ASHRAE Journal - July 2022 - 35

TECHNICAL FEATURE
TABLE 3 Extrapolation results.
We modifi ed the thermal
BUILDING TYPE AVERAGE OF ALL
THREE METHODS
(Btu/h)
BIM LOADS
SOFTWARE 1 BUILT-IN LOADS
TOOL (RTSM)
LOAD
(Btu/h)
Heavy
Light
8,266 7,980
8,362 9,313
% DIFFERENCE
FROM AVERAGE
DEFAULT METHODOLOGY
(TETD-TA1)
LOAD
(Btu/h)
-3.46% 8,689
11.37% 8,333
% DIFFERENCE
FROM AVERAGE
LOADS USING SOFTWARE 2
RTS (HEAT BALANCE) METHOD
LOAD
Btu/h)
5.12% 8,129
-0.35% 7,439
on the rigorous heat balance method described in the 2001
ASHRAE Handbook-Fundamentals.
The RTS (Heat Balance) engine also accounts for the
amount of solar gain lost through retransmission back
out of windows.8 Software 2 does not import construction
types from gbXML,9 so the thermal properties had
to be rebuilt in the software.
In each case the weather fi les were modifi ed to account
for the actual testing weather conditions and to force
the peak load to occur in September so the solar angles
matched the test day. Given the limitations on how
weather fi les can be entered into both Software 2 and
Software 1, we were unable to account for the slightly
cloudy morning of the test day, so both engines will
slightly overestimate the true solar radiation.
The results of the calculations can be viewed in Table 2.
Software 1's built-in load calculation tool, which uses
an RTSM method, overestimated the cooling loads in
the heavy building by 35.3% and the light building by
39.4%. This is approximately the amount of overestimation
from the RTSM methods used in RP-1117 conducted
by Dr. Fisher and Dr. Spitler. Software 2's default engine
overestimated the true tested loads in the heavy building
by 28% and the light building by 37.5%. Software 2's RTS
(Heat Balance) engine was the most accurate engine,
overestimating the heavy building by only 0.1% and the
light building by 14.2%.
While these may seem like overly conservative results,
remember that the test was set up to exaggerate the
inaccuracies of the RTSM. All results generally agree
with the results of the RP-1117 calculations.
Extrapolation to A More Likely Scenario
Now that we've validated the BIM load calculations for
the exaggerated scenario, we can remove the parameters
that exaggerate the difference between the RTSM and
HBM to determine what a more realistic scenario would
look like.
% DIFFERENCE
FROM AVERAGE
-1.66%
-11.04%
properties of the materials to
represent a 2012 IECC code minimum
construction for Oklahoma
Climate Zone 3A as well as added
internal loads indicative of a
modern offi ce environment,
including 340 Btu/h (100 W) of
lighting load, 170 Btu/h (50 W) of
plug load, and 275 Btu/h (80 W) of sensible person load.
We then calculated loads using 2017 ASHRAE climatic
design data for Stillwater regional airport.
As expected, the results from all three methods converged
to a much more reasonable difference. All results
were within about 12% of the average. The results can be
viewed in Table 3.
Conclusion
BIM load calculations can be comparable to traditional
methods of calculating loads. Using BIM to calculate
accurate cooling loads faster and with fewer errors is
a viable path for the industry that can improve profi tability,
help retain talent and allow better systems to be
designed.
References
1. Bruning, S. 2012. " Load calculation spreadsheets: quick
answers without relying on rules of thumb. " ASHRAE Journal (1).
2. gbXML. 2022. " gbXML-About. " https://tinyurl.com/bdd8kxc4
3. Roth, S., S. Bruning, K. Gowri, D. Crawley. 2017. " The Current
State of the Art for an Automated Pathway from BIM Building
Models Direct to HVAC Heating/Cooling Load Calculations. "
ASHRAE Seminar Recording, 2017 ASHRAE Winter Conference.
4. Fisher, D., J. Spitler. 2002. " Experimental Validation of Heat
Balance/RTS Cooling Load Calculation Procedure. " ASHRAE
Research Project RP-1117, Final Report.
5. Rees, S.J., J.D. Spitler, P. Haves. 1998. " Quantitative
comparison of North American and U.K. cooling load calculation
procedures-results. " ASHRAE Transactions 104(2)36 - 46.
6. Autodesk. 2021. " Heating and Cooling Calculation Methods
in Revit MEP. " Calculation Section, p. 9. Autodesk. https://tinyurl.
com/43dacnnv
7. Trane. 2021. " Trace 700 Building Energy and Economic
Analysis User's Manual, Version 6.2. " Program Overview, 2.1. Trane.
https://tinyurl.com/2p8vy869
8. Trane. 2021 " Trace 700 Application Notes. " Trace 700 Help File.
Trane.
9. Bohnert, J. 2019. Email Correspondence. Trane Customer
Service.
J U LY 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
35
https://www.tinyurl.com/bdd8kxc4 https://tinyurl.com/43dacnnv https://tinyurl.com/43dacnnv https://www.tinyurl.com/2p8vy869 http://ashrae.org
ASHRAE Journal - July 2022

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