A Combination Boiler – or Combi boiler as they are more commonly referred to as, is an ingenious space saving device which is able to provide central heating and hot water on demand.
A Combi is essentially an instantaneous water heater which can also provide central heating. It is important to note that one of its limitations is that it can only deliver hot water or heating at any given time – not both, and it always gives priority to the hot water. Because the hot water is warmed up instantly it negates the need for a cylinder etc hence saving space.
In general the bear minimum power output for a Combi boiler is 24KW, although some 23KW models have been produced. The average power of a Combi ranges between 24KW to 35KW.
A Combi boiler can be of condensing or standard efficiency(non-condensing) design. All domestic boilers installed in the U.K. from 2005 onwards are condensing boilers.
First Combi Boiler
In 1967 Vaillant conceived a device which for the first time combined heating and a hot water supply in a single combined unit – which is now commonly referred to as the ‘combi’ boiler. Domestic boilers in general have become increasingly complex over the last two decades or so – especially Combination boilers.
Types of Combination Boiler
This is the most common design and just about all modern Combi’s use this design. As far as central heating goes all Combi’s work near enough the same. The hot water is how the two designs differ. On an Indirect design you essentially you have two heat exchangers, one is the Primary heat exchanger – referred to as a Gas to Water heat exchanger – which generates all the heat for both heating and hot water. You have a second heat exchanger which is generally a plate to plate heat exchanger referred to as a Water to Water heat exchanger.
For hot water delivery the heat from the Primary heat exchanger is then circulated through the plate heat exchanger, which in turn indirectly(doesn’t physically touch) transfers its heat to the mains water supply running through the boiler on its way to the tap.
The Indirect Combi design is more expensive to build as it requires more parts and also requires a diverter valve or an additional pump. I have only every come across one boiler with the twin pump design which was a Ferroli. That idea never caught on. Because of the additional components it also makes boilers of this design generally less reliable than the other design. Less parts generally means less problems.
The Indirect method is generally regarded as a more efficient way of transferring the heat from the main heat exchanger to the hot water which is why most manufacturers have adopted this design. One exception is the range of boilers manufactured by Intergas. Their trademark is the diverter valve-less design.
Thats how an Indirect Combi Boiler design works.
The other way of heating up the hot water is by using the Direct method. Here you only have one heat exchanger, the Primary one. The are two channels through this heat exchanger, one for the heating and one for the hot water. The pump on the boiler only runs when the heating is on. When a hot tap is running the pump stops running and the mains water running through the boiler on its way to the tap is heated up.
Thats how a Direct Combi Boiler design works. Its a very simple design which works well. Less parts generally means less problems.