An air-to-water heat pump is a thermal system that extracts energy from outside air and transfers it to the water of a hydraulic circuit to provide heating, cooling and, often, domestic hot water (DHW). It is considered a highly energy efficient technology[1]​ and a key vector for the decarbonization of building heating by replacing fossil fuel boilers.

The system operates through a vapor compression cycle with four main elements: evaporator, compressor, condenser and expansion valve. The heat captured from the air is raised to a useful temperature by the compressor and transferred to the water in the circuit. Electronic modulation "Modulation (telecommunication)") (inverter) allows power to be adapted to demand, improving partial performance and comfort.[2]​.

Performance is measured by COP (instantaneous coefficient) and SCOP (seasonal performance). In temperate climates and with low temperature emitters, air-water systems achieve high SCOP and reduce annual consumption compared to conventional systems. Seasonal yields depend on outside temperature, system design and control. International reports estimate a substantial potential for CO₂ reduction through the massive implementation of heat pumps.[1]​.

In the European Union, air-water heat pumps are incorporated into the Energy Efficiency of Buildings Directive (EPBD) as an instrument to improve the performance of the built stock. The 2024 review reinforces efficiency and electrification objectives. Regulation (EU) 517/2014 on fluorinated gases regulates refrigerants, leak control and labeling requirements; In addition, ecodesign and energy labeling policies set minimum levels of efficiency.[3]​.