Forum discussion
CS-2009 EAp2: Minimum Energy Performance
If a mechanical design engineer adds a note on the mechanical schedules stating "AHU capacities shown are based upon 4Watts/S.F. for tenant power usage (excluding lights)", does the 4W/SF become a process load modeling requirement in the tenant space?
Or can the process load simply be whatever power density equals to 25% of the baseline energy costs per ASHRAE 90.1-2007?
Would there be an issue if the model only requires 2W/SF tenant space to meet the 25% process load requirement?
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 7, 2014 - 9:22 am
There is no requirement for a 25% process load. Never arbitrarily set the process to be 25% of the baseline energy cost.
Model what you expect the usage in the space to be as accurately as you can. If that is 4 W/sf then model that. If the process is less than 25% provide a narrative explanation as to why it is less.
Dave Hubka
Practice Leader - SustainabilityEUA
LEEDuser Expert
530 thumbs up
May 7, 2014 - 11:02 am
Hello Marcus,
I have struggled with LEED's interprertation of the requirement for process energy loads. Past LEED reviewers had me arbitrarily adjust the process energy inputs "until the process energy accounts for at least 25% of the baseline energy cost".
The energy modeling table in the LEED reference guide states "total process energy cost MUST be equal to at least 25% of baseline building performance".
The requirements section of EAp2 state "default process energy cost is 25%".
If minimum process energy cost threshold is not a requirement, not a default, and not a must, why do we need to explain it? (i believe this to be a "it is atypical for a building....." comment)
Any further clarification you can provide is greatly appreciated.
Thank you.
Matt Humphreys
May 7, 2014 - 11:44 am
Marcus:
Considering we are talking about a Core and Shell project that could have any number of a variety of tenants occupy the space in the future, how could one possibly know what usage to expect then?
I guess my question is how would a LEED reviewer interpret such a note on the mechanical schedule. Just because an HVAC system is designed to handle x number of W/SF does that mean the reviewer would interpret as the tenant will have that expected usage? I should think not but the implications of increased process load would be reduced energy cost savings, especially when modeling purchased chilled water under option 1 of the LEED district energy guide.
David:
Regarding the 25% of baseline energy cost requirement routinely enforced by LEED reviewers; the ASHRAE 90.1-2007 User's Manual Table G-B note#3 (which provides default receptacle power densities by space type and is I believe where LEED is getting the 25% minimum requirement) states that the receptacle power density should be increased (from the default value) to 25% of the total energy consumption (not energy cost) if you don't account for other equipment in the space such as computers, cooking, refrigeration, etc. However it does not indicate if it is 25% of baseline or proposed building energy consumption.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 7, 2014 - 12:12 pm
David,
The reviewer was wrong.
The credit language has always caused confusion. This came from a typical office building upon which much in LEED has been based from the very beginning. A typical office has about 25% process. Essentially the mandatory sounding language resulted from the fact that many folks were ignoring this load since it is not part of the "design". LEED wants to make sure you account for all energy use, not just what is regulated by 90.1.
You need to explain it so that LEED can make sure you have accounted for all process loads. there are building types that are typically lower than 25% and as long as you provide a logical explanation it will be accepted.
The 25% is baseline energy cost.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5909 thumbs up
May 7, 2014 - 12:16 pm
Matt,
Most spaces in a CS project have a general idea about the type of likely tenant. Base the expected process loads on that is all you can often do. As I said model it as accurately as you can. The Users Manual Table you cited is often used.
You are right to be concerned about the note on the schedules. Anything that raises a doubt about your modeling inputs can be questioned by the reviewer. The best way to deal with these issues is to be proactive and explain it in a narrative. Explain the note and how you have modeled the process and why you modeled it that way.