All about heating: knowing your system

In the first instalment of this new three-part series, learn what to consider before upgrading and investing in new heating

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Heating systems are complicated, so in order to work out how to make savings on your bills and improve your home, it’s important to understand how they work. It’s the area of property maintenance that homeowners often know least about, however, and the single fastest-changing aspect of the housing world.

So it’s no wonder that the majority of homeowners leave it to others to decide which option to choose or, even worse, pick a standard solution. Often, though, they come to regret not spending more time researching potential systems.

Home energy bills are estimated to rise by around 12 per cent for gas and 30 per cent for electricity between 2013 and 2020.* There is a little to be saved by switching suppliers but, in the long term, the only way to significantly control your costs is to manage your home’s energy usage more effectively.

*Source Department of Energy and Climate Change

The fabric first approach

Before you begin considering which heating system is right for your home, it’s wise to review your property’s overall energy efficiency. A ‘fabric-first’ approach involves maximising the building’s performance by focusing primarily on reducing heat loss through the roof, walls and floor, draughts and cold spots (thermal bridging) — therefore reducing requirements for heat generation.

It is best achieved through effective insulation, good windows, and airtightness (with a good ventilation system). An optimised design can passively capture and retain as much of the sun’s heat as possible while also avoiding overheating.

The basic heating system

At its simplest level, think of a heating system in two parts: the bit that generates the heat, and the bit that distributes this output around your home.

Heat is required in two forms — for space heating and for hot water. So, the simplest of all systems would have: a boiler, which uses power to heat up water and incorporates a pump to move it around; piping, to move that warm water around your house; emitters, whether radiators or underfloor heating; and a cylinder to store hot water for use as required, although these are not required with a combination (combi) boiler.

Choosing a boiler

The most important part of the heating system is the boiler, and your choice will impact on how the system operates and its efficiency. Your new boiler will, by definition, be at least 90 per cent efficient, meaning it will convert 90 per cent of the energy it uses into heat, and if powered by gas it has to be a condensing boiler, which utilises the heat from the exhaust gases within the boiler for added efficiency.

Regular or conventional

Regular boilers are now largely bought as replacements for homes with an open-vented heating system (supplied by means of a feed-and-expansion cistern in the roof space, meaning the system is open to air). As with system boilers (see below), they work on the principle of stored water and require a separate hot-water cylinder. The water from the taps will be at a good flow rate (not to be confused with pressure) and hot water can be supplied instantaneously from the cylinder.

They’re more expensive to install than unvented ‘sealed’ systems, as they need more components and pipework, and they also take up more space, especially the tanks. Hot water from a gravity-fed system can suffer from low pressure if the cistern is not positioned high enough relative to the outlets. Pressure can be improved for power showers by adding booster pumps.

Regular boilers produce hot water more efficiently than combi boilers. However, hot-water cylinders do lose heat over time, meaning a combi boiler may be the more efficient option overall for infrequent use.


Most people will opt for a regular ‘system’ boiler, but you could choose a ‘combination’ boiler — or ‘combi’ for short. Combis work as sealed systems, so the primary heating circuit is under pressure, and able to store more heat than an unvented system, with no need for a cistern. As the name suggests, combis can provide instant hot water as well as central heating, via a secondary heat exchanger that warms up incoming mains water directly. Combis are quicker, easier and cheaper to fit than system boilers, as well as space-saving, due to the lack of a hot-water storage cylinder or cistern.

The output of a combi boiler needs to be sized correctly to match hot-water demand in order to provide a sufficient flow rate. Larger households with a higher level of demand are likely to need some form of stored hot water instead, such as with a correctly sized hot-water cylinder.

It’s also worth considering that active solar panels need a cylinder to store the hot water they produce and so will not work in tandem with a combi boiler.


System boilers are fitted to sealed heating systems but, unlike combis, store hot water in a cylinder, so they can feed several outlets at once at mains pressure.

There’s no need for a cistern in the loft, and the expansion vessel is built in.

System boilers are ideal for larger homes with higher demands and, as most of their major components are built in (such as an expansion vessel and pump), installation is quicker, cheaper and neater. Flow rates are usually high as water is delivered at mains pressure.

The systems are easy to install, deliver heat and hot water quickly, and are economical to run.

Jargon buster

Hot-water cylinder

This is the storage vessel that supplies hot water on demand to taps and showers, usually located in an airing cupboard. Depending on the type of system, it either heats up cold water supplied by the cold water storage tank (a vented system), or heats up cold water supplied directly from the mains (unvented system).

The water in the cylinder is usually heated by a coil heat exchanger within the cylinder (an indirect cylinder), although many also have an electric immersion heater installed within the body of hot water. A cylinder powered only by the electric immersion heater, or a backboiler, is known as a direct cylinder.

Expansion tank

Because water expands when it heats up, there needs to be room in the heating system to accommodate the additional capacity. In traditional systems this would be in the form of an expansion tank in the loft, but on more modern ‘sealed’ systems that function under pressure, it is in the form of an expansion vessel, located either next to or within the boiler or hot water cylinder.


A domestic boiler provides hot water for central heating and domestic hot water through the combustion of fuel (usually either gas, oil or LPG, but also biomass fuel). The energy released by the combustion of fuel is transferred into water via a heat exchanger.

The amount of heat produced by a boiler is measured in kilowatts (kW) or British Thermal Units (Btu), and typically boilers range in size from 15kW to 40kW for domestic use. A combi boiler has a secondary heat exchanger that provides instant domestic hot water on demand, doing away with the need to store hot water.



Boiler installation

To find a Gas Safe-registered installer, visit, — or for Scotland and Northern Ireland.

What size of boiler do I need?

Boilers come in different sizes, measured in kW, and you need to specify the right one — a boiler that’s too large will not only be more expensive, but will operate less efficiently than a correctly sized model. Bear in mind that plumbers will be more likely to oversize as they don’t want callbacks from problems relating to a small model, and the capital cost is passed on to you anyway.

Many of the boiler suppliers offer online guides for choosing the right size. You can work this out yourself by adding up the required heat output from the radiators or underfloor heating (this can usually be calculated on radiator company websites), then adding 3kW for hot water, plus a 10 per cent buffer. Typical boiler requirements for a large detached house would be in the region of 30kW.

What will it cost?

Boiler + 3–5 radiators (including labour and accessories):£2,700–£4,200
Boiler + 7 radiators (including labour and accessories):£3,800–£5,000
Boiler + underfloor heating (including labour and accessories): £4,000–£7,000

The Kit

  • Boiler: £500–£1,500
  • Hot-water cylinder (standard): £100–£500
  • Hot-water cylinder (unvented): £1,000–£1,500
  • Radiators (single panel, 600W): £50–£200
  • Radiators (single panel, 1,800W): £150–£300
  • Underfloor heating (water, whole house): £2,000–£7,000
  • Underfloor heating (electric, 10m2 or so): £300–£700
  • Controls (thermostats/programmer): £70–£200

Individual jobs (supply and fit)

  • Fitting a new radiator (2–3 hours): £150–£300
  • New boiler + flue (1–2 days): £700–£2,000

Price comparison

*Excluding distribution system such as radiators or underfloor heating


Fuel types

Natural gas

Natural gas is by far the most widely used heating fuel across the UK and is still the cheapest option available. Because gas is so commonly used, the systems for it (boilers and hobs) are typically cheaper than systems for other fuels, too. However, if you’re not on mains gas, choosing a new heating system can be complicated. You’ll need to consider the costs, the paybacks and how the different options stack up. Around four million homes across the UK are not connected to the mains gas network*.

For many years, the choice of fuel for heating your home was relatively straightforward — the options being oil, LPG or electricity. A few may also have used solid fuel such as coal or logs. The vast majority of homeowners in areas on the mains gas network would struggle to justify (in purely financial terms) moving away from gas. It is the cheapest fuel and is likely to remain so.


Around 1.5million homes rely on domestic heating by oil**. A little more expensive than gas, oil-fired central heating is often the cheapest option available in rural areas where there is no gas supply. Heating with oil requires a boiler, and some domestic fires and cookers also use oil as fuel. An oil-fired boiler will often be more costly than an equivalent gas one.

Oil is ordered and delivered in bulk and stored in a tank, so you will need to monitor your usage carefully to ensure you always have an uninterrupted supply. To find a registered installer for oil boilers, visit or

Liquefied petroleum gas (LPG)

The main difference between mains gas and LPG central heating is that LPG is delivered by road and stored in a tank, which is typically rented from a gas supplier such as Calor Gas. An LPG system is the same as a mains gas system only the boiler will have a special burner for LPG. Gas hobs, fires and heaters are also available configured for LPG. LPG is a relatively expensive option compared to mains gas, or oil, largely because of the cost of transport.


As the use of fossil fuels, such as coal, gas and oil, declines, and electricity generation ‘decarbonises’ – in other words, electricity is generated more from renewable sources such as wind, and less from fossil fuels – more heating will come from electricity. It is currently an expensive option relative to mains gas or oil, although like gas it has the advantage of not requiring storage.

The cost of heating with electricity can be reduced by using a cheaper off-peak supply such as Economy 7. Most UK homes that don’t have a boiler and radiator system have electric storage heaters designed to use the Economy 7 tariff.

One thing to bear in mind is that, as there is little control over the release of the heat from the system, stored heat will sometimes need to be topped up with peak-rate electricity heat. The systems are quick and easy to store and don’t require annual maintenance.


Until recent decades, this was the most commonly used fuel in the UK. Burning coal can be a cost-effective way to heat your home and hot water, particularly in rural areas where mains gas is not available. Many older homes still have fireplaces, and the allure of an open fire remains an attractive option. However, the practical reality is that fuel burnt in a hearth will lose up to 90 per cent of its heat up the chimney.

Some homes, particularly in urban areas, are within smoke control areas — meaning that the burning of coal and logs is usually banned. Coal can also be used in solid-fuel stoves to provide a smokeless form of heat. The introduction of smoke control areas means that here, only smokeless varieties can be burnt, so you’ll need to check your area at

Renewable energy

Over the past decade, previously niche heating options such as heat pumps and biomass have become viable mainstream options, thanks in part to government incentives such as the Renewable Heat Incentive, or RHI, offering a genuine alternative to fossil-fuel choices of oil, electricity and LPG. For information, see, or Focus on Heating in November’s issue of Real Homes.

Wood pellet

Wood-fuelled (biomass) boiler systems burn wood pellets, chips or logs, to power central heating. This type of system is usually set up to power thermal stores — large, very well-insulated vessels of pressurised primary hot water, which then function as the heat source for a conventional central heating and domestic hot-water system. The boiler need only fire when the temperature in the thermal store has dropped by a set amount, reducing the frequency and improving the efficiency of each boiler firing.

Biomass boilers tend to be larger than regular boilers and also require on-site fuel storage, and consequently more space is required than for other domestic heating systems. The fuel store also needs to be reasonably accessible to delivery trucks. Its high capital cost and specific requirements may put some off, but generous RHI payments, realistically paying you back within four to six years, make wood boilers a genuine option for homeowners with sufficient space available.

*Source: YouGen