First we consider a 4-pipe system. While this type of installation gives the user the most freedom, it is a more expensive setup. The 4-pipe fan coil unit requires 2 separate coils, one for heating and one for cooling. Two pipes are used for heating and two for cooling (inlet and outlet respectively). A valve connected to the room thermostat determines which circuit is used, heated or cooled.
When heating is required, the valve opens the heating circuit. Hot water from the boiler flows through the coil. The fan coil motor rotates the fan blowing through the coil and spreads warm air throughout the room. When cold air is requested, the cold water valve of the radiator opens, the coil is filled with cold water and the fan circulates the cold air. Have a peek over this to know more about fan coil.
The most common device for fan convection is 2 tubes. Sometimes they are used for cooling only, heating only, or both. In a two-pipe system there is only one snake. When using a two-pipe system for heating and cooling, civil engineers manually switch the heating system (boiler) to cooler (chiller). With a two-pipe system you can only work in one mode, heating OR cooling, not both. This can be a bit cumbersome during seasonal transitions when heat is needed one day and cooling the next.
The fan coil unit is controlled by turning the fan on and off with a thermostat, or by a thermostatic water control valve. The thermostat can be switched from heating to cooling manually or automatically with the aquastat tube.
The thermostat can be mounted on the return air or on the wall to control the room temperature. A controller to control multiple fan coil units from a single thermostat is also available. Built-in or individual controls offer more flexibility as the comfort level can be adjusted for each room or area or turned off completely if needed. For areas that require a constant level of comfort everywhere, a single thermostat to control multiple units is the way to go.