Building Pressure in a Plenum Return System
Building Pressure in a Plenum Return System
(OP)
I'm having trouble wrapping my mind around building air pressure in a plunum return system. Let say that I am trying to positively pressurize a building in order to keep out infiltration. To do so, I am bringing in more outside air throught my AHU than I am exhausting.
But at some point in the plenum as I get near the return duct shaft, isn't my pressure negative to outside? I have a 2 story building with the AHU in the penthouse. Both floors are plenum ceilings with a return duct shaft that pulls from both plenums and goes directly into the AHU.
So am I really positively pressurizing the plenum with repect to outside? Am I really keeping out infiltration, or am I actually pulling it into the plenum with the return fan?
But at some point in the plenum as I get near the return duct shaft, isn't my pressure negative to outside? I have a 2 story building with the AHU in the penthouse. Both floors are plenum ceilings with a return duct shaft that pulls from both plenums and goes directly into the AHU.
So am I really positively pressurizing the plenum with repect to outside? Am I really keeping out infiltration, or am I actually pulling it into the plenum with the return fan?





RE: Building Pressure in a Plenum Return System
RE: Building Pressure in a Plenum Return System
RE: Building Pressure in a Plenum Return System
RE: Building Pressure in a Plenum Return System
Therefore Outdoor air = Exhaust Air + Pressurization Air.
Rule of the thumb pressurization air = 0.05 x Sq. Ft. Floor Area.
If you have presurization air then the net is there is no infiltration in theory. However actually because of wind, the windward side is at positive pressure but the leeward side is at negative pressure. Then there is also stack effect for tall buildings. During winter, the top floor is positive but the bottom floor is negative. That is why we put 10% safety factor.
RE: Building Pressure in a Plenum Return System
If you are maintaining +0.25" pressure in the controlled space then pressure at inlet to the plenum should also be +0.25". Any other pressure contradicts the maintained room pressure.
Once the air enters the plenum at positive pressure, you should check the pressure gradient in the return system. It may happen in the plenum itself or somewhere along the return duct.
RE: Building Pressure in a Plenum Return System
My overall concern is will the return opening in the ceiling plenum pull in infiltration even though in theory I'm bringing in more outside air than exhaust in order to positively pressurize the space.
RE: Building Pressure in a Plenum Return System
RE: Building Pressure in a Plenum Return System
Example, motels pressurize a corridor with a 100% outside air unit and utilize a steady exhaust from the bathrooms of the hotel rooms.
The localized negative in that hotel room, 'pulls' just as hard on an exterior wall/window frame as it does a corridor door frame or transfer grille.
I think the best pressure relief is a barometric damper on a roof top unit.
Down here I do not worry about freezing and it does not bother me to make the GC adjust all the door closers. A year round positive pressure is desirable here.
Take the "V" out of HVAC and you are left with a HAC(k) job.