Boilers Pt 1 – Regular Vs Combi

Existing boilers in UK homes fall into two main types – regular boilers (which might also be called “heat only” or in some cases “system” boilers) and combination or “combi” boilers.

A regular boiler delivers heat to both the heating system (usually radiators) and a separate hot water system which takes heat to the hot water cylinder. A hot water cylinder consists of a metal tank, hopefully well insulated, inside of which is a coil of tubing. The boiler passes hot water through this tube and this indirectly heats the water in the cylinder.

So instead of taking the water to the boiler you take the heat to the cylinder. When you turn on the hot tap, hot water is taken from the top of the tank and replaced with cold water at the bottom.

A combi boiler on the other hand, supplies heat directly to both the radiators and to the cold mains water so it directly heats the water itself and delivers it on demand to the hot taps. Combi boilers store no water and so they avoid the problem of sometimes heating up water that  just sits there cooling down (known as standing losses).

Now you may be thinking “why bother with a regular boiler and hot water cylinder if you can avoid those losses?”, but there’s a catch with combis: they heat water less efficiently than regular boilers. So there’s a trade off between the benefit from avoiding standing losses and the penalty from reduced hot water efficiency.

The upshot of all this is that a combi is most suitable for smaller homes with a low hot water demand while a regular boiler is most suitable for a home with high hot water demand. Combis are also more suitable in smaller homes because they don’t need the space for a cylinder.

In my flat I unfortunately have a cylinder which means that I don’t use anywhere near the full amount of hot water. But it is very well insulated with factory fitted foam which means that I just have it come on once before I get up and there’ll still be hot, well warm, water when I get back in the evening.

There is a trend at the moment towards the inappropriate replacement of regular boilers with combis but it should be resisted. If anything, removing the cylinder makes it that little bit harder and more expensive to install a solar hot water system, which needs as much storage capacity as possible, so if you’re considering going down this route you should try and keep hold of your cylinder.

Next up we’ll cover high efficiency condensing boilers. If you want to read in more detail about gas heating systems then your best bet is to check out the Energy Saving Trust publication Domestic heating by gas: boiler systems (CE30)