If you looking to replace your existing gas boiler you now have to buy a Condensing Boiler.

What is a Condensing Boiler?
A condensing boiler is a high efficiency modern boiler that incorporates an extra heat exchanger so that the hot exhaust gases lose much of their energy to pre-heat the water in the boiler system. When working at peak efficiency, the water vapour produced in the combustion process condenses back into liquid form releasing the latent heat of vaporisation. A side effect is that this water, known as condensate, which is usually acidic, has to be piped away to a drain or soak away.

What size of Boiler do I need?
Most existing boilers are measured by their output in British Thermal Units per hour (BTU/h), but all current boilers are sold in the metric equivalent of kilowatts (kW). You should bear in mind that in the past central heating engineers often installed boilers that had a higher output than strictly necessary. Although this meant there was no possibility of the boiler failing to meet the demand for heat, even in the most arctic of conditions, it also meant that they were mainly operating at a part load, and so running below their maximum efficiency. If you have installed additional loft or cavity wall insulation since the last boiler was fitted, it is highly likely that you will need a smaller boiler than before.
We recommend seeking professional advice from a Gas safe registered installer before choosing an appropriate replacement boiler.