What does the term differential dry-bulb control refer to?

• A. A control strategy that compares the dry-bulb temperature of outside air to return air.
• B. A control strategy that uses humidity ratio to set humidification.
• C. A control that adjusts based on dew point only.
• D. A strategy that uses outside air velocity to regulate flow.

Answer: (C)

The idea behind differential control is using a signal that represents how far two conditions are from each other to drive the system. In the context of differential dry-bulb control, the system uses a signal derived from a moisture-related criterion rather than just plain air temperature differences. Saying it adjusts based on dew point only means the control responds to the moisture content of the air (dew point) as the governing parameter, rather than relying on dry-bulb temperature differences alone. This approach targets humidity and condensation risk directly, which is why it’s described as a dew-point–based differential control.

In practice, this would mean the outdoor-air intake, humidification, or related controls are modulated primarily by dew point information to maintain acceptable humidity levels and avoid condensation. The other options describe different types of control: comparing outside and return air dry-bulb temperatures is a temperature-difference approach typical of economizers; using humidity ratio to set humidification targets focuses on latent moisture without involving a differential signal; and regulating flow based on outside air velocity is about flow rate control rather than a differential humidity signal.