5.3 Cooling Tower Water Use, Process Water, and Metering

Why Cooling Towers Matter

A cooling tower rejects heat from a building's chilled-water loop by evaporating a portion of the recirculating water. As pure water evaporates, dissolved solids (calcium, magnesium, silica, chloride, sulfate) concentrate in the remaining water. If the project lets concentration climb too high, scale and corrosion damage equipment. To prevent that, the system bleeds (blows down) concentrated water to the drain and replaces it with fresh makeup water.

The ratio of dissolved solids in the blowdown to dissolved solids in the makeup is the cycles of concentration (CoC) — also called concentration ratio. Higher CoC = less blowdown = less makeup water = less waste. But push too high and you damage the tower.

WE Credit: Cooling Tower Water Use (1-2 Points)

Step 1 — Test the Makeup Water

The project team analyzes the potable makeup water for at least the following constituents:

(Maximums reflect LEED reference-guide thresholds for the credit; actual limits depend on tower materials and chemical treatment.)

Step 2 — Calculate Maximum Cycles

For each constituent:

Maximum cycles = maximum allowable concentration in recirculating water ÷ concentration in makeup water

The smallest result across all four constituents is the maximum cycles of concentration the tower can safely run.

Worked Example

Makeup water analysis:

• Calcium: 80 ppm → 1,000 ÷ 80 = 12.5 cycles
• Alkalinity: 200 ppm → 1,000 ÷ 200 = 5.0 cycles
• Silica: 20 ppm → 100 ÷ 20 = 5.0 cycles
• Chloride: 25 ppm → 250 ÷ 25 = 10.0 cycles

Limiting constituents are alkalinity and silica — maximum cycles = 5.

Step 3 — Earn the Points

Required controls for either point:

• Conductivity controller that automatically opens the blowdown valve when conductivity (a stand-in for dissolved-solids concentration) exceeds the setpoint and closes it when conductivity drops below.
• Overflow alarm on the tower sump to flag stuck float valves and prevent runaway makeup demand.
• Makeup and blowdown meters to verify performance.

Where Process Water Fits

Beyond cooling towers, LEED also recognizes other process loads:

• Boilers — closed-loop, low makeup but periodic blowdown.
• Steam systems — high makeup if condensate is not returned; condensate recovery cuts makeup dramatically.
• Commercial kitchen equipment — combination ovens, food steamers (boilerless preferred), pre-rinse spray valves (must be ≤1.28 gpm under federal law).
• Laundry equipment — Water Factor ≤4.0 gal/cu ft for commercial washers.
• Medical and lab equipment — sterilizers, autoclaves, vacuum systems (water-ring vs. dry).

Process water generally does not count toward the Indoor Water Use Reduction credit's fixture math, but it does count toward the Cooling Tower credit (for towers) and the Water Metering credit (for sub-metering).

WE Credit: Water Metering (1 Point)

What to Meter

The credit asks the project to install permanent water meters for at least two of the following subsystems that are present in the building, in addition to the building-level meter required by the prerequisite:

1. Irrigation — separate meter for landscape irrigation systems serving >10% of landscaped area.
2. Indoor plumbing fixtures and fittings — aggregate meter for all interior plumbing fixtures.
3. Domestic hot water — meter for water entering the domestic hot-water system (boiler or heater).
4. Boilers — sub-meter on boilers with aggregate projected annual makeup water >100,000 gallons.
5. Reclaimed water — meter on reclaimed water serving the project, regardless of source.
6. Other process water — for example, humidification, laundry, pools.

How Meters Must Work

• Permanent (not portable clamp-ons).
• Record data at least monthly and calculate annual totals.
• Data must connect to a building automation system (BAS), web-based utility platform, or LEED reporting tool — not just a manual analog dial requiring on-site reads.
• Meters can be on either side of a backflow preventer; LEED does not specify location.

Putting It Together

flowchart LR
  A[Municipal supply] --> B[Building-level meter — prerequisite]
  B --> C[Indoor fixture sub-meter]
  B --> D[Irrigation sub-meter]
  B --> E[Cooling tower makeup meter]
  B --> F[Domestic hot water meter]
  C --> G[BAS / dashboard]
  D --> G
  E --> G
  F --> G

Sub-metering any two of the six subsystems satisfies the credit. Most projects pick irrigation and indoor fixtures because the data is most actionable for operations.