What is the ideal combination of control methods for humidification based on return dew point relative to coil dew point?

• A. Use enthalpy control if the return dew point is above the coil dew point; use dry bulb control if it is below.
• B. Use humidity ratio control if the return dew point is above the coil dew point; use enthalpy control if it is below.
• C. Always use dry bulb control regardless of dew point.
• D. Always use enthalpy control regardless of dew point.

Answer: (B)

The key idea is to choose the humidification control variable based on how much latent load the air is carrying as it enters and how the coil will affect moisture removal or addition. When the return air dew point is higher than the coil dew point, the air already contains a relatively high moisture content and the cooling coil will tend to remove latent moisture as air is cooled. In this situation, controlling the moisture content directly—i.e., using humidity ratio control—keeps the supply air’s moisture level on target without overshooting, because the latent load is the primary driver and you want precise moisture content independent of temperature changes.

If the return air dew point is lower than the coil dew point, the air is comparatively dry and the humidifier will be adding moisture with a noticeable energy effect. Here, using a control that accounts for both temperature and moisture—enthalpy control—helps balance the combined sensible and latent loads, preventing over-humidification while still meeting moisture requirements.

Choosing dry-bulb control all the time ignores moisture content and can lead to poor humidity control, while using enthalpy control all the time can be less precise when the moisture content is the dominant factor. The pairing described ensures the control strategy matches the prevailing latent load indicated by the dew point relationship.