Condensing boilers are water heaters typically used for heating systems that are fueled by gas or oil. When operated in the correct circumstances, a heating system can achieve high efficiency by condensing water vapour found in the exhaust gases in a heat exchanger to preheat the circulating water. This recovers the latent heat of vaporisation, which would otherwise have been wasted. The condensate is sent to a drain. In many countries, the use of condensing boilers is compulsory or encouraged with financial incentives.

A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning systems, which can both cool and warm interior spaces.