Boilers work by using fuel to heat water. As the water is heated carbon dioxide and water vapour is produced as a chemical reaction. The gases produced are extracted through the boilers flue and escape into the outside atmosphere. A condensing boiler recovers the heat from the gases and water vapour that would otherwise escape through the flue. The heat is then used to warm the cool water on the return flow from the radiators to the boiler.
In a condensing boiler the waste gases and water vapour travel through a heat exchanger which cools and condenses them to a liquid form, extracting the latent heat of vaporisation. The liquid is then drained away as the boiler works.
A non-condensing boiler is only 70-80% efficient due to 20-30% of energy produced escaping through the flue. A condensing boiler is over 90% efficient in comparison and therefore cheaper to run, using less fuel.
The lifespan of a condensing boiler is far shorter than the old style boilers and they are more fragile. Whilst it was not unheard of a boiler lasting 30 years in the past, condensing boilers have a far shorter life span of approx 10-15 years.
It’s important that the boiler receives regular maintenance and services to extend the lifespan. One of the things that shortens lifespan of a boiler is its stop/start frequency so its vital when choosing the boiler that it is the correct size for the home. Choose a boiler that is too big means it will constantly be stopping and starting, shortening its life span considerably. Other factors that can effect boiler life are debris that clogs up the system, so regular power flushes are recommended.
Due to the energy efficiency of condensing boilers they became mandatory in 2005 and all new boilers are condensing (except in special circumstances).