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.
Life is like a jar of jalapenos. What you do today might burn your ass tomorrow.
"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