Economizer Testing

What an Economizer Does

An airside economizer is a system of dampers, sensors, and controls on a commercial rooftop unit (RTU) or air handler that brings in outdoor air for free cooling when outdoor conditions are favorable. Instead of running the compressor to cool the building, the economizer opens the outdoor air damper to admit cool outdoor air directly into the supply duct.

When outdoor air is cooler and drier than the return air from the building, it makes no sense to recirculate warm indoor air and run the compressor to cool it. The economizer recognizes this condition and automatically opens the outdoor air damper, closes the return air damper, and uses the outdoor air to cool the building directly - often without running the compressor at all.

A properly functioning economizer can reduce annual cooling energy by 20-40% in climates with cool mornings, mild shoulder seasons, and moderate summer nights. In northern climates, the savings can be even greater.

Economizer Operating Modes

The economizer operates in three modes depending on outdoor conditions:

Minimum outdoor air (OA) mode: When outdoor conditions are too warm or too humid for free cooling, the OA damper opens to its minimum position to provide required ventilation air per ASHRAE 62.1. The return air (RA) damper is mostly open, and the compressor handles all cooling. This is normal summer operation.

Free cooling (economizer) mode: When outdoor air is cool enough and dry enough, the OA damper opens beyond the minimum position - potentially to 100% - to provide cooling without mechanical refrigeration. The RA damper closes proportionally. The compressor may be off entirely or may run at reduced capacity to supplement the outdoor air.

Integrated economizer mode: In this advanced mode, the economizer and compressor work together. The outdoor air damper opens to provide partial cooling, and the compressor handles the remaining load. This mode is used when outdoor air alone cannot fully meet the cooling requirement but can still reduce compressor workload.

Dry Bulb Changeover

The simplest economizer control strategy uses outdoor air dry bulb temperature to determine when to switch between modes. A changeover setpoint (also called high-limit setpoint) is programmed into the controller - typically 55-65 degrees F depending on climate and system design.

• When outdoor air temperature is below the changeover setpoint, the economizer is enabled
• When outdoor air temperature is above the changeover setpoint, the economizer returns to minimum OA position

Enthalpy Changeover

Enthalpy changeover considers both temperature and humidity of the outdoor air. Enthalpy (measured in BTU per pound of air) represents the total heat content of the air, including both sensible heat (temperature) and latent heat (moisture). On a humid day, 65-degree outdoor air may have more total heat content than 75-degree indoor air because of the moisture it carries.

The enthalpy changeover setpoint is typically 28 BTU per pound of dry air. When outdoor enthalpy is below this value, economizer operation is enabled.

Some systems use differential enthalpy control, which compares outdoor air enthalpy directly to return air enthalpy. When outdoor enthalpy is lower than return air enthalpy, the economizer opens. This is the most energy-efficient strategy because it always uses outdoor air when it has lower total heat content than the return air, regardless of the absolute values.

Economizers provide free cooling by using outdoor air when conditions are favorable, saving 20-40% of cooling energy. Dry bulb changeover (55-65 degrees F) is simpler but ignores humidity. Enthalpy changeover (28 BTU/lb) accounts for humidity and is more energy-efficient in humid climates. Over 60% of economizers in the field are malfunctioning - testing is essential.