A calorifier is an indirect-fired water heater to provide hot water in a heating and hot water system. Indirect fired means the water heater does not contain a burner. It is a storage water cylinder with one or more heat exchanger coils which contain hot liquids (water or solar fluid).

As an indirect fired water heater cannot produce the heat itself due to the missing burner, it relies on an external heat source such as e. g. a boiler or solar collectors. These heat up the liquid in the heat exchanger coil which in turn heat up the water contained in the vessel.