Fan coil units are often used in apartments, hotels, offices, and classrooms. They provide efficient space conditioning of single zones with local control. A typical fan coil unit (FCU) has a single coil, a fan, filter, and a thermostat linked to a two-way valve at the coil. The fan is usually a three-speed device (which often runs around the clock). When the thermostat calls for cooling (or heating), the valve cracks open, and the air blowing across the coil is tempered.
The system is typically a two-pipe system. Either hot water or chilled water is pumped in the loop, so the system cannot heat and cool any zones simultaneously, as often is required in large buildings with both interior and exterior zones. Using a single coil for both heating and cooling poses a unique design problem, but also unique energy-saving potential.
The heat-exchange surface of a cooling coil is much larger than that of a typical heating coil, so the hot water circulated in the loop has to be at a lower temperature—110 to 140°F water is typical. The most severe limitation of this system is the inability to operate during swing seasons when it is cool in the morning and warm by late afternoon. The two-pipe system cannot change from heating to cooling easily. The entire system is usually changed over twice a year, at considerable expense and trouble. It’s not practical to change it more often, much less daily, as would be necessary for some arid climates.
There’s not much that can be done to improve the energy use of two- pipe fan coil units, other than to isolate one or two zones on the system that cause a premature changeover between seasons. In a school, for example, the load on classrooms is different from that of administrative offices. A split DDX system for the latter would be likely to allow more consistent use by the classrooms.