Boilers Explained

The government is committed to reducing the level of CO2 to levels below those produced in 1990. To help with this, all boiler installed must be “A rated” in their efficiency (above 88%) which we call condensing boilers, together with upgraded system controls.

What type of condensing boiler is for me?
There are 3 different types of high-efficiency condensing boiler:

  • Combination
  • Regular (traditional/conventional)
  • System

Each type delivers all the benefits of high-efficiency technology, but which one you choose will depend on several factors, including your property, your lifestyle and the professional advice of your Corgi registered Heating Advisor.


Combination boiler system
Conventional stored water system
Combination boiler system
Conventional stored water system


What is a combi boiler?

The high efficiency condensing combination boiler is an ingenious space-saving idea. Combis are the majority of all the new domestic boilers installed in Britain every year.

The major difference between a combi and any other type of boiler is that a combi eliminates the need to store hot water — so no hot water cylinder in the airing cupboard. It is both a high-efficiency water heater and central heating boiler, combined (hence the name) within one compact unit which usually sits in the kitchen or utility room, or sometimes in the airing cupboard. The space savings result from the fact there is no hot water storage cylinder, cold water storage cistern or other familiar components of a regular (conventional) heating system.

The further benefits of this are a significant saving on hot water costs, and the fact that hot water is delivered through your taps or shower at mains pressure. So you can enjoy powerful showering* without the need for a pump. Another combi benefit is that it can generally save you money on installation time and costs — no tanks in the roof space means less pipe work and a shorter installation time

What is a regular boiler?

If you are replacing an older model of boiler, the chances are that you will have a regular (conventional) boiler. A typical conventional system incorporates a boiler and extended controls, a feed and expansion cistern, and a hot water cylinder (usually in the airing cupboard) which is often fed by a cold water storage cistern located in the loft.

What is a system boiler?

Unlike a combi, both a system boiler and a regular (conventional) boiler work on the principle of stored hot water — but a system boiler differs from a regular boiler in some important respects.
Many of the major individual components of the heating and hot water system are built in, which means that installation is quicker, neater, easier and more efficient.

The boiler includes an expansion vessel so there is no requirement for a feed and expansion tank in the loft, which allows the installation to save space.



Most suitable system type

You want to use your loft space for a room conversion or other purposes Combi
You live in a flat or bungalow (i.e. have very little or no roof space) Combi
Your home has more than 2 bathrooms Regular or System
Your mains water pressure is low Regular or System
You want to replace an old boiler to improve an existing conventional central heating system Regular, System or Combi
The number of people in your household means that there is regular demand for hot water on tap, but waiting for the tank to constantly refill and heat up is, or would be, inconvenient Combi