Hmmmm.. I've never heard of a heat load referred to as CFM..
Lets see, how does the letters CFM relate to 'heat load"?
Roman numerals maybe?
C= 100 ( if I remember correctly)
F= Cant remember any Roman numerals with that letter.
M= isn't that a 1,000?
So, I guess that's correct, 1000 + 100 = 1100.
Learn something new every day!
using same size will do fine
The Contractor did the right thing. The Practice for return duct size
is to have the
Return Duct at least 20 percent larger than the supply air duct? Why is
Simply because the pressure in the return section of every AC units is
(negative). At this percentage of incremental area, the duct borne
minimize in the return side of the AC units.
One more thing about return is to have the shortest possible length of
to avoid excessive pressure drop.
Hope this helps.
wow...I got my hopes up with the first
answer and the second response trashed
The software program I got "HVAC CALC"
had the RA velocity too high with the 20x10
RA PKG ( 900 FPM ). The size that brought
the velocity to 700 FPM was 25x10. This was
"recommended" by the program as the correct
speed and subsequent size.
I've been following this post for days
trying to get an answer. Small matter though,
I bought the 25x10 RA PKG. I can reduce or
expand on cavities.
Wrong. Return is still 14.7 psi.??????????
Atmospheric pressure for a closed system? Iis this what we learned from
Thermodynamics? I will agree the most if this is the outdoor air... but
it is not the outdoor air dude, not the mixing boxn, RA, Return air
Check, the Building Code about sizing the return air to 20% of the
sectional area of the supply air duct. I'm not The one who said that,
expert who made the building code...
My solution for your return duct sizing is:
If you have 20" x 10" Supply air,
Area of RA = 1.2 x 20" x 10" (20% Area increment)
Area of RA = 240 square inch (cross-sectional Area)
RA Duct sizing:
If you have depth of 10" the width shall be 24" (24"x10")
If you brought the 25"x 10" as recommended by the program you have
Then all I can conclude is that the program complies with the 20%
incremental area in return duct as per Mechanical section of the
You're on the right track. Install it, and see the silent result of
your AC system.
Sizing the Return duct to a smalller size will increase the velocity of
the Return air
and results into Noise (Experience talks). I Don't believe that the
pressure at the
return air is 14.7 psi because this is the atmospheric pressure at
The pressure at the intake of fan for the AC units shall always be
(Because pressure is induced). Oscar may use a velometer or pitot tube
to test if the
pressure is 14.7 psi at RA duct.
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