What is an economizer and how are its controls configured?

An economizer uses cool outdoor air in place of mechanical cooling when outdoor conditions are favorable, and per ASHRAE 90.1-2019, it is mandatory in most climate zones for air handlers over 54,000 BTU/h (4.5 tons) cooling capacity. The control sequence compares outdoor air enthalpy or dry-bulb temperature against return air conditions using a high-limit changeover strategy — differential dry-bulb control (simplest), differential enthalpy, or fixed dry-bulb with adjustable setpoint. The economizer modulates the outdoor air, return air, and exhaust dampers in sequence to maintain mixed air at the desired supply air temperature setpoint, with the outdoor damper opening as cooling demand increases and the return damper proportionally closing. Mechanical cooling is only enabled when the outdoor air damper is fully open and additional cooling is needed — per ASHRAE Guideline 36, the changeover setpoint should be at least 2°F below the mixed air low-limit to prevent hunting.