[bldg-sim] VAV control of underfloor air

[Kirchhoff, Eric]

Although I have always been interested in designing such a system I have yet
to do so.  However, that doesn't seem to stop me from adding in some
anecdotal information from friends who have designed such systems.  First
off (not from friends, this is my own observation), a VFD for an air handler
tends to be nearly the same cost as a starter, except for the additional
control point-that being the differential pressure sensor.  Additionally, to
keep costs down, I would let the VFD control the situation, not the EMCS.
Just have the VFD update the EMCS, and allow the EMCS to reset the
differential pressure sensor.  Go ahead and monitor all the points you want,
and at least do start / stop, status, and DP sensor reset.  Like Robert
says, the VFD is a cost wise investment, even if you end up later on setting
it up to be a constant speed system, the cost risk is low.

Now I would like some additional information.  The IRR of 30% to 50%; is
that just for the mechanical investment or everything?  Since this type of
system uses a raised floor system, all the utilities end up running under
the floor.  Code will dictate the use of plenum rated cable (even when run
inside conduit), the floor system itself tends to be proprietary-or at best
limited in number of suppliers--hence supply and demand comes into play
driving the cost of the floor up and that is beyond the fact that it is
already more expensive since it needs to be structurally capable of carrying
the loads (including areas for very large filing systems), there are a
limited number of suppliers of underfloor vav systems (and the quality
varies), as a positive there is a greatly reduced amount of sheet metal used
for ductwork, there usually is not a realistic coordination between cubical
/ office layout and diffuser placement (in large jobs, the eventual leasee
is unknown hence their layout needs are unknown) causing problems by a
maintenance staff that is unfamiliar with the system (despite training), and
another maintenance problem are users who do not understand nor properly
utilize the system hence they will block the diffusers (more often then the
overhead type).  Plumbing may also be run in the floor space as well as
other utilities causing a need for additional wall furrings then may be in a
more traditional layout. Was there any account of these items in the IRR, or
was it a theoretical value based only on the mechanical components?

I do not mean to discourage anyone from using this system, I actually am
very interested and excited to implement this type of system, and believe
(instinctually) that it is an energy efficient alternative to the
traditional diffusion method of airside systems.  The points listed above
are things you should be aware of, and are questions to ask so as to
minimize their impact.

As Rob says Good Luck, and have some fun!

Eric Kirchhoff, PE

-----Original Message-----
From: Robert Lord [mailto:rgl.lsbris at lincolne.com.au]
Sent: Sunday, November 03, 2002 2:55 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV control of underfloor air

A typical design is to zone the floor compartment according to aspect and
allow the individuals to adjust the swirls.
There are only a handful of large VAVs and the fan speed is varied according
to duct static pressure (not plenum pressure).

In most instances, I have found that the internal rate of return is between
30% & 50% and hence is a good investment. I am sure VSD pricing and energy
pricing varies but a point to note is that the RH control is improved.

Good luck!

Rob Lord


-----Original Message-----
From: Molinini, Louis [mailto:lmolinini at syska.com]
Sent: Saturday, 2 November 2002 8:37 AM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV control of underfloor air


Do any of you out there have any experience designing an underfloor air
system with VAV control.  The particular system I am looking at is a plenum
based system with manually adjustable swirl diffusers (Titus/Krantz).  I am
curious if it ends up operating like a constant volume system or is it worth
the expense of installing the freq drive and the Static pressure sensor or
differential pressure sensor.  Any help is greatly appreciated.
Thanks,
Louis

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