Reducing Potable Water Use: Calculating LEED Water Efficiency Credit 3

When striving for LEED certification, how do you bring to bear water conservation measures? The U.S. Green Building Council offers up to five LEED water efficiency credits: two for reducing potable water use for irrigation; one for reducing potable water use for sewage conveyance; and two for reducing potable water use inside a building. This tutorial demonstrates the methodology for calculating the last two credits, which comprise LEED Water Efficiency Credit 3.

How to get started

The first step is to establish a baseline water use. This is calculated based on the water consumption of fixtures in a standard building as mandated under the federal Energy Policy Act (EPAct) of 1992. Next, the engineer must calculate the projected water use for the design building. Of course, the design case typically includes fixtures that use less potable water than minimally compliant EPAct fixtures, and also might use gray or recovered non-potable water for sewage conveyance (flushing). It’s important to keep in mind that the calculation basis is denominated by the number of building occupants, not the number of fixtures; no water use reduction can be claimed by simply reducing the number of fixtures.

So how does one establish these numbers? The LEED version 2.1 Reference Guide—available from USGBC online at www.usgbc.org —provides information for both flush and flow fixture types (see the tables below).

Furthermore, it’s important to note that the guide allows a 20% reduction in time of each use for automatic motion-control or metering sensors on lavatory and sink faucets. In other words, instead of 15 seconds in the case of a standard faucet, a motion-control faucet is assumed to be in use for only 12 seconds.

Let’s look at some given conditions. For the purposes of this calculation, building occupants are considered to be full-time equivalent staff—visitors and guests are not counted (unless you choose to do so). Building occupants are assumed to be 50% male and 50% female. It is assumed that each occupant will urinate twice a day and defecate once a day. They’ll also wash their hands at a lavatory three times a day and use a kitchen sink once a day. If there are showers in a building, it is assumed that on any given day, 10% of the building occupants will take a five-minute shower.

That being said, the equation changes slightly if a building uses a non-potable water source for sewage conveyance or for clothes washing, for instance. In such cases, the annual use of non-potable water may be subtracted from the annual flush volume.

Design case

All right, so let’s now apply this to a green building scenario. In this case, let’s say there are 300 occupants who each works a five-day schedule (260 annual workdays) in a medical office building. It includes shower facilities; public toilets, where men’s restrooms have both water closets and urinals; and private toilets, with just one water closet and lavatory. All restrooms are fitted with conventional 1.6-gallons-per-flush (gpf) water closets, but the men’s public toilets also include waterless urinals. The public restrooms account for about 10% of the total number of fixtures. Private restrooms are equipped with dual-level flush water closets that allow 0.8 gpf for liquid waste and 1.6 gpf for solid waste. The private restrooms account for the balance (90%) of the flush fixtures. There are several kitchenette areas in staff lounges, each with a sink. Lavatories in both public and private bathrooms are equipped with low-flow 1.8-gpm faucets and motion sensors.

No non-potable water is being used for sewage conveyance or to otherwise substitute for potable water, nor are any waterless, composting-type water closets being used in this example. These technologies, however, are being adoped more. That being said, designers should always consult with local code authorities about any restrictions on the use of waterless fixtures or the use of non-potable water inside a building.

On to the main event

Use a spreadsheet to help calculate the usage, by fixture type, of potable water. The tables on the previous page (p. 51) show how a typical spreadsheet would be set up. In this case, calculating the baseline case first (top table), where only EPAct compliant fixtures are used, the total annual consumption is 947,700 gallons. Next, add in the “green” fixtures for the design case (bottom table). In this case, consumption drops to 729,690 gallons—a 23% reduction below the baseline case.

This satisfies the first point requirement of Water Efficiency Credit 3—a 20% water reduction.

A second point is available if a 30% reduction is achieved. In this example, the design case would have to reduce water usage to 663,390 gallons. This is achievable in a number of ways. For instance, if the public restrooms are equipped with ultra-low-flow water closets (1.1 gpf), the kitchen sinks are equipped with low-flow 1.8-gpm sensor faucets—limiting each use to 12 seconds—and the showerheads are also low-flow 1.8-gpm, the savings can be reached.

As the tables demonstrate, achieving LEED plumbing points is really as simple as plugging in the right fixtures at the right places.

This is the second installment of our new How To department. Each month we’ll provide a short tutorial on the rotating subjects of system integration, system retrofits and designing certifiable LEED systems. This month, we tackle the latter.

Baseline Case

Design Case

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