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Heating, Venilation & Air Conditioning

Constance Air Volume

The simplest type of air system is a single zoned constant volume air handling unit satisfying only one zone sensor. These units are designed to condition the space temperature and humidity closely and efficiently and can be shut down when desired temperature has been reached to conserve energy. This type of unit can be either refrigerant cooled or by chilled water. A unit like this would be installed on a rooftop, floor mounted, or above the ceiling. These units would most likely be serving areas were the heat loads are even throughout, and in open areas such as classrooms, department stores, and computer rooms. They can also be effective in larger office buildings were multiple smaller units are used.

The supply air temperature set point for a single-duct constant air volume air handling unit (AHU) is often reset based on either return air temperature or outside air temperature in order to reduce simultaneous cooling and heating energy consumption. Both reset strategies make engineering sense as long as the reset schedules are reasonable. Quite often the decision to use one over the other is made with the assumption that they will both achieve some sorts of energy savings. However, the impact of these two strategies on AHU energy consumption could be very different. A comparison of these two commonly used supply air temperature reset strategies for a single-duct constant air volume system is presented in this paper. It is shown that from either the building energy consumption or building comfort point of view, the reset strategy based on outside air temperature is inherently better than that based on return air temperature. Significant amount of heating energy savings can be achieved by switching from return air temperature based reset to outside air temperature based reset.

Ventilating maintains an adequate mixture of gases in the air we breathe (e.g. not too much CO2), controls odors, and removes contaminants from occupied spaces. Clean air helps keep occupants healthy and productive. Ventilation can be accomplished passively through natural ventilation, or actively through mechanical distribution systems powered by fans.