An airside economizer saves energy by using cool outside air to cool a building for free, bypassing the compressor whenever outdoor air is cool and dry enough. When the economizer switches on, mechanical cooling can be cut by 30–70% during mild weather, and in cold climates it can supply 100% outside air for hundreds of hours each year. The device is a set of motorized dampers plus controls that compare outdoor and return air enthalpy or temperature and choose the cheaper cooling source.
How It Works
The controller reads outdoor air temperature and humidity and the return air condition. If outside air can carry the cooling load, the return and exhaust dampers modulate to bring in more outside air while the compressor unloads or stops. A single-zone system may need 100% outside air at 10 °C to hold 24 °C indoors; a larger built-up AHU modulates between minimum ventilation and full economizer based on a dry-bulb or enthalpy setpoint.
Dry-Bulb vs. Enthalpy Control
| Factor | Dry-Bulb (temperature) control | Enthalpy (temp + humidity) control |
|---|---|---|
| Sensors | One temperature sensor | Temperature and humidity sensors |
| Cost | Low | Moderate |
| Best climate | Cold, dry winters | Humid, mixed climates |
| Risk | May admit moist air | Slightly complex control |
| Energy saving | Good | Best in humid regions |
A dry-bulb economizer is cheaper and works well where humidity is low; an enthalpy economizer avoids pulling warm, damp air that would raise latent load, so it saves more in humid climates even though it needs two sensors per point.
How Many Free-Cooling Hours
The savings depend on climate. A building in a northern climate with 2,500–3,500 annual free-cooling hours may run the compressor only 30% of the year; a southern humid site with 800–1,500 hours still earns meaningful savings. As a rule, if the outdoor air is below about 12 °C dry-bulb or below the return-air enthalpy for more than 1,000 hours a year, an airside economizer pays back in 1–4 years.
When to Use an Airside Economizer
- Use it when cooling capacity exceeds the code threshold, typically about 11 kW for air-cooled units.
- Use it in cold or mixed climates with more than 1,000 free-cooling hours per year.
- Use enthalpy control in humid regions to avoid adding moisture load.
- Avoid it where outdoor air is consistently hot and humid with under 500 free-cooling hours.
- Pair it with a MERV-13 or better filter since volume of outside air is high.
Decision Checklist
- Check the local climate free-cooling hours before specifying; below 500 hours, justify the cost.
- Choose enthalpy control when outdoor humidity regularly exceeds 60% in the cooling season.
- Size the return and exhaust fans for full economizer airflow, not just minimum ventilation.
- Add a freeze-protection strategy (preheat or modulating damper) for sub-zero climates.
- Confirm the economizer sequence with the building management system during commissioning.
Integration and Limits
An airside economizer shares the AHU with the cooling coil, so duct and fan must handle the higher airflow; expect fan power to rise 5–15% at full economizer compared with minimum outside air. The payoff is large compressor savings. Equipment that is balanced to G2.5 and performance-tested before shipment keeps the damper and fan sequence stable under these swings.
Control Sequence in Detail
The economizer controller reads outdoor air temperature and humidity and the return air condition every minute. When outdoor enthalpy is below the return setpoint, it modulates the outdoor-air and return dampers toward 100% while the cooling coil unloads; when outdoor air warms, it reverses and the compressor resumes. A 3–5 °C deadband around the changeover point stops the dampers hunting, and a minimum-position stop keeps ventilation above code even at full economizer.
Pairing with Demand Control Ventilation
An airside economizer and CO₂-based DCV work together: the economizer sets how much outside air is cheap to use, and DCV sets how much is needed. When both are active, the system brings in the DCV-required amount and, if the economizer allows, more for free cooling with no extra compressor cost. The combined control can cut total HVAC energy 25–45% in cold mixed climates with long shoulder seasons.
Humidification and Free-Cooling
In winter, free cooling with very cold, dry air can over-dry a space, so add a humidifier or limit minimum outside air when indoor RH drops below 30%. An enthalpy economizer naturally avoids the driest air by comparing moisture, while a dry-bulb unit may admit 2 °C desert air that needs steam added back. Coordinate the economizer setpoint with the humidity setpoint to avoid paying for both cooling and humidification at once.
Retrofit and Commissioning
Adding an economizer to an existing AHU means new dampers, a mixed-air sensor, and a control rewrite; budget for reinforcing the return fan, which must pull full economizer airflow. At commissioning, force the unit into economizer mode and confirm the compressor drops to zero load while the space holds setpoint, then check freeze protection at −10 °C. A working economizer typically pays back in 1–4 years on a 20–40 kW system.
Common Setpoint Errors
Designers often set the changeover 2–3 °C too high, so the compressor runs when free cooling was available, or too low, so the space overheats before the economizer engages. A good start is a dry-bulb changeover near 12–13 °C or an enthalpy changeover matched to the return-air condition; tune it during the first cooling season using trend data rather than leaving the factory default in place.
Measuring the Saving
To prove the economizer's value, trend the outdoor-air fraction and the compressor kWh through a full year and compare with a baseline of no economizer. Most buildings show 200–1,200 equivalent full-load hours removed from the compressor, with the largest gains in spring and autumn. Report these hours to the owner so the feature is maintained; an economizer disabled during a controls patch quietly loses its whole saving.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
FAQ
How much energy does an airside economizer save?
An airside economizer can cut compressor cooling energy by 30–70% during mild weather and supply 100% outside air for hundreds of hours a year. In cold climates with 2,500–3,500 free-cooling hours the payback is often 1–4 years, while humid sites with under 500 hours rarely justify the added damper and control cost.
Dry-bulb or enthalpy control—which is better?
Dry-bulb control is cheaper and suits cold, dry climates; enthalpy control adds humidity sensing but avoids admitting moist air, so it saves more in humid regions. Choose enthalpy when outdoor humidity regularly exceeds 60% in the cooling season to prevent raising the indoor latent load.
Is an economizer required by code?
In most U.S. climates, ASHRAE 90.1 requires an economizer above a cooling-capacity threshold of about 11 kW for air-cooled equipment, with waivers only for very few free-cooling-hour climates. Local codes adopt specific editions, so confirm the threshold and any climate exceptions enforced in your jurisdiction.
Does an economizer increase fan energy?
Yes, because the AHU must move full outside-air volume instead of minimum ventilation, raising fan power roughly 5–15% at full economizer. The far larger compressor savings still make it worthwhile in most climates; size the return and exhaust fans for the peak economizer airflow to avoid bottlenecking the sequence.