechanicals on

Upon further reading about return air, would it be wrong to think this way?
The furnace has 2-16×20 return air grilles of which are actually 2-17×14.= 476 sq” of free air space.
Adding filters, may allow as much as 75% air flow = 357 sq” of free air space for return air.
Could it be that the return air grille on the upper half of the closet door needs to be of size to allow 357 sq” of free air space?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Q. "Would it be wrong to think this way?"

    A. I think it would. The free air flow of a grille is not equal to its area. One source says "less than half" its area, another says 75%. (These are not technical sources; here they are:)
    http://hvac-talk.com/vbb/showthread.php?t=179531
    http://forum.doityourself.com/archive/index.php/t-306669.html

    The basic idea is, if the grilles add up to 476 square inches, the free air flow may be somewhere in the range of 238 sq. in. to 357 sq. in. -- before even discussing filters.

  2. James Anderson | | #2

    A different thought? (Trying to understand return air). Say a furnace is installed in the hallway and there are no ra ducts. The ra grill is on the front of the furnace. and there is no closet door in front of the furnace. Say the ra grilles on the front of the furnace are two, 16x20 but, the louvered area is actually 14x17 = 238 x 2 = 476 sq " (of size) and then that would be reduced say by 75% because of the louveres = 357sq", and then reduced again say by 75% when adding filters leaves only 267sq " for return air. Would this 267 sq " be the maximum amount for ra (or of return air ?) that this furnace would have for this type of system? Providing there is no air blockage in the hallwall.

  3. John Brooks | | #3

    James,
    Are you confusing combustion air with return air?

  4. John Brooks | | #4

    James,
    never mind
    I see that you are talking about return air.

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