Strategies for Success: Energy

July 1, 2008

The ability to independently control humidity and temperature levels in education institutions is becoming increasingly important.

Jeff Hartnett

The ability to independently control humidity and temperature levels in education institutions is becoming increasingly important. Typical HVAC equipment with temperature-only control can overcool a space in an attempt to reduce humidity. Maintaining relative humidity (RH) levels below 65 percent per ASHRAE's recommendation helps improve indoor air quality that can positively affect the health and productivity of occupants.

Some factors to consider when selecting either rooftop or split systems for optimum temperature and humidity control:

Stage compressor(s) to match cooling capacity to cooling load requirements. Equipment with staging capability, to better match capacity and load requirements, will be less likely to overcool the space in an attempt to meet reduced RH levels. For single-compressor systems, look for two-stage compressors. For multi-compressor systems, look for the HVAC controls to stage compressors on and off as the load changes.
Select HVAC equipment with OEM-designed and integrated dehumidification systems. HVAC equipment can be integrated with dehumidification systems, such as hot gas or liquid reheat. Generally, the air is cooled first, pulling out humidity. Then the reheat coil, placed downstream from the evaporator, uses either the hot gas from the compressor or hot liquid from the condenser coil to heat up the cooled and dehumidified air to prevent overcooling. Third-party add-on dehumidification systems can be complicated to install, and difficult to control and troubleshoot.
Choose the appropriate sensible-to-latent ratio for dehumidification. The sensible-to-latent ratio (S/T ratio) determines the ratio of capacity available for cooling and dehumidification. The lower the S/T ratio, the greater capacity for dehumidification. Look for HVAC systems that can vary the S/T ratio, depending on the cooling and humidity loads, to optimize comfort. Look for engineering data from the OEM to match equipment and space S/T ratios appropriately.
Choose simple controls for integrated dehumidification systems. Selecting HVAC systems with integrated controls for independent temperature and humidity controls simplifies installation and troubleshooting. Integrated controls also provide a lower total cost of ownership, with a lower installation cost and fewer parts to repair and maintain.

For rooftop units, systems are available that use a humidistat to sense RH levels, and an intelligent DDC to control the staging of compressors in reheat and cooling modes, varying the S/T ratio to match space conditions. For split systems, a field accessory can be mounted downstream of the air handler or furnace, requiring only a thermostat to sense and control RH levels, with no additional control boards or sensors.