[Equest-users] Actual Building Energy Cost

Paul Erickson perickson at aeieng.com
Thu Jun 17 06:11:47 PDT 2010


Eric/Nick,

I think it's important for anyone talking about modeling an existing building to qualify such/any levels of prediction with the amount of data that's used to calibrate the model.  Utility bills and a simple walk through type audit (say a Level 2) will find you feeling pretty cheery if you're at +/- 10%.  If you're at +/- 1-2%, it's most likely happenstance and you should trust that the model is dead-on accurate.  If you've heavily instrumented the various systems throughout the building and your calibration finds you within 1-2% of actual consumption, you're doing your job well and have spent a fair bit of change on the effort.

As for new buildings and predictive modeling, this is where benchmarking and feedback to the modelers and design teams grows ever more important (and talking about 1-2% is nowhere near practical at this point).  There are beginning to be more cases of litigation where large discrepancies exist, so be sure you know what you're promising and that you cya.  It does seem that there will likely be a harder, faster push to focus on operational performance in terms of ratings/classifications as opposed to making the modeling better, but I do foresee the market trending towards the need for us to demonstrate that our models are getting better as time goes on if we want to keep market share.

My two cents...
Paul

Paul Erickson  LEED(r) AP
SR. Sustainable Design COnsultant



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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Eric O'Neill
Sent: Thursday, June 17, 2010 7:11 AM
To: Nick Caton; David Bastow; aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Actual Building Energy Cost

Nick,

I should make it clear that my initial questions weren't meant to drill David. I'm genuinely interested if the DOE projects required an extremely high level of technical accuracy on the input side. (Although they were also somewhat of a reaction to those requirements.) We have this discussion in our office every so often, and I'm usually somewhat on my own, but I'm still not convinced.

Would you mind elaborating a little more about why you feel this to be true:

"HOWEVER:  The concept of building a model for an existing building, whose modeled energy consumption/costs fall within a 1-2% of historical utility records, is entirely feasible - and a reasonable requirement if the goal is to generate a model for predictive purposes."

So let me ask it this way. Let's say you've got a model, and it's coming in about 6% low (both heating and electric), but to the best of your knowledge everything is set up properly. You've got to start making some educated guesses to close the gap. Do you assume the boiler needs a tuning (drop your efficiency a few percent) and the AHU coils need to be cleaned (bump up your static)? Or do you bump up your infiltration slightly (because it's probably not a very tight building). Or did some maintenance guy came along and switched on both the boiler pump manually on when he got a cold complaint, and no one returned them to auto? Or maybe you have data from a few years with abnormally warm summers and cold winters. I could go on practically ad nauseum. There's just so many things that can be wrong with an existing building, as all of you probably know.

Whatever you choose, that guess you make may interact (or lack an interaction) with some measures you're predicting. So, predicatively, what makes a 1-2% off model better than a 5-6% off model unless you really spend the time and find out whether your inputs are accurate?

Cheers,

Eric

From: Nick Caton [mailto:ncaton at smithboucher.com]
Sent: Wednesday, June 16, 2010 5:43 PM
To: Eric O'Neill; David Bastow; aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Actual Building Energy Cost

While I (and I am sure many others) have books to write regarding my thoughts on the topic, I think I might suggest a simple step back:  The concept of predictively modeling to within a few percentage points of accuracy is ridiculous.  There are simply way to many things that could not be known.  HOWEVER:  The concept of building a model for an existing building, whose modeled energy consumption/costs fall within a 1-2% of historical utility records, is entirely feasible - and a reasonable requirement if the goal is to generate a model for predictive purposes.  How often such predictive models' accuracy is misinterpreted is something I get depressed to think about.

I haven't done the DOE-modeling work that's being referenced with such requirements, but I have done work for educational clients (physics/building science departments) who wish to get these models set up for ongoing study/tweaking purposes.  The exercise is challenging, and as close as energy modeling gets to "fun," when you can rest at ease knowing the client is fully on board with what the model is and isn't.

It's fully rational to squirm and cringe when you have to make models that you know will be mis-used and mis-understood, despite your best efforts.  Fortunate is the practicing energy modeler who gets to work for fully educated clients and design teams all the time =).

Rather than drill David, I think it would be safe to assume those DOE requirements exist to calibrate a model to a given degree to whatever historical records are available, NOT to mandate a level of accuracy for predictive purposes.  If I'm wrong, I hope that work never crosses my desk!

~Nick
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NICK CATON, E.I.T.
PROJECT ENGINEER
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Check out our new web-site @ www.smithboucher.com

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Eric O'Neill
Sent: Wednesday, June 16, 2010 4:52 PM
To: David Bastow; aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Actual Building Energy Cost

How much effort was put into minimizing the error of your inputs to justify that level of accuracy in your output? For example, did you convert real weather data for the two years and use an average of the simulations? Were you modeling small buildings so you could get a fairly reasonable infiltration rate empirically? Were occupancy schedules trended, and for how long? Were all the systems and controls working correctly, with all sensors calibrated regularly?

1 to 2% seems to me to be fairly unreasonable. Unless you do an amazing job verifying your inputs, in my opinion that level of precision doesn't get you a better model. If you set everything up as best you can and it comes in there, great! But a correctly set up model can be off by over 2% because of a couple "El Nino" years, a facilities guy locking a humidity high limit to 50% for a summer, or any number of operational factors. On the other side of it, we've tried to match a simple DOE2.2 model to a DOE2.1e model with limited success (I don't think we got within 3%, although we didn't spend too much time with it). Who knows what the difference would be with E+.

I know I always squirm when I'm asked to do existing building models, and it may be irrational. Heck, if the DOE is asking for 1-2%, I probably am being irrational. But it seems to me that the error from assumptions could easily swing a model 1-2% (what would a 30% error on your infiltration do to an otherwise correct building?). What are other people's thoughts?

Eric

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of David Bastow
Sent: Wednesday, June 16, 2010 3:48 PM
To: aazhari at jainconsultants.com; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Actual Building Energy Cost

On all the DOE modeling projects that we have done, of an existing building, they have required the model to be within 1 to 2% or less of the actual energy usage, based on an average two year history.  Is that what you are talking about?

David A. Bastow
McClure Engineering, Inc.


________________________________
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Ahmed Azhari
Sent: Wednesday, June 16, 2010 12:10 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Actual Building Energy Cost
Hi all,

Does anyone have a ballpark percentage of the actual annual energy cost versus the modeled annual energy cost for a building?

Thanks,
__________________________________________________________________________
Ahmed Azhari, B.Eng., LEED(r) AP
Energy Analyst

Jain Sustainability Consultants Inc.

2260 Argentia Road, 2nd Floor
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Tel:  (905) 542 7211 Ext 234
Fax: (905) 542 7622
Email: aazhari at jainconsultants.com<mailto:aazhari at jainconsultants.com>
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