Return Air sizing

[Inspector 102]

Homeowner installed his own system with 8 - 6" heat runs and 1- 14" return. I am not very good with HVAC calculations, however the return air seems a little on the small size. Typical size for this size home around here has been 18". Is there a simple formula to help me determine if this is acceptable? 1100 SF home, 75K Btu 85% effecient furnace.

 

[raider1]

Re: Return Air sizing

Was this system designed in accordance with ACCA Manual J and Manual D as required by IRC section M1401.3?

Chris

 

[Inspector 102]

Re: Return Air sizing

No- the homeowner did not use any design manuals to install the system, only what he thought was common sense. He states the heat is balanced, but it just does not seem right to me. Was simply looking fora rule-of-thumb response. Thank

 

[beach]

Re: Return Air sizing

I am in no way an expert on this......however.....as far as I know, the area of the return should match or be larger than the total area of the supply (and whatever the installation instructions require)..... I'm sure the experts will chime in.

Edit: Eight six inch ducts equal 226.192 square inches (28.274 X 8 = 226.192 sq inches) a 14" duct is 153.938 sq. inches (less than the 6" duct total), an 18" duct is 254.469 sq. inches........ which is probably why you see 18" ducts!

 

[Rider Rick]

Re: Return Air sizing

Beach,

Just trying to follow along:

How did you get the square inches of a six inch duct?

Thank you,

Rick

 

[beach]

Re: Return Air sizing

Rick,

Pi R squared = area

But I cheated and used an online calculator.....http://www.csgnetwork.com/circlecalc.html

Edit: I didn't account for friction loss....

 

[Rider Rick]

Re: Return Air sizing

Thanks Beach.

 

[Paul Sweet]

Re: Return Air sizing

A 14" return duct is actually OK, as long as it's not flex duct, or exceptionally long.

Ductwork is usually sized for simple systems using a friction of 0.1" of water per 100 ft. of duct. Based on this, a 6" duct is good for 110 CFM (cubic feet per minute), and a 14" duct is good for 1000 CFM, which is greater than 880 CFM (the capacity of (8) 6" ducts). The air velocity would be 935 feet per minute, which might be a little noisy.

As a check, the output of a 75 MBH furnace at 85% efficiency is 64,000 BTUH. For a 60 degree temperature rise, you would need 64,000 / (60 deg. temp rise x 1.08) = 984 CFM.

I don't know of a simple formula to calculate CFM vs. friction. I use a duct sizing slide rule that an HVAC supplier gave me. HVAC textbooks have charts that give the same results.

 

[D a v e W]

Re: Return Air sizing

Good charts and calc methods here

http://www.udarrell.com/proper_cfm_btuh ... stems.html

 

[peach]

Re: Return Air sizing

No easy way to calculate it.. kind of depends on if heat or cooling dominates.. undersizing the returns leads to heat & humidity in a lot of climates (it's usually bigger than the supply... it's easier for the AHU to push than pull).