Short answer
An economizer is a set of motorized dampers on a rooftop unit that brings in more outside air when it is cool enough to do some of the cooling, reducing compressor runtime.
What does an economizer do?
An economizer lets a rooftop unit cool the store with outside air when that air is cool and dry enough, so the compressor runs less or not at all during those hours.
A rooftop unit always brings in some outside air for ventilation. An economizer adds the ability to bring in much more, up to the full airflow of the unit, when outside air is cool and dry enough to help cool the store. On those hours the compressor runs less or not at all, and the unit is moving outdoor air through the sales floor instead of mechanically cooling recirculated air.
In coastal Southern California, mild mornings, evenings, and much of the cooler season are economizer hours. Even in hotter inland and desert locations, overnight and early-morning hours can contribute. That is why a working economizer matters to energy use, and why a broken one is so easy to miss: the store still gets cool, just more expensively.
For a one-line definition of the economizer and related controls terms such as setpoint, see the glossary.
What are the parts of an economizer?
An economizer has five working parts: linked outside and return air dampers, an actuator that drives them, sensors, controller logic, and a relief path that lets extra air leave the building.
- Outside air damper and return air damper. Linked blades that open and close in opposition, so as outside air increases, recirculated air decreases.
- Actuator. The motor that drives the dampers. It is exposed to roof heat and weather and is one of the most common failure points.
- Sensors. At minimum an outdoor air temperature sensor; many units also use humidity or enthalpy sensors and a mixed or supply air sensor.
- Controller logic. Decides when outside air is suitable, how far to open, and when to lock out and return to minimum ventilation position.
- Relief or exhaust path. Bringing in more outside air means air must leave the building somewhere, through barometric relief dampers or a powered exhaust.
How does the control decide when to use outside air?
The two common strategies are dry-bulb and enthalpy. Dry-bulb control compares outdoor temperature to a setpoint or to return air temperature. Enthalpy control also considers moisture, so it avoids bringing in air that is cool but humid.
In dry climates, dry-bulb control is often appropriate; in more humid periods, such as Arizona's monsoon season or coastal fog, enthalpy logic can avoid adding moisture load to the space.
Whatever the strategy, the setpoints need to suit the site. A controller left at a factory default, or set without regard to the local climate, can lock the economizer out on hours when it should be working.
How do economizers fail?
Economizers rarely fail loudly. The common failure modes are:
- Stuck closed. The unit never uses free cooling, and the compressor carries every cooling hour.
- Stuck open. The unit brings in large volumes of hot or humid outdoor air on peak days, overloading the coil, raising energy use, and causing comfort complaints that look like an undersized unit.
- Failed or drifted sensor. The controller makes decisions based on a false reading.
- Disconnected linkage. The actuator moves but the dampers do not.
- Wiring or controller fault. Often after another repair, when the economizer was left disconnected and never reconnected.
Because the store usually stays within temperature, none of these generates a service call on its own. They show up in energy bills, in compressor runtime, and sometimes in a failure that arrives years earlier than it should.
What should a maintenance visit check?
A visit should go further than a glance at the damper. Ask that the scope include:
- Commanding the economizer through its full stroke and watching the dampers follow.
- Checking actuator operation, linkage tightness, and blade seals.
- Verifying sensor readings against a reference instrument.
- Confirming changeover setpoints suit the site's climate.
- Confirming the minimum ventilation position.
- Recording any fault codes the controller has logged.
ANSI/ASHRAE/ACCA Standard 180 includes economizer and outside air damper inspection tasks within its maintenance framework, which is part of why REE Mechanical Inc. builds its maintenance programs on that standard. How often those checks happen depends on visit frequency, covered in quarterly vs semiannual PM visits for retail.
Is it worth repairing an economizer on an older unit?
Often, yes, because actuators and sensors are modest repairs relative to the energy and compressor wear they affect.
The exception is a unit that is already near the end of its life; there, the economizer condition becomes one factor in the wider repair-or-replace decision. On a replacement, confirm the new unit's economizer type and controls match current energy code and your building management approach.
How do you tell whether economizers are working across a portfolio?
Compare like with like. Units of similar size, age, and use should show similar compressor runtimes in mild weather, and an economizer whose damper position never changes is probably stuck.
Start with the asset register: which units have economizers, and when were they last verified? Then compare. Units of similar size and age serving similar spaces should show similar compressor runtimes in mild weather; one that runs much longer than its neighbors is a candidate for an economizer check. Where trend data exists, look for damper positions that never change across the day. A short verification pass across a region, done before the mild season begins, often finds more stuck economizers than any single complaint would.
Recording economizer presence and last verification date per unit is part of building an HVAC asset register, and the pre-season pass belongs on the peak-season readiness checklist.
How do economizers fit into a building management system?
When RTUs are connected to a BMS, economizer position, outdoor air readings, and fault alarms can be trended remotely. That turns a silent failure into a visible one.
If your portfolio has controls but no one is watching economizer data, start there. For more on the RTU itself, read how a commercial rooftop unit works.
Methodology: Written by REE Mechanical's team from field experience on multi-site commercial portfolios; standards cited link to the issuing body.