Four-pipe system consists of two-pipe circuit (supply and return) for chilled water and two-pipe circuit (supply and return) for heating hot water. The terminal unit is usually provided with two independent secondary water coils, one served by hot water, the other one by chilled water. The primary air is cold and remains at the same temperature year-round. During the period between seasons, any unit can be operated at capacity level from maximum cooling to maximum heating without regard to the operation of any other unit. During peak cooling and heating, the four pipe system performs in a manner similar to the two-pipe system.
