Air-side economizers are one of the best tools commercial HVAC systems use to save energy. When outdoor conditions are right, an air-side economizer allows a system to cool a building using outside air instead of running compressors. This can lower energy bills and reduce wear on equipment.
But air-side economizers do not all work the same way. How well an economizer performs depends on how it decides when outside air should be used. That decision is controlled by sensors and logic inside the system.
At Harold Brothers Mechanical Contractors, we often see air-side economizers blamed for comfort problems or rising energy costs. In many cases, the real issue is not broken equipment, but how the air-side economizer is controlled. In this article, we’ll explain the three most common air-side economizer control types: dry-bulb, enthalpy, and differential control, and when each one makes sense.
An HVAC economizer allows a system to bring in more outdoor air for cooling when conditions are favorable. Instead of relying on mechanical cooling, the system uses cooler outside air to help meet the building’s needs.
An air-side economizer is not just one part. It includes dampers, sensors, and control programming. If any of these parts are not working correctly, the air-side economizer may waste energy or cause comfort issues, even if the system seems to be running.
Economizers depend completely on their control strategy. If the logic is wrong, the system may bring in air when it shouldn’t, or fail to use outdoor air when it could.
Poor air-side economizer control can lead to:
Choosing the right control strategy helps avoid these problems. In practice, many of these issues develop slowly. Setpoints get adjusted, sensors drift, or seasonal changes expose weaknesses in the control logic. Without periodic review, an economizer can continue operating in a way that looks normal but quietly increases energy use and discomfort.
Dry-bulb control uses outdoor air temperature only. If the outdoor temperature is below a set value, the economizer opens and allows outside air to cool the building.
For example, if the system is set to enable air-side economizer mode below 65°F, outdoor air will be used whenever the temperature drops below that point.
Dry-bulb control is simple and easy to understand. It requires fewer sensors and less programming, which makes it less expensive and easier to maintain.
Temperature alone does not tell the full story. Outdoor air can be cool but still very humid. In humid or coastal areas, this can cause moisture problems and discomfort inside the building.
Dry-bulb control works best for:
Enthalpy control looks at both temperature and moisture in the outdoor air. Instead of using a fixed temperature, the system compares the total heat in outdoor air to the air inside the building.
If the outdoor air has less heat energy, the air-side economizer opens. If not, it stays closed.
Because it accounts for humidity, enthalpy control provides better comfort in many buildings. It helps prevent damp or sticky indoor conditions and improves how the system responds to real weather conditions.
Enthalpy systems need more sensors and more detailed programming. If sensors are not accurate or well-maintained, the air-side economizer may not work as intended. Even small calibration errors can change when the economizer enables or locks out, especially during shoulder seasons when outdoor conditions are close to decision thresholds.
Enthalpy control is often used in:
Differential control compares outdoor air directly to return air from inside the building. The air-side economizer only opens when outdoor air is better than indoor air for cooling.
This comparison can be based on temperature, enthalpy, or both.
Differential control adjusts to real building conditions instead of relying on fixed setpoints. This often leads to better comfort and more consistent energy savings.
This approach requires more sensors, stronger control programming, and ongoing review of system data. Without proper setup, troubleshooting can be more difficult. Because differential strategies compare multiple data points, performance depends on all sensors reading accurately and logic remaining consistent over time. When one input drifts, the system can make the wrong decision without triggering an obvious fault.
Differential control works best in:
|
Control Type |
What It Measures |
Strengths |
Limitations |
Best Fit |
|
Dry-Bulb |
Outdoor temperature |
Simple, low cost |
Ignores humidity |
Dry climates, simple systems |
|
Enthalpy |
Temperature + moisture |
Better comfort control |
Sensor accuracy critical |
Humid or mixed climates |
|
Differential |
Outdoor vs return conditions |
Most adaptive |
Higher controls complexity |
BMS-driven buildings |
Many economizer problems are caused by control issues, not broken parts. These issues include:
These types of issues rarely appear as hard failures. Instead, they accumulate gradually as overrides are left in place, sensors fall out of calibration, or control sequences are changed without documentation. As a result, economizers often appear functional while delivering inconsistent comfort and higher energy costs.
When this happens, the air-side economizer may appear to work but still waste energy.
At a mid-size office building in Plymouth, the facility team kept getting comfort complaints every spring and fall. The air-side economizer was set up using dry-bulb control only, so the system pulled in cool outside air even on damp coastal days.
Offices felt clammy, and the HVAC units ran longer trying to remove moisture, which led to repeat service calls even though no equipment had failed. Once the air-side economizer control strategy was reviewed and corrected, the comfort issues and callbacks dropped significantly.
Follow-up reviews helped ensure the updated control strategy stayed aligned as outdoor conditions changed throughout the year.
Economizers rely heavily on the building automation system. Sensors, schedules, and control logic all affect when and how outdoor air is used.
When these elements fall out of alignment, economizer performance often suffers. Fixing the problem usually requires reviewing controls, not replacing equipment.
Economizers can save energy and improve comfort, but only when the control strategy fits the building and climate.
Dry-bulb, enthalpy, and differential economizers all have a place. Problems usually start when the wrong strategy is used or controls are not maintained.
If your economizer feels unreliable, the cause is often inside the building automation system. Understanding how controls and sensors work together is key.
At Harold Brothers Mechanical Contractors, we help building owners and facility teams identify control issues that affect comfort and energy use. To learn how economizers connect to the bigger picture, read our article on Commercial BMS & Controls Optimization for Comfort and Efficiency to see how proper controls can bring systems back into balance.