Any HVAC Guys in Here

So I am re-doing the ductwork in my girlfriends house and just need a little help sizing the main and supply branches. The house is an 1100 sq ft pre-fab rancher and has that mobile home type ductwork where there is square duct running through every floor joist and it’s all interconnected.

Well whoever had the house before her decided that that wasn’t good enough for them and cut into the existing ducts and ran flex duct off of it, and did just a terrible job, so basically both systems are trash now.

What I want to do is use round metal pipe supple pipe that runs down the middle of the house and use reducing wyes to run to the individual registers. The AC is a 2 ton Weatherking and the heat is a 76k BTU propane furnace, and they use the same ductwork.

My question is can I use 10" round duct as the main running down the middle of the crawl and use 6" pipe as the branch lines? I want to take advantage of the weather and get this all done before the heat is needed. Any help would be greatly appreciated. Thanks.


For a 2 Ton system which is 800 CFM, you should start with 12" round then reduce as you branch off.
6" round duct will flow 100 CFM.

Sounds like it would be easier to get some sheet metal and seal up all the holes that were wrong. Self tapping sheet metal screws and sealant are a lot cheaper and less time consuming than total replacement.

Brian, Scott is well versed in HVAC and, if for some odd reason he isn’t sure about something, he will get the facts. You can take that to the bank…Scott’s one of the good ones around here. :slight_smile:

I tried that first, but there are entire sections that are ripped out and there are holes poked in the old duct intermittently. Basically the deeper I dug the more problems I found. I do agree with the logic though.

Ok great so 12" main duct and then branches of 6"? Or branches of 8"?

Take model, size, blower size, etc of HVAC unit to local Tech School, Community College or HVAC dealer and ask them.

Also have rough floorplan and room sizes of house for them to work with.

Thanks Larry:)

On your branches - 6" round is typical for small rooms and 8" for larger areas.
A 2 ton system isn’t large and neither is the house you’re describing, so you may use both sizes.

If you have a tight area that you only have room for one run and its beyond 12’ x 12’, use an 8". That will be good for 220 CFM.

There are other nuances/rules to sizing duct but this should be pretty simple.

If you prefer, send a PM or call (989-370-3683). Happy to help.