What are the three control strategies for outside air economizers?

• A. Dry-bulb control, humidity ratio control, and dew point control.
• B. Dry-bulb control, dew point control, and enthalpy control.
• C. Enthalpy control, humidity ratio control, and dew point control.
• D. Dry-bulb control, enthalpy control, and optimized control considering both temperature and humidity.

Answer: (D)

Outside air economizers are driven by how favorable the outdoor conditions are for cooling without adding unnecessary energy or moisture load. The three recognized control approaches use different ways to decide when to bring in outside air.

First, dry-bulb temperature control uses a simple temperature threshold. If the outdoor dry-bulb temperature is below a set point, the economizer opens to bring in more outside air. It’s easy to implement but only considers temperature and can ignore how humidity affects cooling and comfort, which can lead to moisture issues in humid climates.

Second, enthalpy control looks at the total energy content of the air, combining both sensible and latent components. By comparing the outside air enthalpy to the return air enthalpy, the system admits outdoor air only when it actually lowers the total energy needed for conditioning. This handles humidity effects better than dry-bulb control and generally improves energy savings, especially in climates where moisture matters.

Third, optimized control that considers both temperature and humidity takes a more advanced approach. It uses information on how both variables interact to minimize energy use while maintaining comfort and IAQ. This method evaluates conditions more holistically and can adjust outdoor-air intake dynamically to balance latent and sensible loads, providing the best overall performance among economizer strategies.

The other options mention dew point or humidity ratio in ways that aren’t standard standalone economizer strategies, or pair them with unrelated controls. Dew point alone doesn’t capture energy implications as effectively, and humidity ratio alone doesn’t reflect how energy use changes with outdoor air. Hence, the combined, optimized approach alongside dry-bulb and enthalpy control best represents the three control strategies.