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

David Eldridge DEldridge at grummanbutkus.com
Thu May 21 11:25:10 PDT 2015


I’d like to add that although 90.1 Appendix G includes scope language as pointed out by several board members, that its purpose is “to make an apples-to-apples comparison for rating” it doesn’t mean that the model couldn’t also reflect the expected energy use of the building subject to some of the major assumptions.

That Appendix G scope statement shouldn’t be a “get out of jail free” card. I don’t think people on this list mean it that way, but I can imagine some confusion if an owner uses Google to search for “accuracy of building energy models using 90.1 Appendix G” and this thread comes up.

The NBI chart in my opinion is provocative and out of date with current industry practices and standards – some of the buildings use double the energy of the baseline building! How could this happen? There must have been some significant variances between the modeled equipment and what was actually constructed, and the building may be operating much differently than the original assumptions – 200% energy use doesn’t happen from faulty zoning, slightly incorrect setpoints, default equipment curves, 0.5 W/sf plug load equipment when it was actually 0.75 W/sf, etc. There must be major equipment variances, process loads, and/or extended operating hours.

I look forward to an updated study in this regard based on current modeling practices, USGBC’s review process which has evolved since that study was published, availability of Appendix G as a resource as opposed to using ECB which is designed for pass/fail analysis, and USGBC collecting data from buildings going forward…hopefully this study is in the works.

I’ll suggest that our message to those retaining consultants for modeling services should be:


·         The 90.1 comparison for LEED or Green Globes has some constraints in order to make it a model to model comparison across many buildings that are submitted. Appendix G doesn’t have a primary intention of predicting actual energy use. Energy efficiency investments made to the building based on modeled energy savings are likely to remain valid (or perform even better in terms of payback) if actual energy use comes in higher than the 90.1 model…if the systems are verified to be operating properly, and the extra energy use may be coming from higher plug loads and operating hours.



·         Yes the models can provide a predictive estimate of energy use, but the level of coordination between the owner, design team, contractor, and modeler requires extra effort from all parties. An experienced modeler will have a higher probability of asking the right questions from these parties to close the gap between modeled and actual energy use. The energy model is not able to account for changes that take place after the fact unless information is exchanged and the modeler’s scope includes updating the model at that point. This should be emphasized for any core and shell buildings, and any building that will have high EUI tenants that aren’t known at the time of the energy model’s creation.



·         In order to verify the cumulative performance of ECMs or investigate higher than expected energy use, an M&V process is recommended after the building is occupied in order to refine the model to correct for building occupancy factors, especially plug loads and operating hours, as well as actual weather. Comparing a pre-occupancy energy model to post-occupancy energy use data includes bias errors that need to be addressed through a calibrated energy model. The calibrated model may (will) provide different results than what was submitted to LEED or Green Globes.

This discussion also hinges on what someone considers to be “close” – is that within 5%, 10%, or 25%? An expectation needs to be set before the project.

If anyone wants to propose some ASHRAE research to provide better plug load and operating schedule data, let me know! TC 7.6 research subcommittee (among others) wants your input…

David

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
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From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Alfonso Hernandez
Sent: Thursday, May 21, 2015 10:34 AM
To: Maria Karpman; 'Brooks, Alamelu'; 'Jim Dirkes'; 'Nathan Kegel'; Jacob Dunn
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Comparing ASHRAE 90.1 App G Models to Real Buildings

Jacob et al:

Some more points to the mix:

1) Increased infiltration/exfiltration due to seasonal changes in wind patterns, which mostly affect high-rises, is not modeled very often. The changing nature of wind (more or less cold or hot wind coming from a given direction) will affect cooling and heating loads.

2) Not every software can model the thermal mass of concrete or brick.

3) Microclimatic conditions (the urban overshadowing Jacob was mentioning) have to do also with air movement (urban canyon effect), and are not modeled often.

4) Buildings with Occupancy/Vacancy sensors are usually not modeled with accuracy. App G just gives a 10% LPD reduction when using them (in theory you could model them if you know the exact patterns of occupancy before construction, which never happens).

5) Daylight Harvesting and Electrochromic glass are too dynamic systems to be modeled accurately (unpredictable occupancy again).

However, I do agree with Prof. Reinhart about calibrating design models with as much real time data as possible. That way we could recreate other scenarios (climate change, systems decay, the impact of a change of occupancy, etc.).

Alfonso E. Hernandez, LEED AP
Kirksey | Architecture

From: Bldg-sim [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Maria Karpman
Sent: Thursday, May 21, 2015 9:55 AM
To: 'Brooks, Alamelu'; 'Jim Dirkes'; '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

There is a section in Appendix G (G1.2) addressing this:

Neither the proposed building performance nor the
baseline building performance are predictions of
actual energy consumption or costs for the proposed
design after construction. Actual experience will differ
from these calculations due to variations such as occupancy,
building operation and maintenance, weather,
energy use not covered by this procedure, changes in
energy rates between design of the building and occupancy,
and the precision of the calculation tool.

You may also want to check out articles on building labeling systems that discuss differences between asset (theoretical, modeling-based) and operational (based on actual utility bills) ratings, such as the one attached (see p.4-6). Both have value as long as they are interpreted correctly.

Maria

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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<mailto: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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