# RA Duct question

My supply air trunk is 20"x10" with a heat load of 1100CFM. What size should the return air duct be?
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huh??
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wrote:

9278.3 square miles
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8" X 8"...smaller the better....it might whistle. But you can tune in a song you want

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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

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Damn...and I exchanged that return air package for huge one.
It took 3days to get this answer.
tx

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Richard wrote:

Mr. Richard,
Don't worry.
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 this?
Simply because the pressure in the return section of every AC units is induced (negative). At this percentage of incremental area, the duct borne noise are minimize in the return side of the AC units.
One more thing about return is to have the shortest possible length of the system to avoid excessive pressure drop.
Hope this helps.
God Bless.
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Wrong. Return is still 14.7 psi.
Return can be smaller because cold air is denser,
Supply should be larger because air expands when heated.

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wow...I got my hopes up with the first answer and the second response trashed it.
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.
Thanks guys Richard

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Richard wrote:

Umm dont listen to Oscar, hes a troll. One of many here.
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Tell me how I am wrong.
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Oscar_Lives wrote:

Oscar,
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 Duct.
Check, the Building Code about sizing the return air to 20% of the cross
sectional area of the supply air duct. I'm not The one who said that, it's the
expert who made the building code...
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Oscar_Lives wrote:

In a word, volume. A cubic foot is a cubic foot.
Now a question for you. What is it that you do for a living?
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Bust unions.
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Richard wrote:

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 Building Code.
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 zero level.
The pressure at the intake of fan for the AC units shall always be negative
(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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Oscar_Lives wrote:

lol, or supply should be smaller when you run the AC?
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Hell, it probably should be, especially if the cold air is blowing on the old lady!
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