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4.4 Single-Duct Reheat Systems

This part of the Energy Efficiency Manual presents the energy conservation measures that apply to single-duct reheat systems. In reheat systems, air is cooled at the air handling unit. The temperature of individual spaces is controlled by reheating the supply air in terminal units. The terminal units are controlled by the space thermostats.

By far the largest energy saving opportunity in single-duct reheat systems is minimizing reheat. Reheat is very wasteful. First, excess energy is expended to cool the supply air. Then, still more energy is expended to partially re-warm the same air before it enters the space. During low-load conditions, much more energy may be cancelled out in this manner than actually enters the space to provide cooling or heating.

You can save both reheat energy and fan energy by adjusting the fan output as the load changes. Several methods are explained, including variable-speed fan drives.

In new construction, none of these methods adds much cost, unless the systems are small. In existing systems, you can save a lot of energy with simple adjustments. You will learn how to radically reduce reheat energy consumption by adjusting the discharge temperature or supply air temperature of the cooling coil. You can do this automatically with supply air temperature reset controls.

However, achieving the highest level of efficiency requires major system changes. The most common is conversion to a variable-air-volume or VAV system. You will learn about temperature control, selection of the heating method, selection of terminal units, diffusers, humidity control, ventilation, and other essential design features.