Boiler/System Types

There are three main types of boilers that feed heating systems found in today’s homes; these are vented, unvented and combination. A brief description of each can be found below, you may have one and want to change to one of the alternatives but chosing the right type for your property and your use is vital.


The fully vented boiler and system



These types of system are very much a British invention. An open vent boiler (also known as heat only boilers) will provide heating to the radiators and to the hot water cylinder, typically houses in an airing cupboard, both the heating circuit and the cylinder are fed by separate tanks in the loft; the hot water cylinder will be fed by a large cold water tank whilst the heating circuit will have a much smaller tank, more commonly known as a feed and expansion tank or header tank. Converting an open vent system to a sealed system can be achieved easily and in two ways; one to utilise the existing boiler and add additional auxiliary equipment, or alternatively install an unvented or sealed system boiler; both would then cause the header tank to become redundant.

Typically, these types of systems are more commonly found in larger homes, boilers with heat outputs ranging from 12 to 42kW providing the source of heat. As technology improves however, more and more combination boilers are coming onto the market that are capable of delivering sufficient heat and hot water on demand where there are often three bathrooms and/or shower rooms in a property.

High domestic hot water flow rate at taps
No discernable loss in flow or pressure when more than one hot tap or shower is used at the same time.

Limited amount of hot water available, once the stored water is used reheat time will be required.
Space required siting a hot water cylinder, its components and also cold water storage in the loft.



The Unvented or Sealed System



A system boiler (sometimes referred to as a sealed system) provides heating much the same way as a vented boiler; however, the difference is that there is no need for a feed and expansion tank in the loft as there is with an open vent boiler. Sealed system boilers with heat outputs ranging from 12 to 42kW are capable of providing the source of heat to many homes regardless of size.

There will of course still be a need for a large cold water tank to feed the hot water cylinder, but this too can be made redundant by changing the vented hot water cylinder to a mains fed unvented cylinder – more on that will be discussed later. Alternatively, the installation of a combination boiler would also mean that there would be no need for cold water storage.

There will still be a high flow rate for domestic hot water
No discernable loss in flow or pressure when more than one hot tap or shower is used at the same time.
No header tank needed in the loft

Same as for vented boiler.



The Combination Boiler



Often referred to as a “combi” boiler, technology has improved dramatically over the course of time with these types of boilers and so has performance. It is estimated that around 70% of homes in the UK now have a combination boiler as their primary source of heating and hot water. These boilers will provide heating and hot water without the need for any tanks in the loft and will provide a huge amount of hot water on demand and at mains pressure too, giving a very powerful shower. They can be sited practically anywhere, though consideration has to be given to the location of hot water draw offs (taps) to ensure that the hot water doesn’t have to run too far.

Although these boilers were initially more commonly found in flats or small one bathroom homes, the development over the years of these boilers has found them to be so reliable and affordable that more and more homes are changing to combination boilers. There are however drawbacks to these types of boilers which can be found below.

Instant, unlimited supply of hot water
Compact, easy to fit almost anywhere
Mains pressure hot water draw off, providing strong and powerful showers

Should the gas supply be disrupted or the boiler breakdown, there will be no hot water
Very noticeable drop in flow and pressure when more than one tap or shower used at the same time
Flow rate greatly reduced in comparison to stored hot water system – boiler has to reduce the flow to allow the water to be heated when passing through the plate heat exchanger.