What a combie boiler is: A combie boiler is where the boiler itself has an extra heat exchanger.
As you open a tap in the bathroom or kitchen the oil or gas boiler runs to heat the water direct. This in turn makes them more efficient for home owners, as there is no storage of water in the cylinder (cylinder is the hot water tank found in the hot press) which can hold a huge amount of water. The energy demand to heat the cylinder in turn heating the amount of water in the tank is huge. Most people heat the cylinder just to wash dishes etc leaving vast amounts of hot water wasted and this happens over and over your life time. With a combie gas or oil boiler you only use the water you need cutting down on your energy consumption and just as important your energy bills and not just gas or oil but electricity too as there is no need for your expensive immersion. I hope this information is helpful.
Philip Marry