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From an alternate perspective, you need to know What the Sensible Load requirments are for day/night based on a selected Inside Air set-point.

CFM = Sensible Load / ( 1.08 * approx. 18)
ie. Room is 12 * 15 = 180 Sq. Feet which requires 2,000 BTU/Hr Daytime .:. 2,000 / 20 = approx. 100 CFM.

If you are targeting a night time set-point, the required load is going to be much less based on both decreased outdoor temperature

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From an alternate perspective, you need to know What the Sensible Load requirments are for day/night based on a selected Inside Air set-point.

x

x

CFM = Sensible Load / ( 1.08 * approx. 18)
ie. Room is 12 * 15 = 180 Sq. Feet which requires 2,000 BTU/Hr Daytime .:. 2,000 / 20 = approx. 100 CFM.

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I want to restrict the flow of the 20" x 30" air return to encourage more air to be pulled from the 3 new air returns located in the bedrooms.

So you are trying to "balance the system by "sucking" the air back. Sounds kinda logical BUT it is always better to Balance the supply air side of the equation. to do otherwise invites low airflow and frozen coils in the summer

4. I think we're making this way too complicated.
I would just block off about 10" of the return grill so it acts like a 20x20 grille. A 20x20 is about what I would use for a 3 ton if I already had the 2 - 12x12's. If the rest of the duct system is not balanced already there's nothing blocking off any returns are going to do any ways. So I say give it a shot & if it doesn't work to your expectations call in the marines(pros).

5. Originally Posted by garyed
I think we're making this way too complicated.
I would just block off about 10" of the return grill so it acts like a 20x20 grille. A 20x20 is about what I would use for a 3 ton if I already had the 2 - 12x12's. If the rest of the duct system is not balanced already there's nothing blocking off any returns are going to do any ways. So I say give it a shot & if it doesn't work to your expectations call in the marines(pros).
Be nice to know what size the supplies are to those rooms.

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