Need duct size advice

17 years ago

Need to know how to calculate what are properly sized ducts for both return and supply for my house. I need some help.

2,500 sq ft built 15 years ago with two ten" returns, one on either side of the house in the wall at floor level with 12x24 grills. The supply ducts are 6" with 7" registers. The original A/C is a 3.5 ton with a 4 ton E Coil, a 5 ton 100,000 BTU furnace, and a constant speed motor (don't know how much air the motor moves).

How can I determine the proper duct sizing to provide the required CFM? How do I determine what CFM is required?

I have had 4 contractors look at the job, all say 4 ton A/C and 80,000 to 90,000 furnace with variable speed fan. Only one has said my returns are too small. Two told me that the 12x24 grills are not enough surface face and I need to add two additional 12x24 grills. The airflow is loud when the system is heating or cooling. The current equipment is the original and 15 years old. The E Coil has a freon leak and that, along with upgrading to a more efficient system is why we are doing this.

If anybody can direct me to a website calculator or provide any help and direction, I would be very appreciative.

Thanks

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Comments (6)

paulbm
17 years ago
To treat the air in your home you need to be able to move all the air in the house through the heating furnace 5 times per.
hr. So that would be 1666 CFM of air. Your two 10" round pipes will move about 760 CFM of return air, 380 each, so they are not even half of what you need. Your hot air ducts should be at least 240 sq. inches. These CFM's will work for your 4 tons of cooling. If your heat runs are 6" you would need a least 14 to 16 opening. later Paulbm
chambery1
Original Author
17 years ago
Thank you for the calculation method, that helps.

Updated information.

House is 2,300 sq. ft. and the 2 R/A ducts are 12" each. Supply ducts are 7".

So 2 12"s should move about 1,100 CFM if I follow your math correctly. 2 14"s should move about 1,490. Adding a 3rd 12" return would allow about 1,640 CFMs.

What would be the best way to go?
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paulbm
17 years ago
3-- 12" round pipes on return air can move about 1800 cfm
depending on turns and restriction. Your dealers should be able to show you a blower performance chart on the furnace
they plan on using.
2--14" round pipes on return air can move about 1800 cfm
also, so go with ever works better for your job.
geothermal
17 years ago
YOUR return and feed need to be 20 x 10 with a 30 x 20 or 24 x 24 return grill .
chambery1
Original Author
17 years ago
paulbm,

I am concerned with HP performance for both heating and cooling, your comments make me think I am undersized by about 1/3 on R/A. What problems will this cause?

geothermal,

You are a square thinker, lol, I have round ducting. But you are correct in that I have the 24 x 24 return grills.

Thank you both for your thoughts!
paulbm
17 years ago
if you put in a Hp and you run it without proper air flow across the coil, you can damage your equipment, and you will have your coils icing up all the time. It is just not the way to do things. later paulbm