A stack economizer is a water-to-air heat exchanger that is designed to use heat from hot boiler flue gases to preheat water. For many decades, economizers have been used on large utility steam boilers to preheat the feedwater using recovered stack heat.

The same principle can be applied to smaller hydronic heating boilers where there is a nearby demand for hot water. These installations have become more economical as energy prices have risen and smaller economizers with light but durable and efficient heat exchangers have been developed.

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