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bonzaco
bonzaco,
Category: HVAC
Satisfied Customers: 1466
Experience:  Owner at Dales Electronic
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I have heat supply and cold air return trunk lines in my

Customer Question

I have heat supply and cold air return trunk lines in my basement that are constructed of duct board. I want to replace them with meatl runs to gain more head room so I can finish my basement. I want to use 6" high metal trunk lines. How do I know how wide I need to go to have enough supply and return?
Submitted: 1 year ago.
Category: HVAC
Expert:  bonzaco replied 1 year ago.

You need to work out the inside area of the existing ducts and then match them up to their metal equivalent. If you would like help with that let me have the original dimensions and I will work it out for you.

Customer: replied 1 year ago.
The inside dimensions of the return is appx: 18" wide x 8" high. The supply is appx; 15" wide x 8" high
Customer: replied 1 year ago.
the supply run starts out a bit wider, then funnels down from the plenum after appx; 4-5 feet
Expert:  bonzaco replied 1 year ago.

Yes by tapering down this ensures that the flow rate stays constant. Supply is 15 x 8 = 120 sq inches / return 18 x 8 = 144 sq inches. So any ducting that when you measure the length x depth equals the two sizes respectively. I would however allow for some insulation to keep the heat/cold inside the ducts.

Customer: replied 1 year ago.
I have found metal duct that is 6 x 24 which equals 144 sq. in. Could I use that on the return. How do you insulate. This will be a finished basement
Customer: replied 1 year ago.
Also, do you have to taper the metal duct the same way as the duct board?
Expert:  bonzaco replied 1 year ago.

You are insulation to stop condensation, so rockwool or glass fibre. I think you will struggle to taper the metal duct so I would try it for a couple of days if needed you could always put a restricter in at the end of the duct. I think however it would work fine, its not like you are going up twenty floors.

Customer: replied 1 year ago.
O.K. What i thought about doing was putting the new runs in place then putting regular house insulation on the sides. I will have suspended ceiling below. The supply run is appx: 24 feet long and the return is appx: 19 feet long. Is there a concern for water leaking out of the runs with condensation? I want to keep the suspended ceiling as close to the duct as possible. I may even drywall over the duct if i need more room and put suspended ceiling on the sides.
Customer: replied 1 year ago.
Sorry, I am not familiar with a restrictor?
Expert:  bonzaco replied 1 year ago.

you can reduce the size of the duct by making the hole smaller right at the end.

Customer: replied 1 year ago.
Sorry, I had to leave for work before I got the last post.
Expert:  bonzaco replied 1 year ago.

Question, why dont you use plastic ducting ?

Customer: replied 1 year ago.
I didn't know it existed. Where can I but it? Do you build the runs as with the fiberglass duct board?
Expert:  bonzaco replied 1 year ago.

I'm not sure that you can use it to code in your state but here is a link to an article that you might care to read --

http://www.achrnews.com/blogs/16-guest-blog/post/126838-pros-and-cons-of-using-pvc-pipes-on-your-hvac-projects