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  • Writer's pictureHobbs & Associates

Enhanced Indoor Air Quality with Advanced Humidity Control Solutions

As an HVAC mechanical contractor or engineer, you understand the critical role indoor air quality (IAQ) plays in the health and comfort of building occupants. One of the significant challenges in maintaining optimal IAQ is controlling humidity levels. High humidity can lead to various issues, including mold growth, condensation, wood rot, and increased susceptibility to sickness and allergic reactions. Addressing these issues effectively requires advanced and precise humidity control solutions.

Hobbs and Associates AAON DX Packaged Unit Hot Gas Reheat

The Challenge of Humidity Control

Traditional cooling systems often struggle to manage humidity effectively, especially when moisture removal is critical. These systems are typically designed to meet dry bulb temperature setpoints. Once these setpoints are reached, the cooling system shuts off, halting moisture removal. This can lead to issues such as moisture re-evaporation from the cooling coil back into the air supply, causing relative humidity to rise again.

This is where innovative approaches to humidity control come into play. By integrating advanced technologies like the modulating hot gas reheat system, HVAC systems can provide precise and efficient humidity control, ensuring occupant comfort and energy efficiency.

Modulating Hot Gas Reheat System

One effective solution to humidity control leverages a design that includes a reheat coil, a modulating 3-way hot gas reheat valve, a supply air temperature sensor, and a simple yet effective controller. This system offers several advantages over traditional on/off solenoid valve systems:

  • Consistent Occupant Comfort: Unlike systems that cause temperature swings, the modulating hot gas reheat system ensures a steady supply of air temperature, maintaining uniform comfort for occupants.

  • Energy Efficiency: By modulating the amount of hot refrigerant gas passing through the reheat coil, the system only uses the energy necessary to achieve the desired humidity and temperature levels. This minimizes energy consumption without compromising performance.

  • Precision Control: The supply air temperature sensor and controller allow for precise temperature and humidity control, which is especially beneficial in applications requiring tight environmental conditions.

Hobbs and Associates AAON DX Split System Hot Gas Reheat

Application Scenarios

The modulating hot gas reheat system is versatile and can be applied in various settings, including:

  • Supermarkets: Supermarkets pose unique challenges, with different areas requiring distinct temperature and humidity controls. From produce sections to dairy and frozen foods, precise humidity control prevents moisture buildup on refrigeration cases and windows while ensuring customer comfort.

  • Underfloor Air Distribution (UFAD) and Variable Air Volume (VAV) Systems: These applications demand precise dehumidification and supply air temperature control, effectively managed by the modulating hot gas reheat system.

  • Ventilation Air with Chilled Beams: This system prevents condensation on chilled beams by controlling the space dew point, ensuring efficient operation and maintaining comfort.

How It Works

The system uses a dehumidistat in the conditioned space to control a 3-way solenoid valve, which bypasses hot refrigerant gas from the compressor discharge to a reheat coil. This allows the unit to continue dehumidifying the air after the dry bulb setpoint is satisfied. The modulating valve ensures the correct amount of reheating, avoiding uncomfortable temperature swings and optimizing energy use.

Benefits for HVAC Engineers

For HVAC mechanical engineers, the benefits of implementing a modulating hot gas reheat system are clear:

  • Improved IAQ: Maintaining optimal humidity levels can significantly improve indoor air quality, reducing the risk of mold and other moisture-related issues.

  • Enhanced Comfort: The system ensures consistent temperature and humidity control, providing a comfortable environment for building occupants.

  • Energy Savings: The modulating control minimizes energy use by only reheating as necessary, helping to meet energy codes and reduce operational costs.

Installation and Configuration

These systems are designed for easy installation and integration into existing HVAC setups. Packaged units typically come with factory-installed refrigerant piping, a modulating hot gas reheat valve, a reheat coil, a liquid line receiver, and a controller. The supply air temperature sensor is also provided for field installation in the supply air ductwork.

For split systems, the air handling unit includes a factory-installed reheat coil and check valves on the liquid and hot gas reheat lines. The condensing unit features a factory-installed modulating hot gas reheat valve, liquid line receiver, and controller. Field connections for the suction line, liquid line, and a single hot gas reheat line are required, along with a recommended suction line accumulator/sub-cooler for VAV and makeup air systems.

Hobbs and Associates AAON 100% OA Unit Hot Gas Reheat

Control Options

Standalone and integrated control options exist for these modulating hot gas reheat systems. The standalone controller is factory-configured and ready to accept inputs from the field-mounted supply air temperature sensor and dehumidistat. It includes built-in logic to ensure positive oil return to compressors and features an LCD display for status monitoring, setpoint adjustments, and valve control during startup and maintenance.

Alternatively, factory-provided unit controllers offer enhanced control capabilities. They can prioritize dehumidification, ensuring the system operates whenever humidity exceeds the setpoint, or operate non-prioritized, maintaining dehumidification only when cooling and heating demands are satisfied. Touchscreen controllers include built-in temperature and humidity sensors, allowing users to select the desired control parameters for both occupied and unoccupied modes.

Advanced Dehumidification for Outdoor Air Systems

The modulating hot gas reheat system is particularly effective for outdoor air systems, which must handle significant volumes of humid outside air. High-capacity evaporator coils feature multiple rows for increased surface area, enhance latent heat transfer, and provide superior dehumidification. This capability is crucial for maintaining neutral supply air temperatures while conditioning large amounts of outside air, making it ideal for compliance with modern ventilation standards.

Hobbs and Associates AAON 100% OA Unit Psychrometrics


Modulating hot gas reheat systems into your HVAC designs can significantly improve humidity control, enhancing indoor air quality and occupant comfort while reducing energy consumption. As HVAC mechanical engineers, leveraging these advanced solutions ensures you can meet the evolving demands of modern buildings and provide healthy and comfortable environments for occupants.

These advanced humidity control solutions address the limitations of traditional systems and offer precise, energy-efficient performance. Modulating hot gas reheat systems is a reliable and effective choice for designing systems for supermarkets, office buildings, or any application requiring stringent humidity control.

Hobbs and Associates Dehumidification Strategies Comparison

This article is based on the advanced equipment and application solutions outlined in the AAON Humidity Control Solutions manual.

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