Indoor comfort and air quality – the APR Control provides modulation for humidity control, the benefit of extended runtimes, and the ability to maintain a system in a dehumidifying mode. These factors allow a simple single-stage DX system to respond to a wide variety of operating conditions to solve a multitude of problems.

By using the APR Control to extend system runtime, you are essentially keeping thermostatic controls from being satisfied as quickly as usual. In addition, because the APR Control keeps a portion of the evaporator coil in a dehumidifying mode at part-load conditions, humidity control is improved. The device will also maintain the average temperature of the discharge air.


A 5-ton package direct expansion (DX) air conditioning system is required to provide space cooling for a meeting room. The occupancy of the space requires that 30% of the system’s capacity be dedicated to conditioning the fresh air necessary to satisfy ventilation codes. The high percentage of outside air will cause large fluctuations in load that will keep standard on/off thermostatic controls from responding accurately or maintaining proper space humidity levels.


The 5-ton unit can be modified with an APR-410-3 Control (3.5 tons capacity control). This will provide 70% capacity control, allowing the system to modulate capacity to match changing load conditions from 5 tons down to about 1.5 tons and all points in between. Because the APR Control allows the system to operate down to 35% of the total capacity, and permits the evaporator coil to stay below the dew point in dehumidifying mode, the system will keep the humidity levels low without the addition of electric reheat.

Additional component solutions can be added in the rare occasion when the APR Control alone is not sufficient for the challenges of the space. These solutions are typically simple and always cost-effective when compared to prepackaged products and their many limitations. They include:

  • Reheat (less than usually necessary) with humidistat control to extend runtime and force a system run
  • Thermostatic controls that allow setpoint flexibility
  • Reduced air flow to improve cycling and latent capacity

As a result of correcting many installed systems, Rawal Devices experts have been called on to provide the system designer with the flexibility to accommodate the calculated peak load/demand, but also provide the system with the necessary capacity modulation to meet a wide variety of load conditions.

To learn more about humidity control at part load, Click Here.