Boilers account for around 60% of what you spend in a year on energy bills, so an efficient boiler makes a big difference. Replacing an old gas boiler with an A-rated high-efficiency condensing boiler and improving your heating controls will significantly cut your home's carbon dioxide emissions.
Modern boilers are more efficient for several reasons, but their main advantage is that they are all condensing boilers. All well-maintained boilers burn fuel very efficiently, but they inevitably lose some heat in the hot gases that escape up the flue. A condensing boiler has a bigger heat exchanger, so it recovers more heat, sends cooler gases up the flue and is more efficient.
Sometimes the flue gases get so cool that the water vapour in the gas condenses out (hence the name). When this happens, even more energy is recovered from the condensing vapour, and the efficiency gets higher still.
When you replace your boiler you have a choice of buying a new regular boiler, keeping your hot water cylinder, or buying a combi boiler that doesn't need a cylinder.
A combi boiler doesn't need a hot water cylinder, so less space is needed.
If you have mains gas, then a gas boiler is usually the cheapest heating system for you.
If you currently have an oil, LPG or coal boiler, then it might be worth looking into the cost of getting gas supplied to your home.