The hot and humid tropical climate makes it necessary to provide year round air-conditioning in Singapore hotels. Except in a boutique hotel, where split units are used throughout the whole building, all the other 28 hotels are centrally air-conditioned, among which 25 hotels operate and maintain their own chiller plants. The remaining three hotels share parts of two district cooling systems; chilled water is pumped to the premises and billed based on the tons of refrigeration supplied. Large public areas like lobby, restaurant are usually conditioned with constant air volume (CAV) or variable air volume (VAV) systems, whereas fan coil units (FCU) are often used to serve guest rooms.