Combustion

• The combustion process takes fuel and combines it with oxygen to make water and carbon dioxide. The water that forms, as a byproduct of combustion, is in the form of vapor, which has a higher energy content than liquid water. This energy represents a loss because it's no longer available to warm the water inside the boiler and is instead carried out through the stack or flue in the exhaust. A condensing hot water boiler attempts to recover this waste heat by adding another step in the heating process.
  
  

Condensation

• Just as evaporation absorbs heat, condensation releases it. A condensing boiler incorporates an additional heat exchanger unit and passes the exhaust through it. The steam in the exhaust condenses in the exchanger, giving up heat that can be recovered and used to heat the water in the boiler. In the meantime, the condensate is piped away into the drain. The exchanger is made from a corrosion-resistant material, such as stainless steel, since the condensate is slightly acidic.
  
  

Benefits

• According to the U.S. Department Of Energy, the efficiency gain associated with a condensing boiler can be very significant -- as much as 10 percentage points when compared with a more conventional, non-condensing system. Condensing boilers are more expensive to install, but they will also save you money on fuel costs; and when you consider that the unit may last 15 to 20 years or more, these savings could be substantial.
  
  

Controversy

• Despite the benefits associated with condensing boilers, they have at times been controversial. In 2005 the British government mandated that homeowners use condensing boilers when replacing old units or installing new ones. Critics charged that condensing boilers were more expensive and required more maintenance. The UK government, however, pointed out that although condensing boilers were more costly to install, they were no less reliable than traditional models and would save money for consumers in the long run.