Main trunk sizing

[Marc46]

Angus,
Yes, that should work fine.
I didn't know it was in the middle of the trunkline. I have a better picture in my head of what your install looks like now.

Basically what you do, is size your trunk for your max CFM..........after you take off X number of drops, you minus the proper amount of CFM used, and step your trunk size down to the new CFM rating remaining.
This is done to maintain your air velocity,..........especially on long trunklines.

Only one suggestion. Since your furnace is sitting in the middle, and you are going to "split" the CFM per se,.............I would not just go straight up into a 12X8 trunk with your supply plenum. It really should be brought in with 2 nice "radius" bends if possible. That is a mighty small trunk, for 1200 CFM to just "slam" head on into.

As the other poster suggested,...........nowadays all of this is usually done on the computer, and several "manuals" are generated for the sake of permits.
I couldn't tell you how many BIG jobs that I designed back in the "old" days, using nothing but a pencil, and a handheld ductulator.
Funny thing is most of them worked as good, or better, than some of the computer generated layouts I have seen. Oh well,........times change!
Good luck!

[jadnashua]

The backpressure and turbulance of a right-angle bend is huge in comparison to a radius bend (dont' quote me, but I think it is something like 5x). Part way in between is to put an angled deflector vane in. Another thing often called for but often omitted, is to use a decoupling band to separate the ductwork from the vibrations of the air handler/furnace from the ducts. It is often just a 2" or so piece of vinal coated canvas connecting the two halves of rigid metal ductwork. This will go a long ways to isolate noise propagation into the ducts.

[angus6]

The backpressure and turbulance of a right-angle bend is huge in comparison to a radius bend (dont' quote me, but I think it is something like 5x). Part way in between is to put an angled deflector vane in. Another thing often called for but often omitted, is to use a decoupling band to separate the ductwork from the vibrations of the air handler/furnace from the ducts. It is often just a 2" or so piece of vinal coated canvas connecting the two halves of rigid metal ductwork. This will go a long ways to isolate noise propagation into the ducts.

I do know he spec'd in the decoupling band as to the transition from the plenum to the duct I'll have to check , I'm guessing the it's a angled deflector as that's what they used in some stuff the did for us at work

Once again thanks guys for the sharing of your knowledge