[Bldg-sim] Comparing ASHRAE 90.1 App G Models to Real Buildings

Christoph Reinhart tito_ at mit.edu
Thu May 21 07:56:47 PDT 2015
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Dear Jacob,

This is an eternal debate and there are many reasons for moving away from the use of 90.1 Appendix G to evaluate the performance of a building designs.  To answer your question directly, we worked a few years ago with Enermodal in Canada on a comparison between design phase building energy models (BEM) prepared for LEED Canada certification (slightly different to Appendix G) to calibrated BEM and measured energy use for 18 buildings. The main findings are quoted below:

Analysis of a Simplified Calibration Procedure for 18 Design-Phase Building Energy Models
H W Samuelson, A Ghorayshi and C F Reinhart
Journal of Building Performance Simulation, DOI: http://dx.doi.org/10.1080/19401493.2014.988752
This paper evaluates the accuracy of 18 design-phase building energy models, built according to LEED Canada protocol, and investigates the effectiveness of model calibration steps to improve simulation predictions with respect to measured energy data. These calibration steps, applied in professional practice, included inputting actual weather data, adding unregulated loads, revising plug loads (often with submetered data), and other simple updates. In sum, the design-phase energy models underpredicted the total measured energy consumption by 36%. Following the calibration steps, this error was reduced to a net 7% underprediction. For the monthly energy use intensity (EUI), the coefficient of variation of the root mean square error improved from 45% to 24%. Revising plug loads made the largest impact in these cases. This step increased the EUI by 15% median (32% mean) in the models. This impact far exceeded that of calibrating the weather data, even in a sensitivity test using extreme weather years.

Best,

Christoph
Christoph Reinhart
Associate Professor
Department of Architecture
Massachusetts Institute of Technology
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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Brooks, Alamelu
Sent: Thursday, May 21, 2015 10:43 AM
To: Jim Dirkes; Nathan Kegel
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Comparing ASHRAE 90.1 App G Models to Real Buildings

I believe Appendix G is not meant to measure the performance of the existing building. ASHRAE 90.1 Appendix G Technical Committee is the right source to answer this question. They can clarify the intention of the APP G modeling methodology.

Best,
Alamelu
Alamelu  Brooks LEED AP (BD+C), HBDP, BEAP, EIT| Senior Associate | +1.443.718.4881 direct | Alamelu.Brooks at icfi.com<mailto:Alamelu.Brooks at icfi.com> | icfi.com
ICF INTERNATIONAL | 7125 Thomas Edison Drive, Suite 100, Columbia, MD 21046 USA
Connect with us on social media<http://www.icfi.com/social>.


From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jim Dirkes
Sent: Thursday, May 21, 2015 10:36 AM
To: Nathan Kegel
Cc: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: Re: [Bldg-sim] Comparing ASHRAE 90.1 App G Models to Real Buildings

I agree fully with all of the above comments and would like to add these:

  *   Even buildings that are commissioned properly will see their performance erode over time.  There are hundreds of reason for this, ranging from poor maintenance to well-intentioned maintenance people not having time to monitor operations well.  There is NO BUILDING that operates well for long.
  *   Buildings often see changes in operation, occupancy and schedule.  These are oftimes gradual changes over a period of years, but can be substantial

On Thu, May 21, 2015 at 10:22 AM, Nathan Kegel <nathan.kegel at iesve.com<mailto:nathan.kegel at iesve.com>> wrote:
Climate files used in the simulations versus the actual weather.

I’m in the midst of a project that shows a variance in EUI of up to 200% just by changing the climate file for the DOE primary school.  Full results to be presented in September.

Add in all the other factors already mentioned, and if your 90.1 model comes anywhere close the real buildings’ it’s far more likely that the 90.1 model was extremely “lucky” than it is that the model used accurate assumptions.

Regards,

Nathan

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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org<mailto:bldg-sim-bounces at lists.onebuilding.org>] On Behalf Of Maria-Lida Kou
Sent: Thursday, May 21, 2015 9:17 AM
To: Jacob Dunn
Cc: bldg-sim at lists.onebuilding.org<mailto:bldg-sim at lists.onebuilding.org>
Subject: Re: [Bldg-sim] Comparing ASHRAE 90.1 App G Models to Real Buildings

Jacob,

Happy to hear that other people are thinking the same.

I was into this subject on my own thoughts recently.

I would like to add in your list: Occupants' behavior actually which is not in the stage to be included into the prediction.
I would add commissioning as well along with controls simulation and controls operation.

Apologies because I haven't worked with LEED projects but I think the above applied in general to "the performance gap".

Really looking forward to hearing more about this subject as I am not that experienced engineer yet, but really interested in "the performance" side of buildings.

Best,
Maria-Lida Kounadi


2015-05-21 15:04 GMT+01:00 Jacob Dunn <jdunn at eskewdumezripple.com<mailto:jdunn at eskewdumezripple.com>>:
Bldg-Sim Community –

I’m trying to compile a list of why it might be inappropriate to compare Appendix G models to actual consumption data.  This comes about because I recently got into a debate with one of my co-workers when looking at the infamous NBI chart/study that shows little correlation to predicted and actual energy values of LEED buildings.  I was trying to explain that the Appendix G model’s intent is NOT to be compared to actual consumption, as it is a modeling protocol aimed at creating consistent relative comparisons for LEED points.

Here are the reasons thus far that support this notion (that App G models shouldn’t be compared to actual data).  Does anyone know of any resources out there that expand upon this?  Or can you think other reasons?


-          Appendix G does not take into account external shading, which can be critical in urban environments for accurate energy predictions

-          Schedules are typically not created with the intent of being predictive.  Overall building hours are adhered to, but detailed schedule creation is not usually in the scope of a LEED model (or is it, in your experience?).  For instance, typical plug load base values during unoccupied times are .3, this is a pretty big assumption.

-          The App G model uses a TMY weather file, which can vary from the current weather year (I wonder on average by how much?)

-          Infiltration values are assumed, unless blower door testing has been done (which is rare for commercial buildings).

-          Thermostat values are modeled as consistent across the building, which is rarely the case in an actual operating building

Any additional insight is much appreciated!


Jacob Dunn LEED AP BD+C
ESKEW+DUMEZ+RIPPLE, APC
2014 AIA National Architecture Firm Award

365 Canal Street Suite 3150
New Orleans LA 70130
504.561.8686<tel:504.561.8686>
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James V Dirkes II, PE, BEMP, LEED AP
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