An economizer is a control system that uses outdoor air for cooling when outdoor conditions are favorable, reducing or eliminating the need for mechanical refrigeration. When outdoor air is cool and dry enough, it can be brought into the building to satisfy cooling loads at essentially zero energy cost (beyond fan power).
How Economizers Work
When the economizer control determines outdoor air is suitable for cooling, the outdoor air damper opens to a position that delivers enough cool air to meet the thermostat setpoint. The refrigeration system is disabled or operates at reduced capacity. When conditions no longer favor free cooling, the damper returns to its minimum ventilation position and mechanical cooling resumes.
Control Strategies
- Dry-bulb temperature: Opens when outdoor temp is below a setpoint (e.g., 65 degrees F). Simple but can bring in humid air.
- Enthalpy: Opens when outdoor air enthalpy (heat content) is below a threshold. Accounts for humidity, preventing moisture problems.
- Differential: Compares indoor vs. outdoor conditions rather than using fixed setpoints.
- Integrated: Allows simultaneous mechanical cooling and economizer operation for partial load conditions.
Where Economizers Are Required
ASHRAE 90.1 and many energy codes require economizers on air-handling units above a certain capacity (typically 54,000 BTU/hr cooling) in climate zones where they are effective. They are most beneficial in temperate climates with significant cooling loads.
Maintenance
Faulty actuators, failed sensors, and stuck dampers are common economizer problems. A stuck-open damper in summer or a stuck-closed damper in spring/fall both waste significant energy. Regular inspection and calibration are essential.