Underfloor air - more questions

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We have this historic building that we are renovating and are
considering underfloor air distribution. The space is a high bay space
(around 50 feet tall) that will be occupied by students with computers.
The design currently has an access floor 6" above the existing floor. We
are thinking of raising it a little more and running the supply air
through underfloor ducts (we can't do a plenum because of the condition
of the existing floor).

As far as I can tell the energy benefits of the underfloor air system

1. Higher supply air temperature (68F)

2. Lower static pressure

3. Stratification (lights, equipment and people heat stratifies
above the return air grills.

The alternate is a supply duct that throws air across the space (60 feet
wide) from a height of 20 feet.

So my questions are:

1. If the UFAD goes through ducts (as opposed to a plenum) is the
reduced static pressure realistic? How much would you expect to see it
reduce by?

2. How much stratification can you expect here? I assume there is
a radiant portion from these things in addition to the convective
portion. Would a 50% reduction in the sensible loads be realistic? I
know the normal procedure would be to assign the rest of the loads to
the return plenum, but it this case the return will be ducted at 8',
while the warm air will be allowed to plume up to 50. Does that change

3. I assume I should model the upper part of the zone as a plenum.
In which case - does the upper plenum start at 20' for the alternate
system (spiral duct at 20'), and at 8' for the UFAD?

All responses are appreciated


Vikram Sami, LEED AP

Vikram Sami's picture
Joined: 2011-09-30
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Check this document:

Chris Jones

Chris Jones2's picture
Joined: 2011-10-01
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Also, do not forget that since you are now able to supply the air at a
higher temperature you can set your outside economizer at a higher
temperature as well. Alot of the savings from displacement type air
distribution systems come from the increased outside air economizer use.
Good luck.

Steve Mignogna

Steve Mignogna's picture
Joined: 2011-09-30
Reputation: 0