ductwork design

Old 10-30-11, 05:34 AM
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Join Date: Oct 2011
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ductwork design

Hello everyone, I am not sure I am posting this in the proper place, but here it goes.

We are installing duct work in my mom's house. Due to a financial bind we are doing it ourselves. We had installed duct work in our own home that was designed by a heating in cooling company years ago so we used the same basis for the design of my mom's. We are not contractors and we really were short on cash. This is the design:

We have a 12" round metal supply duct that travels from her new basement, through the old basement, up to through a chase to the attic. It then stretches from a T in two directions. One direction is short about 10' and then it L's for another 5 feet. The long side off of the T is about 30' then it L's for another 5 feet. The total length for the supply 12" pipe is about 100 feet.

The supply travels the same path, but it is 14" round metal duct. Total length of about 100 feet.

The supply and the return both have about 4 90 degree elbows.

The return trunk has 7 returns, one in each room, except the living room has a vaulted ceiling so we put in 2 returns. The returns have grates that are 6"X12" tied to 8" flex duct.

The supply trunk supplies (11) 4" X12" grates with 6" duct.

The house square footage is 2100.

The heating and cooling in the past was 4 tons.

We were hoping someone could tell us if we have enough returns is it based upon grate size or the flex duct size?

Will the duct work be large enough for a four ton unit?

Thank you in advance and I pray that someone understands this post.
Old 10-30-11, 05:29 PM
airman.1994's Avatar
Join Date: Dec 2005
Location: VA
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For 4 tons you will need about 20 inch hard duct for return. And around 17 for supply. What you have is way under sized.

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