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Exceptional Calculation Method Official Definition?

asked 2018-01-31 12:21:38 -0600

kwalkerman's avatar

Is there an official definition of an "Exceptional Calculation Method"? I think it's clear that exceptional calculation methods include post-processing of energy model results to implement equipment that can't be modeled directly in the software.

It also makes sense that implementation of EnergyManagementSystems in EnergyPlus would constitute an exceptional calculation.

What about pushing software to model things it wasn't designed to model? Adapting efficiency curves with the PVVT system to model VRFs in eQuest is one example. Using a "dummy" zone to distribute air and approximate a DOAS system is another method that is used in eQuest and seems "exceptional" as in the software was not designed to model such a system.

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Are you asking the definition in 90.1 or in LEED?

bbrannon4's avatar bbrannon4  ( 2018-02-01 02:22:38 -0600 )edit

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answered 2018-02-01 10:36:42 -0600

updated 2018-02-01 10:41:56 -0600

This is an interesting question, but I think can easily get a little too philosophical as to what 'modeled directly' means or what a software was 'designed to model' that the code writers were probably deliberately trying to avoid.

I read the key sentences in the sections in 90.1 where 'Exceptional Calculation' is mentioned as (in 90.1-2013):

  • 11.4.5: "Where no simulation program is available that adequately models a design, material, or device...", and

  • G2.5: "When the simulation program does not model a design a design, material, or device..."

(emphasis mine)

Both Chapter 11 and Appendix G give detailed descriptions of what an accepted 'Simulation Program' are and the features they shall have. DOE-2 is explicitly called out in both places, and EnergyPlus explicitly in Appendix G. Therefore in my opinion, the use of EnergyManagmentSystems or use of curves or dummy zones would not necessarily fall under the rubric of Exceptional Calculation Method, since they are done within the capabilities of the simulation program, and are "adequate" (at least in the sense that, if the Rating Authority finds something fishy in the results they can request more information on the inputs, justification, etc.).

So I think the ultimate issue here is: both ECB and PRM provide a framework for demonstrating building performance, but leave the option open to the Rating Authority to define its own specific standards in both approving the simulation program and the calculation methods. If the RA doesn't think the way you're using EMS (or the use of EMS in general) is an acceptable capability of the simulation program EnergyPlus, that's fine, but then you can follow the Exceptional Calculation Methodology to make your case for its use (which they can then accept or reject).

Edit: in summary: ¯\_(ツ)_/¯

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Thanks for this answer. I looked in ASHRAE 90.1 and LEED for a definition and couldn't find one for either. My thinking with calling out EMS is that it is specifically designed for users to do things that are "exceptional", but within the iterative capacities of the program.

Using EMS, you can write your own plant and coil components and it is easy to write code the breaks the laws of physics. Knowing the amount of discussion and review of EnergyPlus source code for such components, it seems irresponsible to consider EMS to be part of the normal capabilities of EnergyPlu

kwalkerman's avatar kwalkerman  ( 2018-02-02 19:55:33 -0600 )edit

Yeah I understand the argument with EMS, but you could make similar ones regarding, e.g. Coil:UserDefined, or SurfaceProperty:OtherSideCoefficients, or even regular performance curve inputs that, as long as they didn't result in conditions outside the physical limits to cause warnings/errors, could represent all manner of 'gamed' output, all within the simulation program. This kind of puts us into modeler ethics territory, but in any case I don't think the standards/rating frameworks want to start splitting those hairs with hard definitions.

Eric Ringold's avatar Eric Ringold  ( 2018-02-03 10:57:38 -0600 )edit

PS regarding LEED, since they are a Rating Authority I think their use of Exceptional Calculation is a little more about documenting savings for equipment not regulated by energy codes, whereas the use in the energy codes is more about calculations not supported by approved software.

Eric Ringold's avatar Eric Ringold  ( 2018-02-03 11:04:17 -0600 )edit

Coil:UserDefined uses EMS in order to run. I agree that there are many other areas where incorrect coefficients could result in a poor model, but for many of these, there are defaults, examples or tools available to help modelers come up with the correct inputs.

kwalkerman's avatar kwalkerman  ( 2018-02-11 15:35:38 -0600 )edit

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Asked: 2018-01-31 12:21:38 -0600

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Last updated: Feb 01 '18