If the return dew point is above the coil dew point, which control method is preferred for humidification?

• A. Enthalpy control
• B. Dry bulb control
• C. Humidity ratio control
• D. Dew point control

Answer: (A)

Enthalpy control focuses on the total energy content of the air, which combines both its temperature (sensible load) and its moisture content (latent load). This makes it the most reliable way to manage humidification across varying moisture conditions, because decisions are made from a single metric that reflects how much energy must be added or removed.

In this scenario, the return air has a dew point higher than the coil’s dew point, meaning it already carries a significant amount of moisture. If you rely on a control method that looks only at temperature or only at moisture content, you can end up either over-humidifying or under-humidifying the space. Dry-bulb (temperature) control ignores moisture, so it can mistake comfort needs for just a temperature target. Humidity ratio control (moisture content) alone doesn’t account for the accompanying temperature, which can lead to inappropriate humidity independent of how much energy the air actually carries. Dew point control can be slow to respond and doesn’t directly balance the energy exchange happening in the mixing and conditioning processes.

Enthalpy control, by evaluating the air’s overall energy content, responds appropriately to high moisture levels in the return air and to the conditioning the coil provides, preventing unnecessary humidification when the air is already moist. That makes it the preferred method for humidification under these conditions.