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Advanced humidity control

Humidity control is vital to achieving acceptable indoor air quality in commercial buildings, but it can be a challenge with many rooftop units, especially on mild days. The Humiditrol® dehumidification system from Lennox was designed to solve that problem, providing superior moisture control over a standard rooftop unit. The result is that it is easier to keep the relative humidity (R.H.) level below 60 percent, which is the ASHRAE recommendation.

The Problem with Standard Rooftop Units

When the weather is hot and there is a high sensible load, a rooftop unit is required to run most of the day. During the normal cooling cycle, a standard rooftop unit will remove high levels of moisture from the outdoor air that enters the building, as well as return air from the occupied space. In many cases, this will keep the building below the recommended 60% R.H. level.

Because standard rooftop units typically only operate when there is a call for cooling, on mild temperature days when the building does not have a large sensible load, the rooftop unit will not run as often or as long. This may not provide sufficient dehumidification, especially if the outdoor air has high relative humidity and the unit has an economizer designed to take advantage of free cooling.

Running a standard rooftop unit for extended periods during mild temperature days will take care of the moisture, but will typically overcool the occupied space. This leads to uncomfortable employees and unhappy customers.

Benefits of a Hot Gas Reheat System

Incorporating a hot gas reheat system that initiates dehumidification operation based on the actual relative humidity level of the space, such as the Humiditrol dehumidification system, can significantly reduce these problems. These units remove moisture by operating the refrigeration cycle just like a standard unit in cooling mode, but then use a hot gas reheat coil to reheat the cooled and dehumidified air to a neutral temperature before returning it to the occupied space. This solution removes moisture and helps prevent overcooling situations.

During mild temperature and high humidity days, hot gas reheat units can remove more moisture than a standard unit because they start operation based on a need to dehumidify the space, and not a need to just control temperature. While a standard unit might run for a very limited amount of time, units with the hot gas reheat system can run until the R.H. set point has been satisfied.

Units featuring the hot gas reheat option provide efficient moisture removal and space dehumidification operation by reheating the cooled and dehumidified air with hot gas from the compressor, which is already operating during the cooling cycle. This is much more efficient than using one compressor to cool and dehumidify the air and another device – such as an electric heat strip or gas furnace – for reheating the air.

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