# Help me save my shop!! Can I put this size duct into my trusses?



## thinktankgroup (Feb 2, 2018)

Hello, new member looking for some guidance. We have a design worked out but I wanted to verify before I call my sheet metal guy. There has been some back and forth as to whether this will actually work or not.

The back story-

This is for a shop that already has a low ceiling at about 15' (even lower if you consider the electrical buses etc...). We need to place the duct work in the trusses in order to maintain the best headroom. These bar trusses will allow a 14” spiral duct to pass and and I am able to get a 10' piece into the web easily so installation isn't a concern.

An old timer on the job is convinced that we need to do a single 20+” duct per bay that will absolutely not fit into the truss work. This translates into a 10-11 foot high ceiling after it passes under all of the electrical buses, fire suppression, etc... He is convinced that 14" diameter is far too small. The owner is understandably not pleased with that solution.

The shop is a simple 120' X 120' square building with 3 bays that are 40'X120'. The plan is to place 3 rooftop units and serve the space with 3 'H' shaped patterns. So HHH is the layout. We have two 5000 CFM units and one 6000 CFM unit. The placement is so that 2/3rds of the building is served by the lower half of the 'H' and 1/3rd by the upper half. I hope that makes sense.

What I am hoping for is that we can do all of this with 14” duct and smaller. Obviously, the plenums will be much larger, but hopefully not when it crosses the trusses that are 80" OC. It can't sound like a jet engine but it doesn't need to be whisper quiet either. I just need as much vertical space as possible.

Our calculations show that for a 14” round duct should be fine for about 1500 CFM at a velocity of 1400 FPM. Our units put out a static pressure of 1.1.

My questions are:

Is this going to be too loud? Is there a solution we aren't thinking of? Do these calculations seem correct?


Thank you very much for any input!


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