The switch from non-condensing to condensing boilers was one of the most significant improvements in home heating efficiency of the past 30 years. Here's how it works.
Non-condensing boilers
Traditional boilers allowed flue gases to exit at 250–300°C. All that heat was wasted up the flue. That's why old boilers were only 65–75% efficient.
How condensing works
A condensing boiler has a secondary heat exchanger. It captures the heat from the flue gases before they exit, cooling them down to 50–55°C. This extracts so much heat that the water vapour in the flue gas condenses back to liquid — which is why there's a white plume from modern boiler flues in cold weather, and why all condensing boilers produce condensate waste water.
The efficiency gain
By capturing the latent heat of the condensing steam, a condensing boiler achieves 90–96% efficiency vs 65–75% for a non-condensing boiler. The saving on a typical annual gas bill can be £200–£350.
The legal requirement
Since 2005, it has been illegal to fit a new non-condensing gas boiler in most domestic applications in England and Wales. All modern replacement boilers are condensing. If your current boiler is pre-2005 and hasn't been replaced, there is a significant efficiency gain available from upgrading.