LEED 3.0 Calculations
By Peter Kraut
Leadership in Energy Efficient Design, better known as the LEED rating system, dictates the way that we judge sustainable construction. Unfortunately, all too often, water use reduction takes a back seat to recycled materials and energy costs. In the past, although unlikely, it has been possible to achieve Platinum status without saving a single drop of water. In April, the U.S. Green Building Council recently introduced LEED version 3.0 putting slightly more importance on water savings. Sadly, it is still rather insignificant in the overall score for a green building. What used to account for 2 out of 69 points, now accounts for 4 out of 110 points. This represents a marginal increase from 2.9% to 3.6%. On a positive note, the credits the reduction of irrigation and waste water also have been increased and a prerequisite for low-flow fixtures has been added.
This prerequisite has changed the baseline by which low-flow fixtures are measured making those 4 coveted points more difficult to achieve, In earlier versions, low- flow lavatories were the key to success since the baseline was 2.5 gallons per minute and in many jurisdictions commercial lavatories were limited to 0.5 gallons per minute. Similarly, showers even slightly below the old baseline of 2.5 gallons per minute showed significant savings. Today, the fixtures that beat the baseline could often be mistaken for evaporative cooling misting nozzles. The maximum flow rates now allowed in any LEED project are shown in Table 1.
Table 1 — Baseline Flow Rates
It should be noted that the EPA, ASME and numerous plumbing codes have spent dozens of pages defining what has been summarized in the above table and those specifics should not be ignored. For example, the maximum flow rates are typically measured at 60 psi except for shower- heads that are measured at 80 psi. Pre-rinse sprayers for food service applications are loosely defined while residential showers in LEED now quantify the flow rate based on the number of allowable showerheads per square inch of shower stall. In case you’re wondering, it’s one per 2,500.
Calculating water savings is simply a matter of comparing the amount of water used in the design with the water that would have been used if baseline fixtures were provided. The USGBC knows that a good engineer can spin the numbers in his favor, so most of the uses and durations are now standardized. For example, if you had a single residence with a shower that saved only three tenths of a gallons per minute, you might want to claim that there were 12 people living there and each of them took two 30 minute showers each day and they only flushed the toilet once. To protect the rating system from such abuse, two things have been standardized.
First, the number of occupants for any LEED project must be used consistently for all calculations. This number of people is used for calculating the number of parking spaces in Sustainable Site credit 4.4 and for calculating the number of air conditioning zones in Indoor Environmental Quality credit 6.2. The first works out best when maximized while the second is best when minimized. Truly, the actual number of occupants should be used.
In addition, for occupancies where people come and go, a fractional number would be needed. This problem is addressed by considering an occupant of an office building, for example, as someone who is there eight hours per day. A lecture hall that sees 40 visitors once a day for two hours would be considered to have 10 occupants (40 5 2 / 8). A break room that serves people accounted for in their offices would be considered to have zero additional occupants. Adding all of these together for the building, you would get the final number to be used in all calculations. This term is referred to as the Full Time Equivalents or FTE. This number can vary greatly from the number of occupants used in egress calculation and the number of occupants used in the fixture calculation.
Second, for water use, all people are considered regular. By this, I mean that every man uses a toilet once per day and a urinal twice per day, even at a chili cook-off. Everyone has good hygiene and washes his hands after visiting the restroom for 15 seconds — even the guy who just exited while you could still hear the flush valve cycling. Occupants never wash their hands at any other time, but all occupants use the kitchen sink once a day for 15 seconds and one tenth of them also use the janitor’s sink for 15 seconds a day. If a shower is present, one tenth of the occupants take a shower. More surprisingly, people only shower for 5 minutes! All occupancies are considered 50% male and 50% female, unless you can prove otherwise.
With the baseline already being so low and all of this rigidity, the only way to demonstrate a savings is by using ultra-low flow fixtures or ULFs. Although not technically correct, I am lumping dual flush toilets and waterless urinals into this category. A dual flush toilet simply uses 1.6 gallons per flush when the handle is pushed one direction and 1.0 gallons per flush or less when the handle is pushed another direction. Waterless urinals use no water at all, instead using a chemical to maintain the trap seal. These fixtures, can have problems with salt build up if not properly coordinated and are quickly being overshadowed by one pint urinals.
Low-flow aerators are now readily available in just about any flow rate to match just about any faucet. Low- flow showerheads have reached an obscene level below one gallon per minute and they seem to be getting even lower every day. I have heard jokes about zero gpm showerheads and have even thought of patenting the idea. I’m not sure if it would be a good novelty item or if we are actually heading there. Time will tell.
Once the fixtures for a job are selected, the savings can be determined. Calculating water savings has become rather simple. Again, this water savings is simply a matter of subtracting the amount of water used by the design fixtures from the water that would have been used if baseline fixtures were provided. Assuming no showers, kitchen sinks or janitor’s sinks, the numbers for a 400 person shell office building work something like those shown in Tables 2 and 3.
Table 2 — Baseline Calculation
Table 3 — Design Calculation
In this example, 1,224 gallons of water per day will be consumed, while the baseline would have been 1,830 gallons of water per day. Subtracting the two and dividing by the baseline, it is clear that a 33.1% water savings can be achieved. (1830-1224) / 1830 = 0.331
The math is no longer a concern for engineers since the calculations are now automated in a PDF controlled by the publishers of LEED. These can be found at http://leedonline.usgbc.org. Unfortunately, for Version 3, also called LEED 2009, the forms are not yet available. Until then, the forms for version 2.2 should be used and the defaults must be changed to match the requirements of Version 3. The form has its nuances, so a little investigating may be necessary the first time you use it. The greatest concern is that each fixture reference in the baseline calculation must correspond to the same fixture reference in the design calculation. When multiple fixture types are combined such as ULF toilets and conventional toilets, a percentage of occupants need to be assigned to each with the total adding up to 100%. When dual flush toilets are used, the “occupants” also are adjusted using 33% for a full flush and 67% for a low flush.
For every building there seems to be a unique case, like a dual flush toilet, that does not fit neatly into the template. The challenge for the engineer is to model the building water consumption accurately without deviating from the accepted normal practice in LEED calculations. For this, the form is flexible and provides a space to explain why you are making a change. Changing occupants and durations will likely attract attention to the calculation causing what could be a long and arduous audit process. Falsifying the consumption could lead to something worse.
LEED online has become a great tool to help newer engineers understand and calculate water savings. As newer versions of the forms are posted, we are likely to see more fields and more flexibility, along with more specific standards. I suspect that future versions will better address mixed use occupancies, commercial kitchens, hospitals and other buildings where the fixtures are more diverse and their use is more governed by program than by rule. And finally, as we try to quantify the low-flow fixtures today, manufacturers are constantly developing new products that break the rules, requiring that our standards be updated and our methods be revised. Perhaps that zero gpm showerhead may become a booming business after all.
To my knowledge, Jeff Atlas of Symmons Industries is the “inventor” of the zero gpm showerhead. Peter A. Kraut, P.E., is a licensed Mechanical Engineer in 22 states. He founded South Coast Engineering Group, near Los Angeles, Calif. in 2001. He can be reached at (818) 224-2700 or via email at pkraut@socoeng.com.