Chemically, METHANE (gas) combines with OXYGEN in the air (combustion) to produce carbon dioxide and water. In a standard boiler, the water leaves the boiler, as steam, mixed with the carbon dioxide, wasting energy. In a condensing boiler, the steam is condensed into water, saving most of that energy. The water produced tends to be slightly acidic, so is not re-usable. The thermal efficiency of a standard boiler is usually between 65% and 75%, depending on its age and condition. A new condensing boiler is likely to acheive around 90%.