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A multi-zone air-handling unit was popular several decades ago due to the convenience of small sized modular units, which were inexpensive to install and easily maintained in a mechanical room. The cost and convenience proved to be of little benefit as the units perform poorly from an energy usage perspective. A “three-deck” multi-zone unit is a hybrid of its kind, and it can be very efficient when controlled properly. In theory, there will not be simultaneous heating and cooling if its heating damper is controlled separately from the control of the cooling damper. When the zone load is neutral (not heating or cooling), for example, all the mixed air will be bypassed through the bypass deck. However, there are opportunities and challenges in this system.
This paper presents theoretical backgrounds of advantages and challenges in the system operation of the three-deck multi-zone unit and methods to optimize temperature and economizer control to improve energy efficiency. A case-study will be presented examining a medical facility’s utilization of 35 three-deck multi-zone units serving most hospital areas, as well as illustrates a 10% savings in total gas and electric consumptions over the period of one year.