The water-side economizer is another option for reducing energy use. ASHRAE Standard 90.1 addresses its application, as do some state energy codes.

The water-side economizer consists of a water coil in a self contained unit upstream of the direct-expansion cooling coil. All economizer control valves, piping between economizer coil and condenser, and economizer control wiring can be factory installed.

The water-side economizer uses the low cooling tower or evaporative condenser water temperature to either (1) precool entering air, (2) assist mechanical cooling, or (3) provide total system cooling if the cooling water is cold enough. If the economizer is unable to maintain the air-handling unit’s supply air or zone set point, factory mounted controls integrate economizer and compressor operation to meet cooling requirements.

For constant condenser water flow control using a economizer energy recovery coil and the unit condenser, two control valves are factory-wired for complementary control, with one valve driven open while the other is driven closed. This keeps water flow through the condenser relatively constant. In variableflow control, condenser water flow varies during unit operation.

The valve in bypass/energy recovery loop is an on/off valve and is closed when the economizer is enabled. Water flow through the economizer coil is modulated by its automatic control valve, allowing variable cooling water flow as cooling load increases (valve opens) and reduced flow on a decrease in cooling demand.

If the economizer is unable to satisfy the cooling requirements, factory-mounted controls integrate economizer and compressor operation. In this operating mode, the economizer valve is fully open. When the self-contained unit is not in cooling mode, both valves are closed. Reducing or eliminating cooling water flow reduces pumping energy.


• Compressor energy is reduced by precooling entering air. Often, building load can be completely satisfied with an entering condenser water temperature of less than 55°F. Because the wet-bulb temperature is always less than or equal to the dry-bulb temperature, a lower discharge air temperature is often available.

• Building humidification does not affect indoor humidity by introducing outside air.

• No external wall penetration is required for exhaust or outside air ducts.

• Controls are less complex than for air-side economizers, because they are often inside the packaged unit.

• The coil can be mechanically cleaned.

• More net usable floor area is available because large outside and relief air ducts are unnecessary.


• Cooling tower water treatment cost is greater.

• Air-side pressure drop may increase with the constant added resistance of a economizer coil in the air stream.

• Condenser water pump pressure may increase slightly.

• The cooling tower must be designed for winter operation.

• The increased operation (including in winter) required of the cooling tower may reduce its life.

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