The Benefits of Hot Gas Reheat in HVAC Systems for Humidity Control in the Midwest
Clint Rabe, P.E.
Clint‘s background designing both mechanical and electrical systems allows him to bring a unique perspective to the design and construction process while helping him to identify potential project opportunities and pitfalls. Clint’s collaborative approach to the construction process is evident as he works to bring all parties to the table in developing the right project solutions. His conscientious and hardworking approach reinforces his personal commitment to making each project a success.
Introduction to Hot Gas Reheat in DX Cooling Systems
Hot gas reheat is an energy-efficient feature in direct expansion (DX) cooling HVAC systems, commonly used in rooftop air handlers (RTUs), that allows precise indoor humidity control without causing overcooling. This feature is especially useful in the Midwest during periods of high humidity and mild outdoor temperatures.
How Hot Gas Reheat Works in HVAC Systems
In a typical DX air conditioning cycle, refrigerant absorbs heat from the indoor air (cooling it) and then rejects that heat outdoors via a condenser. In systems with hot gas reheat, however, some of this hot refrigerant gas bypasses the outdoor condenser coil and is directed to a reheat coil located downstream of the cooling coil in the air handler. Instead of requiring a separate heat source for reheat (which in some cases might be hot water from inside the building), the system recycles waste heat from the refrigeration cycle and reintroduces it into the “cooled” air stream, making hot gas reheat a more energy-efficient way to achieve humidity control. Why reject this heat outdoors when it can be used to improve indoor comfort?
Cooling and Dehumidifying Air Without Overcooling
The hot gas reheat process has two main functions:
- Cooling and dehumidifying the air.
- Reheating the air to a comfortable temperature.
As air passes through the cooling coil, it’s both cooled and dehumidified, causing moisture to condense and drain out of the air handler. This cooling also lowers the air temperature significantly, often to around 55°F—an uncomfortably cool temperature if a person isn’t looking to be chilled. The reheat coil steps in here, using the hot refrigerant gas to slightly raise the temperature of the now-dehumidified air before it’s delivered to the space, avoiding a situation where it “freezes the snot out of Greg in accounting.” This reheat process brings the air closer to the desired indoor temperature while maintaining a lower humidity level.
Why Hot Gas Reheat is Ideal for Midwest Humidity Control
For facilities managers, the advantages are clear, particularly when dealing with the high humidity and mild temperature conditions common in the Midwest. In other words, hot gas reheat is helpful when relative humidity is high, but outdoor temperatures aren’t warm enough to prompt the DX rooftop unit to run for temperature control based on the thermostat setting inside the building.
Comparing Conventional Cooling and Hot Gas Reheat for Comfort
Without hot gas reheat, conventional systems may have to excessively cool the air to remove moisture, leading to an uncomfortable drop in indoor temperature—prompting Greg to ask, “Why am I wearing a coat in June?” Hot gas reheat allows humidity to be controlled independently of the temperature, maintaining comfort without making spaces too cold. Lowering indoor humidity makes the environment feel more comfortable—less “sticky” in humid conditions.
Considerations for Hot Gas Reheat Installation
Hot gas reheat systems deliver precise humidity control, especially beneficial when humidity is high, but outdoor temperatures don’t warrant cooling. This feature can often be integrated at the factory level in RTUs, but lead times may vary. For retrofit options, consider a Rawal valve—a device manufactured by Rawal Devices, Inc.—which can be field-installed to provide air conditioning modulation at a lower cost. (Note: RCE has no affiliation with Rawal Devices, Inc. – we just think it’s a smart option for the right setup!)
