Basic Flex Duct Sizing for simple 1-ton system

[telljcl]

There seems to be so much information out there on this, but I need a sensible approach for a functional system that may not be able to be "by the numbers" because of access, complexity, cost or other factors.

Hopefully I can come close. The particulars are as follows:
* System - 1 ton / 400-500 CFM 3 speed adjustment air handler.
* All ductwork has to be flex
* Planning on a 10" return on the left wall - next to unit. (length will be single run of less than 8').

Looking at the drawing, what size flex should "A", "B", and "C" be (ideally) and what would be the best way to transition from the flex to ceiling registers at these points? I'm having to cut into an existing ceiling from below to do this - I have 18" of height in the joist spaces, with 18" width. There is some fiberglass insulation in there as well, but can be mashed a bit.

I'd like to have fairly quiet supplies, but it isn't a recording studio or anything - just don't want the system running to draw much attention to that fact.

Thanks for any insight!

 

[Dana]

"* All ductwork has to be flex"

Why? Flex duct that isn't stretched tight underperforms hard-piped duct by a huge amount. Take the time to read this bit of lab testing.

If the whole point of using flex is to limit the low frequency noise & vibe propogation of the air handler, a short section of fat flex between the air handler to a hard-piped duct system gets you there. But for the longer runs, the higher friction & turbulence of flex duct INCREASES the noise rather than reducing it. It's like sliding against the sticky rosin on on a violin bow compared to a glass rod.

On the hard piping, use only radiused ells & tees. A sharp 90 degree interior corner adds a lot of turbulence and increases the duct impedance.

 

[Sponsor]

 

[telljcl]

"* All ductwork has to be flex"

Why? Flex duct that isn't stretched tight underperforms hard-piped duct by a huge amount. Take the time to read this bit of lab testing.

If the whole point of using flex is to limit the low frequency noise & vibe propogation of the air handler, a short section of fat flex between the air handler to a hard-piped duct system gets you there. But for the longer runs, the higher friction & turbulence of flex duct INCREASES the noise rather than reducing it. It's like sliding against the sticky rosin on on a violin bow compared to a glass rod.

On the hard piping, use only radiused ells & tees. A sharp 90 degree interior corner adds a lot of turbulence and increases the duct impedance.

Thanks - I understand the value of hard pipe over flex - but as mentioned, there are several reasons that I need to go this route. Noise isn't the only consideration.

 

[Stuff]

Some amateur answers:

Only 3 registers means you want 150 cfm per register.

Using this chart you need at least a 12" for A, 10" for B, and 8" for C. Go up in sizes if you can.
Use these if you can: http://thesheetmetalkid.com/sheet-metal-duct-y-branch/

10" round return is too small. Either make it square or go to 12" Will your filter be on the register?

 

[telljcl]

Some amateur answers:

Only 3 registers means you want 150 cfm per register.

Using this chart you need at least a 12" for A, 10" for B, and 8" for C. Go up in sizes if you can.
Use these if you can: http://thesheetmetalkid.com/sheet-metal-duct-y-branch/

10" round return is too small. Either make it square or go to 12" Will your filter be on the register?

Filter on the return grill if possible. 10" flex won't handle 450cfm in only an 8' run?
Thanks!

 

[Stuff]

The chart shows that 10" won't cut it. Additionally there is a lot of talk that flex rarely delivers so you should up size.

The more corners you cut the worse the system functions. You are already compromising by using flex.