PPD Plenum System



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    David Kaplan


    This is more commonly called a "return air plenum."  Essentially, the space above the ceiling system serves as one, giant "return air duct."  There is actual supply ductwork which supplies all air to the spaces below the ceiling.  But above the ceiling, there is only a large return duct that comes from the bottom of the air handler and just sort of "stubs in" the plenum.  Think of that duct like a large air vacuum.  The fan in the air handler sucks all the air from the spaces into plenum space, and it eventually makes its way over to this return duct stub and back to the air handler.

    This type of design is very, very common, but there are code implications.  For one, the other utilities and construction items that are in that plenum have to be "plenum rated."  You may have heard of "plenum rated cable."  What it essentially boils down to is that the items in the plenum must not contribute to flame and smoke spread, meaning, if they should catch fire in that plenum space they have a flame spread index and smoke developed index that is within code-accepted limits.  Even items like batt insulation fall into this category as well.  

    Hope that helps.

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    Rebekka O'Melia (Edited )

    What do you mean exactly?  The plenum space is typically below the structure, and just above ceiling tiles.  Ducts can be placed in this area. The space between floor joists can also be utilized.  Sometimes the joists can be fabricated into a return air space even.

    And open plenum space can be used as return air.

    Hope this helps.

    Rebekka O'Melia, B.Arch, M. Ed, Registered Architect, NCARB, ​​Step Up ARE Coaching​​​

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    Michael Ermann

    To reduce the cost and building volume associated with return air ductwork, in open-plan offices we often run a single large return air duct into the "plenum" space above the dropped ceiling and below the structural deck. If a duct is analogous to a stream, a plenum is analogous to a lake. The entirety of the plenum is then suctioned so that a return grille can be placed anywhere in the grid and suck air back to the AHU through the plenum. See an example here. Or a video here.

    In "displacement ventilation" systems, we also use a plenum for supply air. See here.

    Displacement ventilation: mechanical cooling, supplied near the floor, with a little-bit-cold air brought in at not-very-fast duct velocities. This displaces the warmer room air near the floor and pushes it upward. Grilles in the ceiling suck out the warmer return air (also slowly). Because our skin is warm, naturally convective plumes form where the people sit, drawing warm air up toward the ceiling grille and replacing that air with the colder pool of heavy air near the floor.

    Benefits: uses less energy (smaller & slower fans, 65 degree supply air instead of 55 degree supply air, more hours available for the economizer cycle "free cooling" because 65 degree outdoor air can be brought directly into the space); is quieter (slower fans); and provides superior air quality (the stalest room air is the warmest, so the stale air hovers at the ceiling near the return grille, where it can be filtered and exhausted outside).

    Limitations: only works in rooms with high ceilings (minimum 9'); doesn't work with heating (so you'll need radiant baseboard heating for winter); doesn't do as well in humid climates (cooling air to 55 degrees removes more of the room humidity as condensation at the cooling coil than cooling the air to only 65 degrees); with high cooling loads, occupants can feel uncomfortable (with cold feet and warm head).

    Used for: high-occupancy rooms (lots of people so lots of stale air to be removed); theaters (tall ceilings, need quiet); cooler climates (popular in northern Europe)

    For an excellent video on the subject, see here. You don't need to watch for more than a few minutes if you are in a hurry. . . you'll get the point quickly.

    This theater, below, uses displacement ventilation. Blowing cold air from the high ceiling hard enough to mix with the air at the orchestra level would necessitate the kind of ducted air velocities that always produce too much noise. Instead, air is ducted to a concrete plenum in the sloped "leftover" wedge-shaped space beneath the seats. Air then seeps up slowly from the pressurized plenum below the seats, like this.

    Below the seats the ducts pressurize the plenum. Note the bored holes in the ceiling:

    Those holes hide beneath the audience seats.


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    Natasha Graves

    Thanks everyone! This was helpful. I've had questions on practice tests that differentiate between a plenum return and an HVAC return system. I had a general idea that it was just ductwork that was the biggest difference but when trying to find material to confirm my thoughts ended up at a dead end. 


    Thanks again!

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