There is a great deal of confusion surrounding boilers and we have tried to cover the basics here. Essentially if you have any doubts you should discuss your installation in detail with your supplier, or your installer who must be Gas Safe registered. In the case of oil boilers this should be done by an OFTEC registered engineer.

The size of your boiler is dictated by the output you require. This is measured in terms of radiators and hot water cylinders. It has been common in the past to oversize boilers to make sure the boiler would heat your house adequately. One result of this is that most boilers installed pre 1989 are as much as 30% oversized. This is an incredible drain on finance as well as energy and these days boiler efficiency calculations are much more advanced.

There are several types of gas boiler from which you can choose, so it's important that you understand how they differ if you're to make the right choice for your individual needs

Types of Central Heating Systems

The Conventional Heating System

Requires two water tanks located high up one a Cold Water Storage Tank the other an Expansion / Feed tank, also requires a Hot Water Cylinder. The storage cylinder holds a large quantity of hot water that is available immediately (or as soon as the feed allows). However when this store has been used up it has to be reheated, this could take anything up to 1 hour. This type of system is considered the least efficient as there are heat losses from the pipe work to and from the boiler to the cylinder and the cylinder itself.

System Boiler Heating Systems

System boilers come in two types, a vented low pressure and an un-vented high pressure system. Both require a Hot Water cylinder. Vented system boilers require a storage tank but most of the other components are held in the unit itself which means that installation is quicker and cheaper. Also, the hot primary water is pumped through the system to the radiators and hot water cylinder resulting in a faster response and more economical running costs.
Un-vented system boilers incorporate the equipment necessary to form a sealed system including a pressure relief valve and a pressure gauge, removing the need for a feed and expansion tank.

Combi Boiler Heating Systems

Combi's are now the most used form of central heating boilers. Most Combi's run on mains pressure water, thus do not require tanks to be placed in the loft. They also eliminate the need for a hot water cylinder as they instantly heat the water when it is needed. Another benefit with the combi is that, due to it running from the mains water supply, you can create an effective power shower without the need for a pump.
Combi boilers require the least amount of plumbing thus offer a cost saving when installing.

Condensing high efficiency boiler

All new central heating boilers have to meet government-set minimum standards of operating efficiency (SEDBUK Band B or above).

Condensing boilers have low emission levels and are very energy efficient. Old conventional boilers run between 50% -  70% efficiency,  modern condensing boilers recover and recycle most of this wasted heat energy and some can operate at up to 95 - 97% efficiency.