Room Pressurization from existing AHU's

[nevinjohn]

Consider a scenario, where I need an additional air of 500CFM to maintain "XX Pascals" positive pressure in the room. The AHU capacity is 3000 CFM, (2500 as per the heat load and the additional 500CFM to maintain the pressure) and the return duct is sized so as to take n 2500CFM. The rest 500 shall be suked in from the Fresh air duct by the AHU blower. Will this work??

Or, should I install aditional fan for this 500CFM air??

Please have a look at the attached pdf.

[Brian_UK]

Yes that will work providing that you balance the airflows of the return air and fresh air correctly.

[install monkey]

go bigger as you have to allow for air leakage through doors,filters clogging and possibly fume cupboards, drains on sinks etc

maybe stick a fan on the fresh air to bring it direct into the room instead of the return duct

[nevinjohn]

What if the AHU is inside the room, and there is no return air duct. Only the fresh air duct is sized according to the fresh air volume required, but this is also not "connected" to the AHU, as the AHU doesnt have any mixing box. The Fresh air duct stops neat the return air filter. In this case, do you think the AHU will suck air from the fresh air duct, instead wont it just suck what ever is available near the return air filters...i.e just recirculating the room air instead of drawing in fresh air?

[Brian_UK]

What if the AHU is inside the room, and there is no return air duct. Only the fresh air duct is sized according to the fresh air volume required, but this is also not "connected" to the AHU, as the AHU doesnt have any mixing box. The Fresh air duct stops neat the return air filter. In this case, do you think the AHU will suck air from the fresh air duct, instead wont it just suck what ever is available near the return air filters...i.e just recirculating the room air instead of drawing in fresh air?

No, absolutely NO CHANCE.

Air movement into and out of sealed rooms requires fans, always has and always will do. Bite the bullet and do a proper job on it. If the client won't pay it then walk away.