Boiler Types Explained

Condensing Boilers

The term ‘condensing boiler’ refers to boilers that produce condense (moisture vapour) from time to time.

Condensing boilers use heat from exhaust gases that would normally be released into the atmosphere. To capture this latent heat, the water vapour from the exhaust gas is turned into liquid condensate.In order to make the most of the latent heat within the condensate, condensing boilers use a larger size heat exchanger or a secondary heat exchanger.

Because of this feature, a condensing boiler is able to extract more heat from the fuel it uses than a standard efficiency boiler. This also means that less heat is lost through the expelled flue gases.

Non-Condensing Boilers

The term ‘non-condensing boiler’ refers to boilers that do not produce condense.

A non condensing boiler is similar to condensing boilers except they do not have a secondary heat exchanger. A modern non-condensing boiler will be around 4% to 8% less efficient than an equivalent condensing model.

Modern UK building regulations usually prohibit the installation of non-condensing boilers in all but a few exceptions. Exemptions must to be obtained from your local council / authority prior to installation.

Combi Boilers

A combi or combination provides hot water and heating directly from the boiler itself.

Combi Boiler Household Image

A combi boiler is an innovative space-saving option and a very popular choice in UK homes. In fact, combis now account for well over half of all the new domestic boilers installed in the UK every year.

A combi boiler is both a high-efficiency water heater and a central heating boiler, combined (hence the name) within one compact unit.There's no separate hot water cylinder which can provide vaulable space saving within your property. Further benefits of a combi boiler includes hot water being delivered, on-demand through your taps or shower at mains pressure levels. This allows you to enjoy invigorating showering without the need for an additional shower pump.

Combi boilers work best for smaller properties with one or two bathrooms. Larger properties should consider either a regular or system boiler.

Regular Boilers

Regular boilers, also known as ‘conventional’ boilers heat your central heating system directly and produce hot water for your hot water cylinder.

Regular Boiler Household Image

Below, you will see an example of a central heating and hot water system layout using a regular boiler.

If you are replacing an older boiler, there's a good chance will have a regular or conventional boiler already. A typical regular boiler system incorporates a boiler, a hot water cylinder and extended controls like a water pump and is often fed by a cold water storage cistern tank located in the attic along with a smaller expansion cistern tank.

System Boilers

A system boiler heats your central heating system directly and produces hot water for storage in your cylinder. These systems come in two configurations, unvented and vented. To the right is an example of an system boiler layout with an open vented hot water cylinder.


Firstly, many of the major individual components of the heating and hot water system are built into a system boiler, which means that installation is quicker, tidier and usually easiler. Secondly, the hot water is pumped from the system boiler through the heating system to the radiators and hot water cylinder, resulting in a fast response and more economical running costs. The system boiler removes the need for a feed and expansion tank in the loft space.

To the right is an example of an system boiler layout with an pressurised - unvented cylinder.

Open Vented System
Regular Boiler Household Image
Unvented System
Regular Boiler Household Image

RW Thomas Quality Gas, Oil & Warm Air Installation Services.
Contact Us For Your Free Quotation TODAY