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Solving High Humidity Challenges in HVAC Systems

Solving High Humidity Challenges in HVAC Systems

One of the biggest contributing factors to high humidity is the lack of compressor run-time, commonly known as short cycling. This is often a result of a system being oversized, providing too much cooling capacity for a space, and satisfying the thermostat too quickly. Having some form of capacity control is the most effective way to prevent short cycling from occurring and the APR Control is the perfect solution!

“What Do You Mean My HVAC System Is Oversized?”

We’ve said this time and time again, all DX air conditioning systems are oversized by design. Lack of “design day conditions” creates this situation. All air conditioning systems for any building or application are designed to a standard that is referred to as a “Design Day” or “Peak Load Conditions.”

Design day conditions refer to the hottest day of the year, the highest indoor cooling load, and ventilation necessary for the occupants, contributing to the maximum that the unit expects to see during the year. Taking into consideration all the things that create load, such as:

  • Full ambient load (highest outside temperature for the region)
  • Full building occupancy
  • Lighting
  • Equipment being used in the space (computers, printers, machinery, etc.)
  • Occupant fresh-air ventilation – newly prioritized and increased

The DX system is then supposed to be able to provide maximum cooling for the expected interior load in that space. The reality is that conditions change day to day and hour to hour, meaning that for most of the cooling season, your properly designed air conditioner is oversized for the conditions of the space.

“Why isn’t my air conditioning system dehumidifying?”

Even with a properly designed and operating piece of HVAC equipment, operating outside of design parameters (during low load conditions) leads to overcooling and short cycling.

An oversized air conditioner will quickly satisfy the thermostat, which turns off the compressor. A compressor that is off, will not dehumidify! The system needs adequate runtime to start effective moisture removal. While the temperature in the space may be at setpoint, the compressor just hasn’t run long enough to remove humidity. Remember that air conditioning isn’t just about temperature, it’s also about removing the moisture from the air (decreasing relative humidity)!

When the compressor shuts off, the evaporator coil becomes warmer than the dew point of the air going across the coil and is unable to remove any moisture. Meanwhile, the fan will stay on and blow ALL that water that is on the coil and in the drain pan right back into the space. THEREFORE, extended runtime is needed to counter-act the RE-humidification from a wet coil and full drain pan.

The solution is simple – the APR Control!

The APR Control will reduce the total capacity of the DX air conditioner to match the lower load demand in the space. Providing up to 80% turndown on any DX system, the APR controls the system’s cooling capacity to the varying space conditions in real time.

To lower the relative humidity in the space, you need the compressor on and evaporator coil below the dew point of the air going across coil. The APR varies the amount of refrigerant flow to the rest of the circuit as needed, resulting in reduced capacity at the evaporator to avoid overcooling. The evaporator continues to remove humidity, even at part-load operation.

This load matching capability extends the runtime to keep the compressor active for a longer dehumidification cycle, consistent discharge air temperatures (as necessary in VAV, VVT, or make-up-air systems), all while reducing the short cycling.

Rawal Devices’ APR Control offers a simple, reliable, and cost-effective solution that can eliminate or prevent high humidity because of compressor short cycling.

If you are experiencing ongoing customer complaints about high humidity, likely due to an oversized system, please give us a call to discuss how the APR Control can eliminate these headaches or prevent them from happening at all!

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The Rawal APR Control is a variable capacity enhancement to DX HVAC systems.