[Bldg-rate] LEED+90.1 Process/Plug Loads Conundrum

James Hess JHess at tmecorp.com
Fri Apr 30 13:42:08 PDT 2010

This is a good question.  On prior projects we have worked on, I have found that you can have less than 25% plug loads, so long as you have some documentation to back it up.

For example, we are currently working on a prison project.  The plug loads are very low for obvious reasons; they don't provide stereos, computers, ipods, TV's, etc. to the inmates (typically, all they get is an alarm clock/radio that uses ~ < 10 watts).  There is no way on this project that we could get the plug loads to come in at 25% unless we artificially jacked them up to levels that would never exist in reality.  We would have a similar problem in that the Proposed Design equipment would not be able to cool the spaces.  So, we just document the loads we do have and I believe we are good.  The project has been through the 1st review and this has not come up as an issue.

I believe we have had other projects where we documented the loads we had and passed the review with no problems.  We will typically develop a simple spreadsheet and document the internal loads that each room has.  That is easy enough to do these days I believe, for most applications.  For example, a typical desktop computer uses about 65 watts on average, monitor = ~ 45 watts (depending on size), clock radio ~ 10 to 15 watts, etc. you get the point.  (get a "Killawatt" power meter from Amazon and have some fun taking some measurements to see approximately how much power stuff uses, guaranteed to provide hours of fun).

Just provide some documentation on some good reasonable estimates for the loads and I think you will be fine.

An alternate method is to use default values from Table G-B from the ASHRAE 90.1-2007 Users Manual.

For example, the Receptacle Power Density for the Office Building Type is 0.75 watts/SF per Table G-5.

If using that value results in the process loads being less than 25%, my comment to the reviewers would be that the 0.75 watts/SF is the value determined by ASHRAE to be appropriate for the building type.

I think the most important thing is to use something reasonable and defendable, and make it the same between the Baseline and Proposed Design energy models.  Our experience indicates that it does not have to be exactly 25%, can be less than 25% or greater than 25%, depending on the building.

One thing that is interesting is that on federal projects, per EPACT requirements, you do not have to factor in the process/recep loads for the purpose of determining whether the 30% savings requirement has been met.  That makes it easier to show 30% savings.




James A.  Hess, PE, CEM
Senior Energy Engineer
TME, Inc.
Little Rock, AR
Mobile: 501-351-4667

From: bldg-rate-bounces at lists.onebuilding.org [mailto:bldg-rate-bounces at lists.onebuilding.org] On Behalf Of Nick Caton
Sent: Friday, April 30, 2010 12:55 PM
To: bldg-rate at lists.onebuilding.org
Subject: [Bldg-rate] LEED+90.1 Process/Plug Loads Conundrum

Something new occurred to me this week and I'd love to hear others' thoughts!

LEED prescribes baseline/proposed energy models to follow ASHRAE 90.1 with a caveat:  the "process energy costs" must total 25% of the baseline energy costs.  By extension, 90.1 requires the same loads/schedules be applied to the proposed model as they have to match.  As an aside, the LEED EAc1 templates appear to calculate/check this using process energy consumption (not cost), but that's not what I'm getting at...

I just had a typical case where I had to inflate the baseline internal miscellaneous equipment loads to get to 25%.  These additional loads were substantial enough that when applied to the proposed model/design, I ran into many unmet cooling hours for the equipment capacities entered.

Then I realized:  We can define additional equipment electricity loads but simultaneously apply a multiplier (in eQuest anyway - I expect this is feasible other programs also) to reduce/negate the corresponding heat load contributions.  This results in the energy consumption/costs showing up correctly in the final results/reports, but does not artificially inflate the internal loads that the baseline/proposed systems must handle.

Would this practice (which incidentally can be a time saver) of accounting for extra process/miscellaneous loads without extra thermal contributions be in line with the intent of ASHRAE/USGBC?  It would still normalize the otherwise unstandardized process consumption/costs of the baseline/proposed models between different building types...

On the other hand, if it really is the intent of USGBC to add arbitrary additional internal heat loads to our models that our actual designs were not designed and sized for, does it follow that we should allow the proposed models to autosize cooling equipment/fan capacities and not specify them (this would seem incongruous with 90.1 to me)?


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