Hydronics is the name for the use of Water as the heat-transfer medium in heating and cooling systems.  some of the oldest and most common examples are steam and hot-water radiators.  In large-scale commercial buildings such as high-rise and campus facilities, a hydronic system may include both a chilled and a heated water loop, to provide for both heating and air conditioning, chillers and cooling towers are used separately or together as means to provide water cooling, while boilers heat water. In addition, many larger cities have a district heating system that provides, through underground piping, publicly available steam and chilled water. By paying a service fee, a building in the service district may be connected to these.  Hydronic systems are of three basic types:

• Steam
• Hot water
• Chilled water

Hydronic systems are classified in five ways:

• Flow generation (forced flow or gravity flow)
• Temperature (low, medium, and high)
• Pressurization (low, medium, and high)
• Piping arrangement
• Pumping arrangement

Hydronic systems may be divided into several general piping arrangement categories:

• Single or one-pipe
• Two pipe steam (direct return or reverse return)
• Three pipe
• Four pipe
• Series loop