Transfer duct sizing 1

[DuctHunter]

What's the fastest velocity a return air transfer duct should be sized for. Or better yet how should transfer ducts be sized in general. If I have a central Roof top unit serving a three story building with a return air plenum and all the transfer ducts going from tennant space to tennant space are undersized how can this effect the performance of the return fan ?

Thanks for your help.

 

[NCDesign]

When you don't have a ducted return your 'designed' return air path competes with all the doors, leaks, and assorted pathways that might be easier for the air to go than the path you want it to. If the transfers are undersized the air will try and find another path back to the unit. The limit on this is the total static pressure and HP of the fan. Some common side effects of undersized transfers are doors that fly open or shut, doors that are very hard to open and shut, and very noisy return grilles closer to the unit's return pickup. My general rule of thumb is to size transfer openings at about 300 FPM. I found that anything greater than that and the air wants to go through doors, hallways, elevator shafts, etc more than my desired unducted return air path. Anything lower and the opening just gets way to big.

 

[mechanicaldude]

What a great answer. Thanks, that's exactly what I was looking for.