A four-pipe fan coil unit (FCU) has much the same internal workings as a two-pipe system, but with both a heating and cooling coil. The thermostat controls both the chilled water and heating water valves. There’s a cooling and heating band, with a dead spot between the temperature settings. This intermediate temperature zone keeps the system from short cycling.
The advantage of the four-pipe system is that the same method can be used in a facility with interior and exterior zones that can call for heating and cooling in different zones at the same time during the day. The only drawback is that it’s a complex system to control and an expensive one to install.
A variation of the dual coils is an FCU that has a single coil through which either hot water or chilled water is valved, at the unit. The control sequence is similar to the two-pipe FCU, with a heating band, a dead zone, and a cooling band. The coil must be sized for the cooling mode, which means the hot-water temperature should be 110 to 140°F. This is the usual temperature of domestic hot water. Some new systems combine the domestic hot-water and hydronic hot-water systems, to avoid having to install a second circulating system, pumps, and valves.
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Excerpt from Energy Conservation Projects.