6.3 Indoor Water Use, Fixtures, and Occupant Demand

Indoor Water Is Occupant Use at Fixtures

Indoor water is the WE topic most candidates meet first, and it is the most numerically defined. The Indoor Water Use Reduction prerequisite requires the project's total fixture water use to be at least 20% below a calculated baseline, and the optional credit awards points for reaching 25%, 30%, 35%, 40%, 45%, and 50% reductions. Every newly installed toilet, urinal, private lavatory faucet, and showerhead must additionally carry the EPA WaterSense label (or local equivalent outside the U.S.).

The EPAct 1992 Fixture Baseline

The baseline comes from the Energy Policy Act of 1992 and related plumbing codes. Memorize the headline rates because the exam quotes them as distractors:

gpf = gallons per flush; gpm = gallons per minute. A flush fixture (toilet, urinal) is rated per flush; a flow fixture (faucet, shower) is rated per minute. They are baselined separately and then aggregated.

How the Calculation Works

LEED does not just compare a fixture's rated flow to the baseline; it weights each fixture by how often building occupants use it. The headcount is built from Full-Time Equivalent (FTE) occupants - an FTE is one person present 8 hours a day - plus transient occupants (retail customers, students, visitors) who use fixtures less often. Standard daily use estimates assign, for example, 3 toilet uses and a fraction of a faucet/shower use per FTE per day. Multiplying use frequency by fixture flow gives a daily gallon total; the percent reduction is (baseline gallons - design gallons) / baseline gallons.

Worked Scenario

An office houses 100 FTE occupants. The team installs 1.28 gpf toilets (vs. 1.6 baseline) and 0.35 gpm public faucets (vs. 0.5 baseline). Both fixtures beat the baseline by 20% on their own, so the aggregated daily total comfortably clears the 20% prerequisite and likely earns an early credit point. To push toward 30-40% the team would add 0.125 gpf pint urinals or dual-flush toilets and offset remaining demand with nonpotable water.

Flush Fixtures Versus Flow Fixtures

The exam expects you to classify fixtures correctly because they are measured in different units and aggregated separately before the percent reduction is computed. Flush fixtures discharge a fixed volume each cycle: water closets (1.6 gpf baseline) and urinals (1.0 gpf baseline). Flow fixtures discharge a rate over time: lavatory faucets, kitchen faucets, and showerheads (measured in gpm). Dual-flush toilets complicate this slightly because they offer a reduced flush for liquid waste and a full flush for solids; LEED uses a blended effective flush volume based on assumed use ratios.

Composting and pint-flush (0.125 gpf) urinals push flush-fixture savings even lower.

Strategies to Climb the Reduction Tiers

Reaching the 20% prerequisite is usually easy with standard WaterSense fixtures. Climbing to the 30-50% credit tiers requires stacking strategies:

• Specify 1.28 gpf high-efficiency toilets or dual-flush units instead of 1.6 gpf models.
• Use 0.125 gpf pint or waterless urinals where appropriate to the use.
• Drop public lavatory faucets to 0.35 gpm with aerators and sensor or metering controls that limit run time.
• Offset remaining demand with nonpotable water (rainwater or graywater) supplied to toilet flushing, which counts as reduction in the calculation.

Common Traps and Reasoning

• A meter does not earn the indoor reduction; only efficient fixtures (and nonpotable substitution) lower the calculated number.
• WaterSense labeling is mandatory for the named fixtures - an unlabeled "low-flow" faucet can fail the prerequisite even if its flow is low.
• Process water (kitchen pre-rinse spray valves, dishwashers, clothes washers) is handled by a separate set of appliance requirements, not the fixture calculation.
• Reducing flow too aggressively can impair function (for example, drain-line carry for toilets); LEED expects WaterSense-rated performance, not arbitrary minimums.
• Transient occupants use fixtures less per day than FTE occupants, so a high-traffic retail space and a same-headcount office can have very different daily totals.

Use the mental sequence reduce, substitute, monitor: choose WaterSense fixtures to reduce, apply nonpotable water where quality fits to substitute, then meter to monitor. Read the first noun in the stem - faucet, toilet, occupant - to confirm you are in indoor fixtures rather than process or outdoor water before you choose a strategy.

Worked Reduction Calculation

Consider a stem that gives a baseline daily indoor use of 1,000 gallons and a designed use of 760 gallons after WaterSense fixtures. The reduction is (1000 minus 760) divided by 1000, which equals 24%. That clears the 20% prerequisite but falls just short of the 25% credit threshold, so the team would add a pint urinal or graywater offset to cross 25%. The exam rarely asks you to compute, but it expects you to read a percentage and know which prerequisite or credit tier it satisfies. Always express savings as a percentage of the baseline, never as raw gallons, because LEED scores the percentage reduction.

Behavior and Controls

Fixture hardware is the main lever, but sensor faucets, metering faucets that shut off automatically, and occupant signage support the savings by limiting run time. These are secondary to choosing efficient fixtures, yet they help a project hold its designed reduction in real operation.