Sizing return and supply over long distance

[RA335]

We are replacing a 2.5 ton air handler in the attic above garage. Unit will be located 30-35 ft from the 'closest air conditioned room'. The farthest room receiving air is 50 ft from the air handler.

Question is - contractors want to run flex pipe from new unit to existing return and supply connections (30 ft run) using 12" diam. flex. I think the pressure loss will be high and that air flow would be better with larger diam. flex (16" or 18"). contractor disagrees but i think he wants to spend less by using smaller ducting.

Any advice on this?

thanks

[beenthere]

12" isn't big enough if its 3" away.

Without seeing the run set up, 18" is probaly a safe bet.

[andserco]

Get an opinion from another contractor....There is alot to go wrong with a setup like that,- equipment selection, duct sizing, comfort, eqiupment's life longevity and etc.. an ESP reading would be interesting on the setup that was suggested...

[Shophound]

18" flex run 30 feet long better be stretched tighter than a drum head or it will be overly restrictive on airflow.

This is the "got'cha" with flexible ducting. Even sized correctly, if it is installed loosely, the end result is the same as undersizing the duct...insufficient air flow.

[RA335]

how many scfm should i expect from a 2.5 ton after 30 ft of 18 in flex

[beenthere]

Besides how teh flex is ran, and connected at each.

That depends on the rst of your duct system also.

Should be around 1000 CFM all other things being equal.

Od course, I'm guessing by the info you gave.

But that 12" will NOT work.