With some decentralized systems, an air-side economizer is an option, if not an energy code requirement (check state code for criteria). The air-side economizer uses cool outside air to either assist mechanical cooling or, if the outside air is cool enough, provide total cooling.

It requires a mixing box designed to allow 100% of the supply air to be drawn from outside. It can be a field-installed accessory that includes an outside air damper, relief damper, return air damper, filter, actuator, and linkage.

Controls are usually a factory-installed option. Self-contained units usually do not include return air fans. A barometric relief, fan-powered relief fan, or return/exhaust fan may be provided as an air-side economizer.

The relief fan is off and discharge/ exhaust dampers are closed when the air-side economizer is inactive.


• Substantially reduces compressor, cooling tower, and condenser water pump energy requirements, generally saving more energy than a water-side economizer.

• Has a lower air-side pressure drop than a water-side economizer.

• Reduces tower makeup water and related water treatment.

• May improve indoor air quality by providing large amounts of outside air during mild weather.


• In systems with larger return air static pressure requirements, return or exhaust fans are needed to properly exhaust building air and take in outside air.

• If the unit’s leaving air temperature is also reset up during the airside economizer cycle, humidity control problems may occur and the fan may use more energy.

• Humidification may be required during winter.

• More and/or larger air intake louvers, ducts, or shafts may be required.

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