Hot Gas Reheat (HGRH) is a dehumidification strategy used in air conditioning systems where hot refrigerant gas discharged from the compressor is diverted to a reheat coil positioned downstream of the evaporator coil. This process allows the system to cool air below its dew point for moisture removal, then warm it back to a comfortable supply temperature without introducing additional moisture. The result is precise, independent control of both temperature and humidity within a conditioned space.
How It Works
In a standard cooling cycle, the compressor sends high-pressure, high-temperature refrigerant gas to the condenser. In an HGRH system, a portion of this hot gas, typically between 10% and 30% of total refrigerant flow, is redirected through a separate reheat coil located after the evaporator. The evaporator first cools the supply air to approximately 48°F to 52°F (9°C to 11°C), extracting moisture as condensation on the coil surface. The subcooled, dehumidified air then passes over the reheat coil, where it is warmed to a target supply temperature, usually between 55°F and 65°F (13°C and 18°C). The reheat coil itself typically operates at surface temperatures ranging from 110°F to 140°F (43°C to 60°C). Modulating valves or solenoid valves control the amount of hot gas diverted, allowing the system to adjust reheat capacity based on real-time humidity sensor feedback.
Applications
Hot Gas Reheat is essential in environments where humidity must be controlled independently of sensible cooling load. Common applications include:
- Healthcare facilities: Operating rooms and sterile processing areas that require relative humidity levels between 30% and 60% per ASHRAE Standard 170.
- Data centers and server rooms: Spaces where condensation poses a risk to sensitive electronic equipment, with humidity targets often held between 40% and 55% RH per ASHRAE TC 9.9 guidelines.
- Clean rooms and laboratories: Environments demanding tight humidity tolerances, sometimes within ±2% RH.
- Dedicated Outdoor Air Systems (DOAS): Units that process 100% outdoor air and must deliver neutral or dry air to the building, particularly in hot and humid climates.
- Museums and archives: Facilities protecting moisture-sensitive collections and materials.
Standards and Codes
ASHRAE Standard 90.1 (Energy Standard for Buildings) places limits on the use of reheat to prevent excessive energy consumption, but provides exceptions for systems using recovered energy such as hot gas reheat. ASHRAE Standard 62.1 governs ventilation requirements that often necessitate dehumidification of outdoor air. In healthcare, ASHRAE Standard 170 and FGI Guidelines define the humidity ranges that frequently make HGRH a practical necessity.
Practical Significance
Compared to electric resistance reheat, HGRH recovers energy already present in the refrigeration cycle, making it a more efficient reheating method. However, overall system efficiency is still lower than cooling without reheat, as the system must overcool air before rewarming it. Modern HGRH systems use modulating hot gas valves and integrated humidity sensors to minimize energy waste by activating reheat only when dehumidification demand requires it. Proper refrigerant piping design, including oil return provisions and pressure drop calculations, is critical to reliable HGRH operation and long-term compressor health.