Ok… So post assumes you’ve just checked out this post about how a boiler works… then what about these condensing boilers then…
Condensing boilers have all the same elements as standard boilers, but have increased efficiency. The efficiency of a boiler is a measure of its ability to convert energy from the fuel that goes in, to heat in the water that circulates around your house. Condensing boilers do this by having an extra heat exchanger in the flue. As the flue gases (fumes, smoke… and the like) heat up the water (or some cases the air) a bit as it is feeding into the boiler. This means that the boiler has less work to do to heat the water in the main body of the boiler. It also means the flue gases are cooled down a bit more than usual, and so some of the particles in the gases will “condense”. The condensed liquid can be a bit nasty, and so higher quality materials are necessary in the boiler. This causes the boilers to be more expensive but the reduction in fuel costs can be significant.
Another key point to note is that the flue gases are at a far lower temperate than the gas straight after the burner. This lower temperature gas is really only good for heating cooler water, and so works best on a lower temperature system. So with condensing boilers, the lower temperature you run the system, the more efficient they run.
Lower temperature systems means the water circulating around your house will be at cooler, meaning that you will either
need larger radiators (or have underfloor heating)
Accept that it will take longer for them to heat up the room (not a lot, just a bit)