What follows is a simple, but detailed explanation of the different boiler types generally available. Click here for Idiots Guide!

Conventional Boilers
In a conventional boiler gas jets play onto a cast iron heat exchanger through which Hot Water Cylinderwater passes to be heated. If used to supply taps, hot water cannot be provided on demand but must be stored, usually in a copper cylinder. This type of boiler has relatively simple controls and tends to be more reliable as less can go wrong. Energy consumption can be high but the cost of this is moderated by the low maintenance costs. They are versatile in that they can be used in almost any type of property and can be pumped or gravity fed.

    Click on diagram to enlarge.

Combination Boilers
A combination boiler heats water for the central heating in the same way as a conventional boiler but can also provide the taps with instant hot water. The main difference is that the hot water for the taps is fed directly from the mains rather than from a hot water storage cylinder. This can also be useful in a small property where space for a cylinder is at a premium. The internal workings are increasingly technical and can be perceived as unreliable but are becoming more accepted.  Other than the lack of an airing cupboard one problem can be that in the event of a breakdown the user can be without both heating and hot water whereas on a conventional system an electric immersion element can usually be inserted into the cylinder to provide hot water for the taps. With a combination boiler, hot water and central heating requirements are provided from the one unit. As well as providing central heating water into the radiators, it provides all of the domestic water for baths, sinks, and showers.

The big advantage with a combination boiler, or a "combi" as it is usually called, is that it not only delivers continuous hot water, but more importantly delivers it at mains pressure too. So, with a combi system you can have a really effective shower without needing an expensive "Power Shower" booster pump.

Combis are also considered to be amongst the easiest systems to install, because they eliminate the need for both an expansion tank in the loft and a hot water cylinder in an airing cupboard.

The combi does have limitations. Most standard combis take 40 seconds to heat water. 

Combis provide maximum pressure through only one tap at a time. If you have two taps running, the powerful flow rate is diminished in one (or both) of the taps. So, if you want high temperature and a high output flow rate from your combi, choose a high capacity or a storage combi
more info

Combi's provide maximum pressure through only one tap at a time. If you have two taps running, the powerful flow rate is diminished in one (or both) of the taps.

Combination Storage Boilers
By adding a storage tank to a combi, the problems of flow rate reduction are progressively overcome, depending upon the size of the tank. Now you can use a couple of taps simultaneously, without an unacceptable drop in performance.

The advantage of this system over a conventional boiler and storage tank is that the hot water never runs out. Even after running a bath, a combi storage boiler doesn't need time to recover before you can use it again. So there's no need to plan your hot water requirements, hot water is always there.

However, the flow rate from an average combi storage boiler does not match the high flow rate from a modern system with an unvented storage tank.

System Boilers
Like conventional boilers, a system boiler can provide central heating and hot water from a cylinder if required. The key difference with a system boiler is that all the major components are built in to the boiler. For example the pump, normally installed remote from the boiler, in built in; so is the expansion vessel, which replaces the feed/expansion tank often installed in the loft. The safety valve, the automatic air vent and even the programmer are also included. With these components built in installation time is reduced significantly, fewer materials are required, costs are reduced and servicing is simplified. The added bonus of a "dry loft" removes the worry of any leak or frost damage to tanks and pipe work.

A Typical System Boiler with Unvented Hot Water Installation

Typical System Boiler Diagram
Click on diagram to enlarge and use back button to return

Back Boilers
Back boilers can be fuelled by gas, oil or solid fuel. All require a lined, natural-draught Back Boileropen flue to expel the potentially harmful gases. Solid fuel back boilers can only provide hot water when the fire which heats the house is lit. Gas and oil back boilers however can work independently of the fire front so are able to provide hot water all year round. An electric immersion heater can be used as with a conventional boiler.


Condensing Boilers
A Condensing boiler uses modern technology to maximise the fuel efficiency of either a Condensing Boilerconventional or combination system.
The boiler is designed so that the cooler water returning from the radiators is passed through a secondary heat exchanger to be warmed by the hot flue gases which are normally expelled in to the air. The warmer water is then sent back to the radiators.
It is known as a Condensing boiler because the water from these flue gases 'condenses' in the secondary heat exchanger and drains away at the bottom of the boiler.
Although condensing boilers will become increasingly popular as dramatic savings can be made on fuel consumption they are at present still in their infancy and may be more prone to breakdowns. They are also more expensive to buy although grants can often be obtained to assist with the extra cost.

Additional information about Condensing Boilers

Storage Tanks
Boilers with a storage tank (hot water cylinder) are better capable of coping with the demands of multiple use and can deliver water at a high temperature and at a high flow rate too. Unvented tanks allow mains pressure water delivery, rather than relying on gravity as traditional open vented systems do.

Unvented tanks remove the need for separate cold water storage in the roof.

They are most suited for family homes, and in properties where roof space is limited.