Maybe I shouldve posted this in 'tech talk' but whatever...

Can a ME explain to me (in layman's terms) what a plenum air system is and how it works?

12/3/2007 11:34:40 AM

That's a pretty general question. Plenum basically means ductless. For example, in an office building the space between the ceiling tiles and the roof deck can be used as a return air plenum. The air returns to the rooftop unit through the plenum and not through a duct. This basically saves money b/c you don't have to pay for additional ductwork.

If that doesn't help, then give a little more info and I can help you out

12/3/2007 11:46:57 AM

Umm where is the plenum and is it plenum return or supply?



With the little bit of information you have given me it means that the air is blown into or sucked out from a large space and the pressure difference between those 2 spaces allows airflow between them at any place there may be a hole in the boundary between them.

Typically most hvac systems are plenum return meaning that the air is ducted directly into the space then there is a return grill going to the ceiling cavity where the return fan draws that air back to the A/C unit.

[Edited on December 3, 2007 at 11:49 AM. Reason : yeah more info needed.]

12/3/2007 11:48:40 AM

im not 100% sure what youre trying to find out. The difference between having a plenum outlet and a straight fan outlet on an air handling unit(AHU) is basically this:

The outlet for a system without a plenum will be ducted the exact same size as the fan outlet and be hooked up directly to the fan outlet.

The outlet(supply side of course) for a plenum setup means the fan blows the air out into another compartment, which has different ducts exiting from different holes in that compartment, meaning the air is sent out in different routes directly from the AHU box.


i added 2 CAD pictures to my gallery for you to sort of see a plenum setup within the AHU itself.


[Edited on December 3, 2007 at 11:51 AM. Reason : ]

12/3/2007 11:49:08 AM

here is a picture of an under floor supply plenum with an above ceiling return plenum.

12/3/2007 11:52:56 AM

since you asked for a 'plenum air system' ^ that is probably what you are looking for. but more info would be nice.

12/3/2007 12:06:59 PM

thanks for the responses...I was not looking for anything in particular, just a general overview of how they work, which I have gotten.

so these systems are cheaper because they are ductless....are they as efficient? i assume you have to use special cables in them (for low voltage).

12/3/2007 2:04:18 PM

A plenum return is cheaper because there is no duct but the combination i showed above which is a ceiling plenum return but an under-floor plenum supply is not because constructing the underfloor plenum usually costs more to construct than the duct would have cost. Also typically larger fan equipment is required because one would generally use air closer to room temp (than a typical ducted overhead supply system) in an underfloor system because the room would be very drafty with 55 degree air blowing up everyone's legs. This means a larger volume of air is required to cool the room thus more fan power and larger fans are required.

Also the supply air would pick up heat from any heat sources located in the supply plenum before it even made it to the room.

ohh and stuff run through the plenum will have to be plenum rated which basically means they do not burn as easily and when they do, they do not smoke as much.
By stuff i mean, wires, insulation etc.

[Edited on December 3, 2007 at 2:48 PM. Reason : .]

12/3/2007 2:42:54 PM