Building pressure relief

[tikitime]

Typically, I've been designing HVAC systems where the return air is about 80 - 90% of the supply air so I have not been worrying about building pressurization. However, now I have a church with a vav ahu and a significantly reduced RA flow. So I have a few questions:

1. At what point do you want to start looking at including some pressure relief into your design?

2. I'm located in Florida and most of the buildings I have seen do not have relief fans or exhaust fans for pressure control. I have seen relief dampers. Why would you choose a fan over a relief damper?

3. For my design I have a ducted return system. The limited mechanical room space is going to eliminate the fan option. So them am I left with a relief air damper and transfer grilles in every room?

Thanks for your help!

 

[BronYrAur]

Is your building makeup air system independent from your main VAV system? If not, your outside air intake will drop as the return fan throttles down.

One side note, are you following an occupancy schedule with your OA or controlling it by CO2 levels? A church typically has high occupancy for a very shorty period of time and then minimal occupancy for the rest of the time.