In simple terms, an economizer is a heat exchanger. specifically: (Steam Boilers) An arrangement of pipes for heating feed water by waste heat in the gases passing to the chimney Economizers, or economisers, are mechanical devices intended to reduce energy consumption, or to perform another useful function such as preheating a fluid.

In simple terms, an economizer is a heat exchanger . Robert Stirling 's innovative contribution to the design of hot air engines of 1816 was what he called the 'Economiser'.

An economizer is a part of a building’s cooling system that uses cool outdoor air to cool the building instead of operating the air conditioning compressor. An economizer is an add-on feature to an HVAC air handler that draws in outdoor air and mixes it with return air from indoors.