废气加热器，是一种减少能量消耗、或者提供液体预热功能的机械设备，属于换热器的一种。通常与锅炉、发电设备、暖通空调设备链接，用液体吸收废气中的热能以供重新利用。

Economizers (US and Oxford spelling), or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, Refrigeration, ventilating, and air conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.