Building Pressure in a Plenum Return System

[BronYrAur]

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?

 

[imok2]

Well you may want to see if the ceiling is sealed by using a T-T Smoke Stick and T-T Smoke Gun also you can use a digital manometer and see if the pressure is about 350-450 Pascals

 

[vannic]

The reason for positively pressurizing a building is to keep air from infiltrating into doorways, windows, openings and into the building. The plenum is negative with respect to the space and to the environment. The thought is that the building construction is completely tight (or tight enough) where infiltrating through the building skin/roof is of no consequence if it occurs at all.

 

[simsd]

Are you using a return fan in the units?

 

[lilliput1]

Supply Air = Exhaust Air + Return Air + Pressurization Air = Outdoor Air + Return Air

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.

 

[quark]

Building pressurisation depends upon net outflow (for +ve pressure) and net inflow (for -ve pressure) of air. For a +ve pressure room, if the leak rate is 100 cfm, say, and the recirculation rate is 1000 cfm then supply air flow is 1100 cfm and return air flow is 1000 cfm.

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.

 

[BronYrAur]

Thanks all for your input. To answer simsd's question, there is a return fan. The is no return ductwork to speak of other than a vertical drop from the AHU and a take off for each floor. The duct penetrats the wall into each plenum and stops. There is no return ductwork above the ceilings which is why I'm confused about the pressure. Is the ceiling plenum pressurized? I'm thinking so, but at some point as you get near the return opening, the pressure must be negative.

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.

 

[lilliput1]

If you return less than what is supplied and if you balance the return by means of adjusting volume damper in the return air duct, the return air plenum would be positive with regards to the outside. It would be the portion of the return duct downstram of the balancing damper that would be negative.

 

[AbbyNormal]

Air will take the path of least resistance.

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.