A fan coil is like a hybrid between air conditioning and radiators. Air-con pipes contain refrigerant gas for cooling or heating, whereas water-circulating radiators can only be used for heating. However, in a fan coil system a fan takes room air and passes it over a coil of pipes circulating water, then gently returns it to the room. This can be driven by an energy-efficient heat pump (looks like an external air-con unit), and switched to heating or cooling. It is also effective for dehumidifying.
Fan coil systems come in different guises. They can look much like radiators and are economic to install. Or they can be more sophisticated vented systems, the units concealed behind walls or ceiling.
Fan coils are also good in tandem with water-based underfloor heating, again powered by a heat pump. The underfloor system can be switched to cooling, but only to a certain point before condensation forms and the floor becomes slippery. However, by adding wall-mounted fan coil units, the water flow can be diverted to them for more effective cooling whilst draining the condensate away.