LEED Water Use Calculations

By Timothy Allinson, P.E.

Much has been written about LEED buildings and Green Design, especially during the past year. I wrote about these topics myself in the January 2005 issue of this magazine. I think that the only more frequently addressed topic during that same time period has been radiant floor heating.

Since writing about "LEED and Plumbing Design" nine months ago, the subject of water use calculations has developed momentum. Back in January, my article summarized the fact that one LEED credit can be achieved by reducing domestic water use by 20%, and a second credit can be achieved by reducing it 30%. This reduction is relative to a baseline dictated by the Energy Policy Act of 1992.

At the time, despite my research, I was unable to determine what that baseline was, and the detailed calculation remained a complete mystery to me.

Since then the fog has lifted, and I now know how this mysterious calculation is performed. Perhaps many of you have known this for months or years, and, if so, you can stop reading now. But for those of you as confused as I was then, please read on, as the simplicity of the mystery is revealed.

The Baseline
The baseline water calculation is an estimate of water usage based on population, fixture flow rate, frequency of use and duration of use.

The project architect generally determines population, which is calculated based on square footage. A one-bedroom condo might have 1, 1.25, or 1.5 people per unit, depending on size. A two-bedroom condo would have two people or more. Retail is frequently calculated based on one person per 500 square feet for an eight-hour period. If the retail operation is 12 hours, then the population must be increased by 50%. Office buildings are based on the actual anticipated population, also known as the Full Time Equivalent, or FTE.

Fixture flow rate for the baseline is taken as follows: Water closet 1.6 gpf; Urinal 1.0 gpf; Lavatory 2.5 gpm; Kitchen sink 2.5 gpm; Shower 2.5 gpm.

Frequency of use varies, depending upon a few factors. Water closets are assumed to have three uses per day. For men, when urinals are present, the urinal is assumed to have two uses per day, and the water closet is figured as one use per day.

Lavatories are calculated as three uses per day for 15 seconds each, kitchen sinks are two uses per day for 15 seconds and showers are used once for 300 seconds (5 minutes).

The Design Case
Since the object of this calculation is to save water, the fixtures that are selected must have reduced flow rates. Fixture selection will depend on occupancy, and practical considerations must be made in their selection.

For example, in an office building the urinals can be selected as waterless and lavatories can be provided with 0.5 gpm aerators and/or sensors. In the women's toilets, dual flush water closets can be used. If so, the calculation might include 1 gpf for two uses and 1.6 gpf for one use for each woman, depending on the fixture/flush valve combination.

For residential applications, it would be unwise to use 0.5 gpm aerators on the lavatories, as those are extremely inconvenient for residential uses such as shaving. A realistic reduction might be 1.5 gpm.

As mentioned for the women's commercial toilet room above, dual flush water closets can be used throughout residential applications. Most tank-type dual flush water closets use 0.8 gallons for the light flush and 1.6 gallons for the main event.

Depending on the selected showerhead, shower flow rates can be reduced. Some manufacturers make showerheads in 2 gpm and 1.75 gpm models; I would not consider a lesser flow rate than that. Be sure to consult with your client, as you do not want to provide showerheads that are going to make the resident unhappy, especially without their knowledge.

Lastly, kitchen faucets can also be specified with reduced flow rates. Most kitchen faucets are based on 2.2 gpm standard, so there is automatically a savings, relative to the 2.5 gpm baseline. Aerators can reduce flow further to 2, or even 1.5 gpm, but again, be sure to consult with your client, as low-flow kitchen faucets might not be considered acceptable for the installation.

An Example
Consider a high-rise condominium with an FTE of 325 residents (162 men and 163 women) and 6,000 square feet of 12-hour retail space. The retail space would have a population of 12 based on one person per 500 square feet, but, since it is a 12-hour operation, this number must be increased to 18. Since there are inherent differences in water use, aspects of the residential and retail water consumption must be calculated separately.

This baseline calculation would be performed as follows. For like fixtures the population can be summed for men, women and retail, totaling 343 people.

Water closets: 3 uses x 1.6 gpf x 343 people = 1,646 gpd (gallons per day)
Lavatories: 3 x 2.5 gpm x 0.25 min. x 343 = 643 gpd
Showers: 1 x 2.5 gpm x 5 min. x 325 = 4,063 gpd
Kitchen sinks: 2 x 2.5 gpm x .25 min. x 325 = 406 gpd
Total: 6,758 gpd x 365 = 2,467,000 gpy (gallons per year)

The design calculation would be performed as follows. Note that this project utilizes dual-flush water closets for the residences.

Water closets (light flush): 2 uses x 0.8 gpf x 325 people = 520 gpd
Water closets (full flush): 1 use x 1.6 gpf x 325 people = 520 gpd
Water closets (retail): 3 uses x 1.6 gpf x 18 people = 86 gpd
Lavatories (residential): 3 x 1.5 gpm x 0.25 min. x 325 = 363 gpd
Lavatories (retail): 3 x 0.5 gpm x 0.25 min. x 18 = 7 gpd
Showers: 1 x 1.75 gpm x 5 min. x 325 = 2,844 gpd
Kitchen sinks: 2 x 2.2 gpm x 0.25 min. x 325 = 358 gpd
Total: 4,698 gpd x 365 = 1,715,000 gpy
Savings: (2.467 - 1.715)/2.467 = 30.5%

You can see from this example that the bulk of the water savings results from the reduction in the shower flow rate. This accounted for nearly 60% of the total water savings.

If you were to perform this calculation for a commercial building, the annual consumption would be based on 250 days per year rather than 365.

Note that, because this project utilizes a conventional cooling tower, nothing was included for cooling tower water consumption. If a high-efficiency cooling tower were used, then the associated water savings could be credited to the calculation. Also, if reclaimed water were used for flush valve fixtures, the domestic savings would serve as a credit to the annual consumption as well.

In Reality
A few aspects of this calculation aren't entirely realistic. First, we all know that a homeowner dissatisfied with a showerhead is going to replace it with one that flows more water. Second, kitchen sinks are used for a wide array of time durations and often more than twice a day by each resident. I would expect the average kitchen sink to be used for a much longer duration than 30 seconds per resident per day.

Although there may be aspects of this calculation that are debatable, at least it defines and quantifies the intended goal of saving water, reducing the load on water treatment plants, and achieving valuable credits toward LEED certification.

Excel Calculation
There is an Excel spreadsheet available for performing this calculation. I obtained a copy from the LEED consultant on the sample project described in this article. The spreadsheet makes manipulation of the population and fixture flow rates much easier than the manual calculation, so if you can get your hands on a copy of the program you should do so. If you are the person responsible for documenting the LEED credits on your project's LEED Web site, you will have to enter numbers into this same program on the site to prove your successful water use reduction.

 

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.