If anyone can answer this it would be great...Im designing an reducing extended plenum system....21/2 ton heat pump airhandler....Most 21/2 ton unit blowers are pushing 1000cfms which means the equivalent 8 inch duct would be 22x8 to start with off the supply end of the handler...Heres the prob...In this particular goodman unit 2.5 ton......Im using a 20kw heatstrip.....which by manufact....requirements, i must run the blower in the high setting which technically can raise the cfm to around 1400cfm...depending on static pressure....so this in turn means that that orignal 22x8 duct would have to be raised to 28x8 for 1400cfm...Im a novice thats installing the duct system myself....I dont want the the duct to start out to big, because with 30,000btus(2.5) the required size of duct is 22x8 but if its sized to 28x8 the heat would dissipate some within that large of an air volume, plus register output may be lacking and for opposite reasons i dont want the trunk to small....I dont know what my static pressure would be...The trunk will be 28ft metal duct and Ill bve reducing down as I go....There will be 11 branch ducts,,,,should I start w/ 22x8,,,26x8(in the middle) or 28x8 Thanks u.Mike.
If your fan coil unit is 2 1/2 ton, the blower itself on high speed should only produce 1000 CFM @ .50" static pressure. A 22" x 8" duct is going to be about 400 CFM short of that capability. You need to be somewhere in the realm of the equivalent of a 16" pipe to carry your velocity @ 700 FPS. A 20" x 12" will accomodate this if you are limited on size. Hope this helps you with your calcultaions. There should be some of the other guys on later, get their input also.