[Bldg-sim] Load Calcs with Energy Simulation Software

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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?

Thanks,

Jonathan Curtin EIT, LEED AP

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Jonathan-
Greetings. I have a little experience in CHVAC and slightly more in eQuest, and here is my $0.02 worth.

If your objective is sizing equipment, developing detailed zone loads for system and equipment sizing, CHVAC is, in my opion the much better way to go. I found it easy to set up zones, create systems, and the output is very well oraganized in terms of providing the design engineer with the information they need.

To try and do the same thing in eQuest is much more difficult and time consuming than CHAVC. Furthermore, the program inputs outputs are focused on energy use. The system sizing information is not as easy to locate and discern as in CHVAC.

Again, bare in mind I am new to both these programs.
This forum (Bldg-sim) will get you many other worthy opinions, I hope mine helps.

Regards and good luck.
W. Peter Anderson, P.E

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Trane's TRACE700 and Carrier's HAP4 Energy programs can also be used independently for loads calculation. The DOE2 based energy programs can be used but they are clumsy for loads.
Automated Procedures for Engineering Consultants (APEC) first released its Loads Program called Heating & Cooling Calculations (HCC) in 1966. It has been updated over the years until around the early 1990s. The last version is called HCC-V version 3.0. HCC-V has a forms (not MS Windows) interface and is easy to use. In my opinion this program is still good enough for design loads calculations which is used to size ducts, pipes and equipment.
APEC was a consortium of M-E and A-E design firms that funded the development of M-E software. Around 2004 APEC folded up. I bought out the APEC Loads, Ducts & Pipes programs as mementos-souvenirs since I was involved in fixing bugs and providing customer support in 1969-70 - my first career job out of college. The programs come with good supporting documentation. The Ducts and Pipes programs are DOS based, and the network has to be defined with numbered sections. They are not worth using since such programs should be CAD based.
There are plans to put up the APEC HCC-V program for free download on a website.
The problem is the program comes on two 3.5" diskettes. Using a diskette drive (Drive-A) you insert Diskette-One and run "setup.exe" and then you get a message saying insert Diskette-Two into Drive-A. I cannot get the program to install from Drive-C or from the CD drive by combining the the contents of both diskettes into one folder and running "setup.exe" because it still asks yo to insert Diskette-Two into Drive-A. I tried sending the fully installed program to another computer but there is an error message (DLL) when you try to run it from the other computer.
Does anyone know how to fix this problem? I can send you the contents of the two diskettes in two folders. Those who have 3.5" diskette Drive-A (about $40 for an external drive) can copy the folders into two diskettes and install from Drive-A. There are plans to put up other information (for educational purposes) on the website for free download. This will include project case studies using DOE2.1E, eQUEST and other programs.
Varkie

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Hi Jonathan,

I also recommend using a good load calculation program for calculating the
loads. Elite CHVAC seems to meet the criteria well and not be so
complicated that errors get introduced merely from confusion. Load
calculation programs are designed for a different purpose than energy
modeling programs. The design intent is to estimate the load on the
building at specific weather and internal conditions and be reasonably close
to the worst case load at those given conditions for the purpose of
selecting equipment. An energy modeling program is designed to give results
that match the energy consumption of the building and thus is more focused
on a dynamic situation. They might use TMY weather data that doesn't match
the % load exceedance that would be the goal of a load calculation (the
ASHRAE 97-1/2% and 2-1/2% conditions for example). Often the program is
tweaked to get the result to match existing utility bills, or to model a
system that it was not programmed to handle. There are a lot of dynamic
variables that cannot be precisely defined, such as occupancy patterns, mass
influence, lighting and equipment loads and schedules, and so on. The
energy modeling programs thus need to be more complicated, and therefore are
more susceptible to errors from confusion, lack of systems understanding and
experience regarding inputs, or programming errors not yet fixed. When
drawings are imported from a CAD program, there is even more potential for
input error as the black box tries to put that into the appropriate format
for a load calculation.

I would certainly be great to have programs that we have more confidence in
to perform both energy and load calculations. I'm hopeful that soon that
will happen. We sure could make good use of tools like that.

Michael Haughey, P.E., LEED AP

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Jonathan,

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

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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
on-line.

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

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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
conditions
4) Schedules (occupant, lighting, equipment, etc.) for design conditions
and average representative conditions typical used in models are
different
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
credits.

I have only spent little time looking at this in detail, and this
discussion has definitely been of great learning experience.

Thanks!

Jonathan Curtin EIT, LEED AP

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Demba Ndiaye's picture
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Good discussion on this topic.

One final point:
Most simulation programs assume that the building is heated up/cooled
down within the first hour of plant operation. This leads to high
predicted loads during that first hour that would not occur in reality.
(in cold climates a 2 hour warm-up on Monday morning is not unusual)

So, it may be useful to filter out the first hour loads, and then
examine the distribution of the remaining loads to pick off the 97.5%
value.

Cheers, Steve.

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