[Bldg-sim] Load Calcs with Energy Simulation Software

Dan Katzenberger dan at eeEnviro.com
Fri Jan 25 22:09:00 PST 2008

There are several differences between load calculation and energy simulation
programs, some of which others have mentioned.  Some other differences
Summer load calculations are done at the worst summer design conditions
(outdoor air temperature, outdoor humidity, solar gain, etc.), and all of
the internal gains are assumed to be on 100% (e.g., people, lights,
equipment, etc.).  Energy programs use schedules and may not have 100% of
the internal gains on during the peak summer conditions, thereby suggesting
smaller equipment sizes.
Winter load calculations are done at the worst winter design conditions
(outdoor air temperature, nighttime, etc.), and the internal gains are all
assumed to be 0% (e.g., no people, no lights, no equipment, etc.).  Energy
programs use schedules, and may not have 100% of the people, lights, and
equipment off during peak winter conditions.
Some load programs allow the user to add a safety factor to the heating and
cooling loads (not just the equipment size).  So the heating/cooling load in
your zone is increased, therefore your air supply cfm is increased, your AHU
is increased in size, your plant size is increased, etc.).  I don't know how
to do this directly in any of the energy analysis programs that I use.
Because of these and other differences, I have found that energy modeling
programs will give smaller equipment sizes than do load programs.  Engineers
typically error on the side of safety, and therefore they prefer to size
equipment for the worst possible scenarios as described above.  When I was
designing building mechanical systems, I would never count on 25% of the
lights being on at night when it's -20 degrees F outside so that I could
reduce my boiler size.  My loads program would give me a larger boiler,
whereas my energy modeling program would give me a boiler sized as if the
lights will be on.  Also, the local TMY2 weather file that I use for energy
modeling doesn't even have -20 degrees F as an outdoor air temperature, and
this is the temperature that many engineers use to design their heating
systems.  M loads programs allow the outdoor design conditions to be input
Load programs require much less input than energy programs, and generally
don't require any special knowledge, art, or workarounds.  Energy modeling
programs require much more input, require very specialized knowledge and
experience, and always require workarounds and creative inputs (art) to get
the correct results.
All that being said, Trane's Trace 700 program does an acceptable job of
being both a loads program and an energy analysis program, although it does
have strengths and weaknesses in both areas.  In Trace, when running loads,
you can disable the energy related inputs.  After you are satisfied that you
have the correct loads, then you can proceed to working on the energy
parameters and inputs.  You can easily switch between running loads only and
loads + energy at any time.
Daniel A. Katzenberger, P.E., CEM, LEED-AP 
Principal & Certified Energy Star Green Building Engineer
Engineering, Energy, and the Environment, LLC
"The Green Group"
420 North 5th Street, Suite 305, Minneapolis, MN 55401-1348
(612) 327-4210

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jonathan Curtin
Sent: Friday, January 25, 2008 3:25 PM 
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Load Calcs with Energy Simulation Software

Hello All,
Would anyone out there like to share experiences or precautions in using
simulation models (particularly eQuest, DOE2.1e, EnergyPlus) for load
calculations? Any advantages/disadvantages, tricks, or warnings in using
these simulation models as opposed to dedicated load calculation software
like Elite's CHVAC? Any studies comparing the two?

Jonathan Curtin EIT, LEED AP


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