Nearly all the new boilers in the UK are of a condensing boiler type where the heat that would have been wasted is condensed and used again to make energy savings. The efficiency of the gas boilers in particular has massively changed over the years and they now use so much less gas per kilowatt of heat produced. The size has diminished and the improvements keep coming.
The original boiler system required a Geen warm water boiler to heat the water, which was then pumped through the central heating system on one system and a second part that went to fill the hot water tank so that a large ready supply of hot water was kept on stream. To keep the system balanced there was a large cold water tank which could replace the hot water drawn off the system immediately. So the system whilst very effective required a boiler, a hot water tank and a cold water tank. The cost was large and the space required was also a major consideration. Also the hot water was always instantly replaced so there was always a large volume of water with a heat loss.
The next stage was obvious in that the three separate parts of boiler and hot and cold water tanks are combined together in the combi or combination boiler. This is now the best selling boiler in the UK market and this is because it supplies a boiler at a cost about the same as a straight heating boiler and does not require the space or cost of the hot and cold water tanks and associated piping.
The combi boiler does the same thing in one boiler. It heats and pumps the central heating water in much the same way as before but the cold water is supplied directly from the mains and the hot water is supplied on demand. That is to say that when hot water is drawn off the mains supplies cold water directly to the boiler. Therefore there is no extra water heated for a hot water tank. There are a number of problems with this system in that the water flow depends solely on how quickly the boiler can heat up the water drawn off.
Drawing off water is usually slower than the old system and a bath can take sometime. There is a problem that filling the sink to wash the dishes cannot be done at the same time as running the bath. However on the whole the system works well and ever improving boilers get the heat into the water quicker and manage the system well.