Duct work question for air return

[chrisc]

Hi. i HAVE A 8X24 main trunk for my cold air return going to the furnace. Off this main is a 8x12 branch running perpindicular off of it. (The 8x24 is vertical and the 8x12 is horizontal making a tee. I want to tap into the top of the 8x12 duct to install another return which will be only a few feet away (upstairs) . The problem is I cant find all the parts I need, Should I use 6" pipe to make it easier to find the parts. When i cut into the top of the branch (8x12) I have to go over about 6-7 feet. So i will need a 90degree starter with the fins going into the top , the 6 or 7 feet of rectangular duct and another 90 degree connection for the inside of my wall. Im not sure exactly what the best way to do this is. So im looking for input. I an definitely not paying someone hundreds to do this easy job. Its just a matter of how to set it up. The system is already unbalanced . But it is starving for air so it needs this. even if it's not done hte way it should be. Thanks

[paul52446m]

Hi. i HAVE A 8X24 main trunk for my cold air return going to the furnace. Off this main is a 8x12 branch running perpindicular off of it. (The 8x24 is vertical and the 8x12 is horizontal making a tee. I want to tap into the top of the 8x12 duct to install another return which will be only a few feet away (upstairs) . The problem is I cant find all the parts I need, Should I use 6" pipe to make it easier to find the parts. When i cut into the top of the branch (8x12) I have to go over about 6-7 feet. So i will need a 90degree starter with the fins going into the top , the 6 or 7 feet of rectangular duct and another 90 degree connection for the inside of my wall. Im not sure exactly what the best way to do this is. So im looking for input. I an definitely not paying someone hundreds to do this easy job. Its just a matter of how to set it up. The system is already unbalanced . But it is starving for air so it needs this. even if it's not done hte way it should be. Thanks

Guess i am not really understanding what you have. Are you saying the return air drop at the furnace is 8x24, but the only thing going into this 8x24 is a 8x12 duct? If this is the way it is then the 8x12 duct is no where near enough return air. Adding a 6" round will only give you another 90 CFM of air.
To engineer return air we would have to know size of house, size of furnace,
and what blower is in the furnace. then we size the ducts for the amount of
air we need to move to each room. later paul

[chrisc]

yes that is the way it is. The furnace is a 75k btu for a 1000 sq ft. house. i know its too big! Im trying to fix the problem the best i can. i bought the house a couple of years ago and didnt really look into the furnace issues. I knew the furnace was only a few years old so i figured it was all set.

[HayZee518]

Do you have access to a press brake? If you do then I can design a snap together fitting or fittings that you can put together to get one duct to another. another way would be to send you to a duct manufacturer DUC-PAC in Massachusetts.

[paul52446m]

yes that is the way it is. The furnace is a 75k btu for a 1000 sq ft. house. i know its too big! Im trying to fix the problem the best i can. i bought the house a couple of years ago and didnt really look into the furnace issues. I knew the furnace was only a few years old so i figured it was all set.

A 8x12 duct is only large enough to move about 450 CFM of air. if your furnace is 75'000 BTU. the low CFM would be about 700 CFM. I don't know how much return you have coming into the 8x12. You really need to add 250to 300 CFM
to the 8x24 duct, and then make sure that what is coming into the 8x12 matches the sq. inches of the 8 x12. The 250 to 300 CFM of air needed to be added to the 8x24 would be about 70 sq. inches. Later paul