Condensing boilers are the most energy efficient heating boilers on the market today. These boilers achieve their high efficiency by incorporating an additional heat exchanger, which uses the heat in the exhaust gases to preheat the water as it enters the boiler. This energy would normally be wasted up the flue. When a condensing boiler is working at peak efficiency the water vapor produced by the consumption of fuel in the boiler condenses back into liquid water - hence the name "condensing boiler.