A waterside economizer saves energy by using a cooling tower and a heat exchanger to reject heat to the atmosphere, letting the chiller shut down when the outdoor wet-bulb is low enough. In the right climate it can displace compressor cooling for 1,000–3,000 hours per year and cut cooling plant energy by 20–50%. Unlike an airside economizer, it cools the water loop, not the supply air directly, so it pairs with a chilled-water system rather than a packaged DX unit.
How the Loop Works
On a cool day the cooling tower produces low-temperature condenser water, which flows through a plate heat exchanger to chill the supply water without the compressor. A typical setup targets 10–15 °C supply water from the exchanger when the tower can deliver 5–10 °C below that. The chiller bypasses and the plant runs on pumps and tower fans alone, which draw a fraction of chiller power. A 500 kW plant might drop from 350 kW to under 80 kW of electrical input in full waterside mode.
Waterside vs. Airside Economizer
| Factor | Waterside economizer | Airside economizer |
|---|---|---|
| Cools | Chilled-water loop | Supply air directly |
| Needs | Cooling tower + plate HX | Outdoor-air dampers |
| Best system | Central chilled water | Packaged DX / AHU |
| Free-cool hours | Based on wet-bulb | Based on dry-bulb/enthalpy |
| Fan impact | Low (water side) | Higher (more OA airflow) |
The two are not rivals; many large campuses use both—waterside for the central plant and airside for distributed units—to stretch free-cooling across more hours and more of the building.
The Wet-Bulb Advantage
Waterside savings track the outdoor wet-bulb, which stays low longer than dry-bulb into the season. A site with a 7 °C winter wet-bulb can run waterside economizer at 12 °C supply water for months; a heat exchanger approach of 2–4 °C means the tower needs only to reach 8–10 °C. Design the plate exchanger for a 5–10 °C temperature approach and size the tower for the economizer duty, not just summer peak.
When to Choose Waterside
- Choose it for central chilled-water plants above roughly 350–500 kW where a tower already exists.
- Choose it in climates with long cool seasons and wet-bulb below 10 °C for many hours.
- Add it when the chiller runs part load for long shoulder periods.
- Avoid it for small packaged systems that have no water loop to cool.
- Combine with airside economizer on distributed AHUs for maximum coverage.
Decision Checklist
- Count annual wet-bulb hours below your supply-water target before investing.
- Size the plate heat exchanger for a 5–10 °C approach and the peak economizer flow.
- Confirm the tower can reach the needed temperature at low ambient without freezing.
- Add a bypass and sequencing control so the chiller and tower hand over smoothly.
- Protect the exchanger from fouling with side-stream filtration on the tower loop.
Plant Integration
Waterside economizer logic sits in the plant controller: when the tower can meet supply temperature, it opens the HX bypass and idles the chiller. Expect 20–50% whole-plant savings during economizer hours and a simple payback of 2–6 years on a mid-size plant. Pumps and towers balanced to G2.5 and performance-tested before shipment keep the loop stable through these seasonal transitions.
Plate Heat Exchanger Selection
Size the brazed or gasketed plate exchanger for the peak economizer flow and a 5–10 °C approach, not for summer condenser duty alone. A 500 kW plant at 10–15 °C supply and 5 °C approach needs a tower delivering 5–10 °C water, so the exchanger area may be 30–50% larger than the summer unit. Oversizing the plate area by 20% costs little and protects performance as fouling rises over the first two years of operation.
Tower Freeze Protection
In sub-zero weather the tower basin and spray must be protected: use a basin heater, run the fan intermittently, or recirculate warm return water. A waterside economizer that freezes is worse than none, so add a low-temperature lockout that drops to compressor mode below the tower's safe limit, often −5 to −10 °C wet-bulb. In very cold sites, a closed-circuit cooler with glycol on the building side is safer than an open tower.
Plant Sequencing Logic
The plant controller watches the tower supply temperature against the chilled-water setpoint. When the tower can meet it within the approach, it opens the HX bypass, starts the tower and pumps, and idles the chiller; when ambient rises, it ramps the chiller back in parallel before the space drifts. A clean handover band of 1–2 °C prevents short cycling; log the changeover dates to prove the free-cooling hours claimed at design.
Water Treatment and Fouling
The tower loop feeds the exchanger, so fouling control matters more than in summer. Add side-stream filtration of 5–10% of flow and a biocide program to keep the plate surfaces clean; a fouled exchanger loses 2–4 °C of approach and silently kills the free-cooling savings. Inspect the plates each season and clean chemically when the approach rises above design; this single task often restores 10–20% of lost economizer hours.
When Waterside Is the Wrong Choice
Skip waterside economizer when there is no central chilled-water plant, when the tower is already maxed in summer, or when the climate has few wet-bulb hours below 10 °C. In those cases an airside economizer on the AHU, or no economizer at all, is the better spend. Match the technology to the loop and the climate rather than adopting it by default on every project you design.
Reporting Free-Cooling Hours
Track the tower-vs-chiller run hours through the BAS and report them annually; a well-tuned plant shows 1,000–3,000 waterside hours, and a drop year over year signals fouling or a failed sequence. Owners who see the number keep the system; those who do not often disable it during a controls change and lose 20–50% of plant savings without noticing the drift for years.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
How much can a waterside economizer save?
A waterside economizer can displace compressor cooling for 1,000–3,000 hours per year and cut whole-plant cooling energy by 20–50%. A 500 kW plant may drop from about 350 kW to under 80 kW of input in full waterside mode, with a simple payback of 2–6 years in a suitable climate.
Waterside or airside—which should I use?
Use waterside for central chilled-water plants with an existing tower and long cool seasons; use airside for packaged DX units and direct supply-air cooling. Large sites often use both, with waterside on the central plant and airside on distributed AHUs, to cover more free-cooling hours.
What temperature can the tower deliver?
In economizer mode the tower targets a supply-water temperature 5–10 °C above the outdoor wet-bulb; with a 2–4 °C plate-exchanger approach it can chill supply water to about 10–15 °C when the wet-bulb is near 7 °C. Size the tower for this duty, not only for summer peak load.
Do I need a cooling tower for waterside free cooling?
Yes, a waterside economizer needs a cooling tower plus a plate heat exchanger to reject heat to the atmosphere without the compressor. If your building has no tower or chilled-water loop, an airside economizer on the AHU is the appropriate free-cooling choice instead.