maximum length of extended plenum

[charles2]

Is it true that an extended plenum can only run about 24' before it loses pressure? Is the same true for a reducing plenum?

[cem-bsee]

one must consider the whole system! just like __ ton/sf! or hp/passenger

[hvac hero]

Originally posted by charles2
Is it true that an extended plenum can only run about 24' before it loses pressure? Is the same true for a reducing plenum?

I'm not sure what your really meaning by extended plenum. Are you wanting to add to your existing plenum? Plenums are usually around 2 ft if the furnace is in a basement or a attic, but a lot of furnaces around here are in closets in the hallway. These setups usually require a 5 ft plenum to get it above the ceiling joices enough to tie your duct on to. I dont think adding 2 ft to a plenum would ever be noticed.

[TLTrex]

I think this is what he is referring to.
http://www.rhvactools.com/techinfo/t...tendplenum.htm

[deejoe]

"extended plenums" have been around for perhaps than 40 years or more ( it was the method of choice in installing duct systems back in the 60's.)
As far as the 24 Ft being the limit, any extra distance can be obtained by installation of a 'tapered' fitting in order to maintain cfm/fpm.
See manual D

[charles2]

OK, thanks. You guys figured out what I was trying to ask.

[hvac hero]

Ahhhh after seeing the pic that the estimator had a link to I see whatcha mean. We call the plenum whats going up right there. We always call the square duct coming off of it a "trunk line".