The Effect of Dehumidification on the Indoor Environment

Conventional HVAC Systems and Humidity Control

Conventional HVAC cooling systems, such as central air conditioners, are primarily designed to control the sensible heat of a space—that is, the air temperature. However, they also play a crucial role in controlling latent heat, which is the energy contained in water vapor (humidity).

The dehumidification process in a standard air conditioner is a byproduct of its cooling function. As warm, humid air is drawn into the system, it passes over a cold evaporator coil. When the temperature of the coil drops below the dew point of the air, the water vapor in the air condenses into liquid. This liquid water then drips into a drain pan and is removed from the building. This process is what "wrings" moisture out of the air, effectively lowering the relative humidity (rH).

The goal of maintaining indoor humidity between 40% and 60% rH is critical. When rH drops below 40%, the air can become too dry, leading to issues like irritated sinuses, dry skin, and static electricity. When rH exceeds 60%, it creates an ideal environment for mold, mildew, dust mites, and bacteria to thrive. High humidity also makes the air feel "muggy," causing occupants to feel warmer and less comfortable even at a lower temperature setting.

By cooling and dehumidifying the air, the HVAC system helps prevent dew point condensation. Condensation forms when a surface temperature drops below the dew point of the surrounding air. In a building, this can happen on cold surfaces like windows, pipes, or ductwork. If the HVAC system successfully lowers the indoor dew point by removing moisture, it reduces the risk of this type of damaging condensation from occurring within the structure.

Infiltration and Latent Humidity Load

In addition to managing humidity generated indoors (from cooking, showering, breathing, etc.), a conventional HVAC system must also contend with infiltration—the flow of outside air into the structure. In humid climates, this infiltration can introduce a significant latent humidity load.

A properly sized and maintained HVAC system is designed to handle this load. It runs long enough to both cool the space and remove a sufficient amount of moisture. However, this is where a challenge can arise. If the unit is oversized for the sensible cooling load, it may cool the space too quickly and shut off before it has had enough time to adequately dehumidify the air. This can lead to a condition where the indoor temperature is comfortable, but the humidity remains high, resulting in that "cold and clammy" feeling.

Supplemental Dehumidification

To address the issue of high humidity without relying on excessive cooling and potentially overcooling the space, a building's HVAC system may incorporate stand-alone dehumidification. These systems are designed specifically to handle the latent humidity load.

A whole-house dehumidifier is often integrated directly into the ductwork of the central HVAC system. It works independently of the cooling cycle, allowing it to remove moisture from the air even when the air conditioner isn't running. This is particularly useful during times when the outdoor temperature is mild but the humidity is high, such as in the spring or fall.

By using a dedicated dehumidifier, the HVAC system can be optimized to focus on its primary role of cooling, while the dehumidifier handles the humidity. This two-part approach ensures that both temperature and humidity are maintained within the optimal range, leading to greater comfort, improved indoor air quality, and more efficient energy use. It prevents the need for the air conditioner to run for extended periods just to dehumidify, which could lead to uncomfortably cold indoor temperatures and wasted energy.