Instead of having a heating system with a big tank with either an indirect coil through it or a heating element inside, a combination (‘combi’) boiler heats up water using a plate heat exchanger.
A plate heat exchanger has a very high heat recovery, which means it can transfer heat from one body of water to another very quickly, almost in real time.
If you imagine you have the boiler pumping heated water into the heat exchanger, the hot water coils round through the heat exchanger and then back to the boiler to be reheated. Cold water enters the heat exchanger via the cold water mains and, without mixing with the hot water already flowing through, the cold water picks up the heat and is then fed out to the hot water taps.
When you switch on your hot tap or shower, the dabble pressure switch within your combi boiler switches on which, in turn, triggers the boiler and the water pump. The pump drags hot water (water you don’t wash in) through the coil within the plate heat exchanger back to the boiler. Cold water (that you will wash in) enters through the mains and picks up heat by flowing through the plate heat exchanger and then flows to your taps or shower as hot water.
In effect, the hot water that is constantly flowing through the system is used to heat up cold water very quickly, making the traditional problem of waiting for or running out of hot water, almost non-existent. The convenience of having almost instantaneous hot water, without having to heat up large amounts each time, is one of the main reasons that combi boilers have become increasingly popular in our homes.
Article by Benjamin Clarke