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How to select the right HVAC equipment for optimal humidity control

The ability to independently control humidity and temperature levels 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% per ASHRAE’s recommendation helps improve indoor air quality that can positively affect the health and productivity of occupants.

Below are some factors to consider when selecting either rooftop or split systems for optimum temperature and humidity control.

  1. 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.
  2. 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 of 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.
  3. 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 appropriately match equipment and space S/T ratios.
  4. Choose simple controls for integrated dehumidification systems.
    Selecting HVAC systems with integrated controls for controlling temperature and humidity independently simplifies installation and troubleshooting. Integrated controls also provide a lower total cost of ownership, with lower installation cost and fewer parts to repair and maintain.

Rooftop units and split system components from Lennox can be ordered with the patented Humiditrol® dehumidification system. The rooftop unit system uses a humidistat to sense RH levels, coupled with an intelligent controller to control the staging of compressors in reheat and cooling modes, varying the S/T ratio to match space conditions. Energence 3-5 ton rooftop units feature an enhanced Humiditrol system that is 20% more effcient.

For split systems, Lennox’ Humiditrol system offers an OEM-designed field accessory to be mounted downstream of the air handler or furnace. Installation requires only Lennox’ thermostat that senses and controls RH levels, and requires no additional control boards or sensors. This system is available for both the T-Class and S-Class 3- to 5-ton air conditioner and heat pump split systems.

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