Hydronics (from Ancient Greek hydro- 'water') is the use of liquid water or gaseous water (steam) or a water solution (usually glycol with water) as heat-transfer medium in heating and cooling systems. The name differentiates such systems from oil and refrigerant systems.

Historically, 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 either separately or together as means to provide water cooling, while boilers heat water. A recent innovation is the chiller boiler system, which provides an efficient form of HVAC for homes and smaller commercial spaces.