[Bldg-sim] Load Calcs with Energy Simulation Software

Jonathan Curtin jcurtin at WylieAssociates.com
Tue Jan 29 06:41:04 PST 2008

Thanks Michael and all who replied.
The responses generally told me what I expected to here, that most
simulation software was not to the point yet where people were
comfortable in using it for load calculation and system sizing, with the
exceptions of Trace700 and Carrier's HAP4 as some have stated.
The following issues seemed to resonate strongest from the responses as
to why not to use software like DOE2 or eQUEST for sizing equipment:
1) Difficulty in setting up model (complicated, clumsy, more susceptible
to errors) and cumbersome organization of output
2) Model zoning generally combines similar HVAC zones and does not
require detailed zoning usually required in load calculations
(simulation models too detailed and will take too long to run)
3) TMY weather data is generally not representative of ASHRAE design
4) Schedules (occupant, lighting, equipment, etc.) for design conditions
and average representative conditions typical used in models are
5) There is optimism that software advances will help improve
capabilities in the future
For my $0.02:
1) I agree that for novice (or even many intermediate) users, you would
be asking for trouble if relying upon the results of a simulation model
for sizing equipment. Though your chances of success improve as you
learn to check, and double check the output, and interpret results.
2) Getting into a hundred+ zones in a building could be difficult to
manage, but for a Core and Shell building with block loads it should not
be excessively burdensome
3) eQUEST does allow for the creation of Design Day weather conditions
(directly adding to the .inp file is the only way I know how to do it
but there may be another way)
4) eQUEST also allows for Cooling Design Day schedules and Heating
Design Day schedules (again by adding to .inp file)
5) I too look forward to advances in the software that can take the
detail that goes into a model and gain more from it that just a few LEED
I have only spent little time looking at this in detail, and this
discussion has definitely been of great learning experience.

Jonathan Curtin EIT, LEED AP


W  Y  L  I  E 
C O N S U L T I  N G
E  N G  I  N  E  E R S

6161 Savoy, Suite 700   Houston, Texas 77036
713.781.2526     713.781.2536 fax

www.wylieassociates.com <http://www.wylieassociates.com/> 


From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Michael
Sent: Saturday, January 26, 2008 01:31 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Load Calcs with Energy Simulation Software

Hi Dan,


I would clarify two things.  First is that the load calculations are
done, or should be done, for the design condition, not the worst
possible condition plus safety factor.  For cooling that normally means
the ASHRAE 97-1/2% condition; for heating, the 2-1/2% condition.  Some
programs list adjustments as safety factors, but there is really a
higher purpose.  One is a morning warm-up factor that allows night
setback energy savings to be realistically achievable.  Similarly, a
cool-down factor can be applied to the cooling load and comes into play
often on Monday morning when the system has been off over the weekend
and the building mass has gotten quite warm.  If you can't cool it down,
then you can't save as much energy by letting it get or stay warm for as
long prior to occupancy.  The warm-up and cool-down factors are largely
a function of the space mass (the greater the mass, the larger the
factor needed).  There is also the possibility of either adding capacity
for future building or process load additions or designing for the
ability to add the capacity in the future.  It is best to do that in a
modular way the does not penalize efficiency until the future load is


Second is that load programs do have schedules for people, lighting,
appliances, and power equipment.  It is proper to set those
realistically for the highest anticipated conditions, but not higher.
If set higher, or at 100%, then the result will have compounding
additional capacity (safety factors multiplied upon each other resulting
in grossly over-sized systems).  Heating is traditionally treated
differently in that internal loads are normally not left on for the
design condition.  It is possible to turn off all lights and internal
gains during a heating requirement.  Cooling is a little different in
that some loads are simply never all on at the same time.  People move
throughout the building and they also come and go, so diversity factors
are also appropriate.


I would expect, or at least want, a program that does both load
calculation and energy modeling to be able to account for morning
warm-up and cool-down (including after a power outage) by sizing the
systems to have sufficient reserve capacity to recover in a reasonable
period of time (and have that time adjustable).  It would need to
consider building mass, and look at setback/setup differentials vs.
recovery time and reserve capacity.


Michael Haughey,P.E., LEED AP

Silvertip Integrated Engineering Consultants


mhaughey at earthlink.net


From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Dan
Sent: Friday, January 25, 2008 11:09 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Load Calcs with Energy Simulation Software




There are several differences between load calculation and energy
simulation programs, some of which others have mentioned.  Some other
differences include:


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 directly.


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
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


W  Y  L  I  E 
C O N S U L T I  N G
E  N G  I  N  E  E R S

6161 Savoy, Suite 700   Houston, Texas 77036
713.781.2526     713.781.2536 fax

www.wylieassociates.com <http://www.wylieassociates.com/> 

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