Return size ??

[toddgotfish]

how many square inches should i have for a 3.5 ton heat pump.
I have 2 grills on same wall(corner), one is 20x20 the other 16x20, but they share the same box(metal) , as it goes into through the floor though the cut out is 14x21, which would be 294 sq in. now once it goes through the floor there is a large 22x22 that leads into thee AH, my question is the 14 x 21 BOOT in the floor....is that enough???how much do i need ???
THX again ..

[wolfstrike]

according to my calculations , it's right on the nose.

18" round return duct(for system size) , translates to a 14 x 20 square return register.

[rickboggs]

wolfstrike,
We use a .05 friction loss for return air ducts and 300FPM grille sizing. It work really well for heat pumps (Rheem use to recommend it)

[dash]

Ductolator can only be used to size the ducts ,AFTER determing the FR(friction Rate) of the actual duct system,using Manual D,from http://www.acca.org


Using .05 or .1 is just a Rule of Thumb.

[rickboggs]

Yeap... rule of thumb... but on the other hand! Ha! (Ya get it... on the other hand!)

dash, you are right.

I've found that most of our applications (the style of homes built in our area) that rule of thumb falls right in line. There are exceptions. But I have never found a case where we had too much return air.

[deejoe]

[QUOTE]Originally posted by wolfstrike
[B]according to my calculations , it's right on the nose.

18" round return duct(for system size) , translates to a 14 x 20 square return register.-----------------
----------------------------------------
WRONG.
18 " diam = 14 x20 in rectangular ducted area, but a 14 x20 REGISTER or GRILLE will have a definite percentage of LOSS depending on its design.
If the register that is installed has a 20 % design loss,then a register having an minimum area of 336 sq inches, or a 14 x 24 would have be installed.

[toddgotfish]

THX for the replies guys. like i said , i got GRILLES....LOL
plenty of filter area. so is the cut out good to go ??or do i need to add another return?

[rickboggs]

toddgotfish,
you have enough.