Process Load & LEED Documentation

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Hello All,

My question is regarding the 25% process load as required by LEED online. I'll give a little background on the building first...

The building is located in northern Minnesota, rendering it a heating dominate building. Our HVAC system is a geothermal water-to-air heat pump system with an electric back up boiler. There will be no gas usage. For this scenario, system type 4 was used for the baseline case. Since a majority of the winter will be spent below the heat pump operating temperature, in heating mode, electric resistance is the main source of heating. This being said, my baseline system has a very large annual heating consumption.

Now to my dilemma. With the large heating consumption in the baseline design, the process load was below the 25% threshold. I was advised to increase the plug load until I was able to surpass the 25%. After multiple iterations, I came to a value of 3.7 W/sqft for the office spaces (the building is primarily offices). Obviously this is much greater than anticipated for offices spaces.

Initially the process load was 30% with my proposed design, but after the modifications the process load was roughly 50% of my total energy consumption. To my knowledge the only other option we have to is document each piece of equipment contributing to the process load.

Has anyone encountered the problem before and have advice on a solution?

Thanks in advance.

Daniel Kaler

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First off - 3.7 is high, but not unreasonable for an office. It depends on what you are doing in the space. I would recommend trying to get a good representation of power loads on a space by space basis rather than spreading a uniform power intensity across the whole building - this makes a big difference with CAV & VAV systems - especially with reheat issues. If you assume 50 ft2 per workstation and around 175 W per station - that's over 3w/ft2. Remember - connected peak power and actual peak power are different - you need to factor in some level of diversity in your usage schedules. If you have any kind of server room - I would definitely model that as a separate space with its own system and equipment load.

Relative to the GBCI & LEED - if you document your equipment assumptions properly and can make the case that you have modeled this close to reality (I would provide a space by space table along with a usage profile), you should be fine. The 25% is a minimum unless you can reasonably document otherwise.

Hope this helps

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Hi Daniel,

My understanding is that if you can document the reasons for the process load being less than 25% (as you have below) you should be fine. I don't think arbitrarily adding process energy to your building is what LEED is looking for. Is this what your reviewer specifically asked you to do?

Cheers,
Dan

?
Daniel Knapp, PhD, LEED? AP O+M

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Daniel(s):

LEED has lamentably not grown to be more transparent on this issue over time, but I can say with some confidence that you really should NOT arbitrarily add plug loads to the project to get to the 25% mark. I used to do exactly that, but found this practice can on occasion "break" a model by introducing internal heat loads the proposed systems simply cannot handle - I tried to work around that issue in various means before thinking to ask the question on the mailing lists and learning there's a better way.

"Sufficient documentation" for something different (often less) than 25% does not normally require a space-by-space table, even. In my experience, when you don't have a some other reference to draw from, a single-line reference to a plug load W/SF value, citing the 90.1 User's Manual or a similar is all that's needed. This is very simply done in the LEED v3 EAp2 spreadsheet. (Offices are 0.75W/SF btw, if you use their plug load schedule).

A space-by-space breakdown would probably be necessary if you're trying to document an exceptional calc to demonstrate energy-efficient computers, timed power outlets, etc... but again not in a normal case where proposed/baseline inputs are matching. Time spent citing and using such W/SF values & scheduling is a fraction of what I used to spend iteratively messing with the proposed/baseline results (and baseline system fan/efficiency calculations) to get to an arbitrary consumption percentage.

Now, Vikram totally correct in that 3+ W/SF can be "correct" if we're talking about a model calibrated to match reality, and it can certainly be higher or lower for an office building depending on the contents/occupants, but if you have nothing beyond the arbitrary 25% requirement driving you to push the numbers so high - I would sooner use plug load power densities suggested by 90.1 as the better assumption.

Best of luck!

~Nick

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I have a few successful LEED models with less than 25% process loads - but
only arrived there after an evaluation and provided a narrative to explain
why. As Vikram pointed out, list all process loads per space, apply a
reasonable run time, and designate a corresponding load in W/sf to that
space (don't forget the elevators).

Regards -

Jeff Ross-Bain, PE, LEED AP BD+C, BEMP

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Dan -

(wrote this up before I saw all the great responses. Will send anyway to provide the page citation you can cite with any reviewer, as needed, to justify why you would like to document your actual process loads.)

I understand from GBCI that the 25% process loads threshold is a default, where the actual process load is unknown. This applies rather clearly to Core and Shell jobs or jobs where the connected process load is unknown / unknowable.

That said, we have found on a case-by-case basis that LEED Reviewers are sympathetic to a well-reasoned and well-documented case showing the actual process loads for a facility, enabling us to use them in both the proposed and baseline. (See LEED 2009 BDC reference manual p273, Baseline Column)

If you have full fitout drawings, you may be able to document the total design electrical power intended for the facility rather easily, via the VA shown on the electrical panel schedules, plus the draw of any larger loads fed direct from the panelboards/switchgear (ie elevators, etc).

If not, does the 3.7W/sf exceed the electrical or air conditioning capacity of the actual designed building? If 3.7 W/sf is far beyond the actual power, you may end up with many un-met load hours in your proposed case. If the artificially high process load exceeds the building's capacity, you may be able to document an assumed lower amount that matches the actual Electrical or HVAC capacity of the facility.

Aaron Dahlstrom , PE, LEED(r) AP

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I agree with what Nick wrote below.

Another thing to keep in mind is that you can also increase your misc loads % by bumping up your misc loads schedule during unoccupied hours, which is a good segway to my next point.

I used to think that 25% process loads was too high and that it could not possibly be that high in the real world. However, in the couple of buildings where I've done M&V on, I was surprised to see that the process loads were actually about 25%, and that was because all kinds of stuff still runs at night and on weekends, as the graph below from one of my M&V projects illustrates. The misc usage is still consistently high at nights and on weekends. In a perfect world, we assume the misc equipment is mostly off at nights and on weekends. Perhaps our schedules need to reflect more real world operation and that will certainly help in hitting the 25% mark.

My point in all of this is that the requirement of 25% misc loads is probably actually in the ballpark for many buildings. Certainly not all, however, since each project is different.

P.S. we have some work to do to see what's going on with all this stuff running when nobody is in the building.

[cid:image002.png at 01CCFBF1.94BE4790]

Thanks!

Regards,

JAH

James A. Hess, PE, CEM, BEMP

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A couple points to add to all the great information stated before me.  

I have had great success with multiple projects using the following two methods: 

Method #1)  As described above, cataloging all the equipment in the building and calculating the "expected" process load based on what is expected to be in the building.

Method #2) In the ASHRAE 90.1, in the Appendix G section in the back ( I don't have page numbers since it is not handy) there is a 'standard' set of values for some typical spaces, one of which is Office.  If you reference the User's Manual in your documentation, you should not have a problem. The Office process load is set at 0.75 W/sf.  

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