[BLDG-SIM] Building Simulation in the U.S.

Daniel H. Nall dannall at mindspring.com
Mon Jun 11 06:58:06 PDT 2001

A couple of thoughts for this discussion.  These thoughts are intended to
be arbitary, argumentative and discussion provoking.
1.  The primary failing I find with most simulation programs is that they
don't allow you to put systems together and to control the systems in
exactly the way they are designed to go together and be controlled.  This
failure may be because the program developer was not familiar with design
practice either in general, or in other regions of the country or the
world, or because design practice changes.  In general, this failure is due
to the fact that the programs are insufficiently flexible to allow systems
to be configured in a variety of ways. 
2.  There is often confusion about sensitivity and accuracy.  We need to
recognize that we are involved in analysis that has an overall accuracy
that may be plus or minus 10% or so.  The most important characteristic of
our programs, however, is sensitivity.  We need to be confident that, while
the analysis may not give us credits or debits to within hundredths of a
percent, it will give us the right credits or debits of the approximate
magnitude, for all the things we want to test. Furthermore, we need to be
confident that the interactions among the various components are evaluated
at approximately the right magnitude.
3.  The interface issue, while important, shouldn't be overstressed.  Until
we incorporate significant amounts of artificial intelligence into the user
interface, an uninformed user will still produce an uninformed analysis.
Building simulation is no different from any other GIGO situation (garbage
in, garbage out).  Some people have spent entire careers at building
simulation.  No current user interface technology can enable a novice user
to extract the value from a simulation program than can be extracted by an
4.  All of the current building simulation programs (with the exception of
thermal network ESP-R derivatives) assume a well-mixed air situation in
each thermal zone.  As long as that assumption holds, there is little
benefit to evaluating the differential radiation transfer among the
surfaces of a zone.  In very few buildings, do the external walls see one
another. If no external walls see one another, and if the zone is assumed
to be well mixed, assuming a combined radiation/convection heat transfer
coefficient of the interior surface of the wall has little penalty in terms
of accuracy, because the interior partitions seen by this external wall
will be very near to the temperature of the well-mixed room air.  If
vertical air nodes are assumed within the zone, to accommodate
stratification, then the individual interior and exterior partitions must
also be vertically divided so that the surface temperature variation over
the vertical expanse of the partitions may be evaluated.  Quickly the
computation overhead for this capability approaches CFD magnitude.
5.  Finally, the very best simulation experts do design, either for new
buildings or retrofit.  Not only does the design process inform the
simulation process, but learning from the simulation process makes superior

Daniel H. Nall AIA, P.E.
Senior Vice President
Director of Advanced Technology
Flack + Kurtz Inc.
475 5th Ave New York, NY 10017
(212) 951-2691 fax (212) 689-7489
Email:       daniel.nall at ny.fk.com

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