Water Balance in a Cooling Tower
A 150,000 ft² office building uses a 500-ton cooling tower and irrigates about one acre of turf. The water bill shows 4.0 million gallons per year for tower makeup and 1.5 million gallons for irrigation. Indoor fixtures were upgraded five years ago to 1.28 gpf toilets and 0.5 gpm aerators. The owner wants the most water savings for a $150,000 budget and is considering a 20,000-gallon rainwater cistern, turf removal, and a cooling-tower conductivity controller. The building is in a humid region with 45 in. of annual rainfall.
1. How should the team prioritize the options?
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Benchmark end uses before buying equipment. Tower makeup is 4.0 million gallons per year, more than twice irrigation, so the cooling tower deserves first analysis. Check cycles of concentration, blowdown control, meter accuracy, and whether the tower is over-cycling or dumping water.
A conductivity controller is often a low-cost, high-return measure if the tower is currently operating at low cycles. Turf removal reduces irrigation demand and may also reduce maintenance, but it changes the landscape and should be sized to actual irrigated area. The 20,000-gallon cistern is small relative to both demands; it should be considered after demand reduction and with a water balance.
Indoor fixtures are already relatively efficient, so further indoor savings are likely limited. Priority: tower optimization and irrigation audit first, then turf conversion, then rainwater for the remaining irrigation demand.
2. What factors determine whether rainwater can be used for the cooling tower versus irrigation?
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Irrigation is usually the simpler first use because it has lower water-quality requirements and can tolerate intermittent supply. Tower makeup is more demanding: it needs filtration, biological control, compatible chemistry, and a reliable continuous supply. A tower cannot be run on a cistern that is empty in dry weather without a potable backup and proper backflow protection.
Estimate the capture volume before sizing the cistern: 1 inch of rain on 10,000 ft² of roof is roughly 6,200 gallons, so 45 inches on a large roof could yield substantial water, but only if storage and demand timing match. A 20,000-gallon cistern may be reasonable for irrigation storage but small for a 4-million-gallon tower demand.
The judgment is to match water quality and reliability to the end use, not to route rainwater to the largest demand first.
3. What metering and verification issues matter for LEED?
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Without sub-metering, the owner cannot prove savings or manage the building. The project should provide:
- A permanent building-level water meter.
- Dedicated meters for irrigation and cooling-tower makeup, plus a way to track blowdown.
- A meter on any rainwater or reclaimed-water supply.
- Commissioning of controls and a leak-detection plan.
- A reporting plan that compares actual use with the baseline.
LEED water credits reward predicted efficiency, but performance depends on operations. If the conductivity controller is installed without a meter, the team cannot verify cycles or catch a stuck blowdown valve. Metering is not a paperwork extra; it is how the savings survive turnover.