Should I use flex duct or sheet metal?

Q: What size duct do I need for a 3-ton AC unit?

A 3-ton system needs roughly 1,200 CFM total. The main trunk typically sizes to 14"×8" or 16" round at 0.08 IWC friction rate, with branch ducts in the 6"–8" round range depending on per-room CFM. Always size from a full Manual D calculation, not by tonnage alone.

Q: What is friction rate in duct design?

Friction rate is the pressure loss per 100 ft of equivalent length, in inches of water column. Lower friction rates produce larger, quieter ducts; higher rates save space but cost more in fan energy and noise.

Question

Should I use flex duct or sheet metal?

Accepted Answer

Sheet metal has lower friction loss and longer life, so it is preferred for trunk lines. Flex duct is acceptable for short branch runs (under 10 ft) where it is fully stretched and properly supported — sagging flex duct can lose 50% of its rated capacity.

Should I use flex duct or sheet metal?

Answers