Hot gas reheat explained. #hvac

While both hot gas bypass and hot gas reheat use the compressor's discharge gas to perform specific functions in HVAC equipment, they are actually very distinct processes. Despite their similar-sounding names, they differ significantly from one another.

The use of hot gas bypass is a method employed to safeguard the refrigeration system from freezing during periods of low load or low suction temperature. Low suction temperatures typically manifest during part load conditions.

The hot gas bypass valve operates by diverting high-pressure, high-temperature discharge gas to the low side of the system, commonly at the inlet of the evaporator coil. By introducing a portion of high-pressure, high-temperature gas to the evaporator/suction side of the system, the hot gas bypass produces an artificial load on the evaporator.

While this technique is a cost-effective and straightforward means of preventing the HVAC system from freezing the coil, it is highly inefficient and can adversely impact oil return.

Hot gas reheat is a process that utilizes high-pressure and high-temperature gas from the compressor discharge to reheat air leaving the evaporator coil. Its primary use is in dehumidification modes to achieve the desired space sensible temperature set point.

During dehumidification mode, the air leaving the evaporator coil typically ranges between 40-55F, which can be too cold for comfort cooling applications at part load conditions. To address this, instead of dissipating the heat rejected to the atmosphere via the condenser, the hot refrigerant gas is directed to a hot gas reheat coil downstream of the cooling coil, where heat transfer occurs, thereby reheating the air. Hence the term "hot gas reheat."


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