building and room pressurization with minimal controls

[sunsentinel]

Hello,

I need to fix an existing design for building/room pressurization.
Here is the current design:
A one storey building with a large room, a small room and a mechanical room. An outdoor filtration unit with a constant volume supply fan and preheat coil supplies OA. An indoor AHU with a constant volume supply fan, heating coil, DX coil and RA inlet provides SA and sits in the mechanical room. RA is ducted and OA and RA mixes before the AHU (no RA or OA damper) and MA temperature is monitored. Building air relieved through the mechanical room, unducted, using a transfer grille, with a wall mount counterweight damper set at +0.1" with respect to outdoors. This is supposed to do the pressurization for the large room. The small room needs to be +0.2" with respect to outdoors but there is only supply and return ducts into/out of this room (no damper on RA). Return air is picked up from large room first then the small room.

1)Will large room pressurization work without RA/OA dampers?(at a minimum)
2)Any problems mixing RA an OA without dampers? (stratification)
3)If I throw a RA damper to the RA ductwork in the small room, controlled by a pressure differential transmitter at a minimum (pressurization does not need to be guaranteed when doors open), will pressurization in the small room be satisfactory?

All ideas are appreciated!
Thanks!

 

[trashcanman]

1)Will large room pressurization work without RA/OA dampers?(at a minimum)
--Doubtful. How is a Test and Balance Contractor supposed to obtain the air flow rates shown without the use of dampers?
2)Any problems mixing RA an OA without dampers? (stratification)
--Yes. See above.

3)If I throw a RA damper to the RA ductwork in the small room, controlled by a pressure differential transmitter at a minimum (pressurization does not need to be guaranteed when doors open), will pressurization in the small room be satisfactory?
--Also doubtful. You are trusting the building envelope (large mechanical room) to be an air tight duct. Since you say it is an existing building, you don't know how it was constructed.

I do not know how you can properly balance such a system without ducting the supply, return and outside air flows, with proper balancing dampers.

 

[Waramanga]

Shutting down the RA wont do anything as far as i can see. the only pressurization you get is from the OA system so if that is running at max then not much you can do without serious changes.