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Inside a typical condensing boiler

Nearly all new boilers are now condensing boilers. Condensing boilers are far more energy-efficient than traditional boilers, as they cleverly re-use heat that would otherwise be wasted.

Some heat generated by traditional boilers disappears up the flue in the form of hot waste gases. But condensing boilers use some of the heat from these flue gases to heat water, making the boiler far more efficient.

Types of boiler

There are three main types of condensing boiler: combination, system heat-only or open-vent heat-only. 

Combination boiler

This is the best-selling boiler type for gas central heating in Britain. A combination - or 'combi' - boiler provides heat for your radiators and domestic hot water on demand. 

The name comes from the fact it combines all the functions of a traditional boiler, hot water cylinder and cold water tank set-up. All the major parts are in one unit, which makes them convenient to install.

There are different types, but they all work by taking cold water straight from the mains supply and heating it as you need it. When you turn on a tap, water is drawn from the mains, heated in the boiler and comes straight out of your tap piping hot.

This means there's no need for a separate hot water storage cylinder or cold water feed cistern. But there's a small time lag between turning on the tap and getting hot water, and the flow rate is limited by how quickly the boiler is able to heat up the water.

The advantage is they require less space than heat-only boilers (because there is no storage cylinder), and avoid heating water that's not used immediately. They also deliver hot water at mains pressure so you can have a nice strong shower straight from your taps.

On the down side, the flow rate limits how much water you can use at one time – no filling the kitchen sink while running a bath, for instance.

You might also miss using the (wasted) heat from your hot water cylinder in your airing cupboard.

Use our boilers product finder tool to filter our test results by boiler type, including system, heat-only and combi boilers.

A heat-only boiler

Conventional heat-only open-vent boiler

Heat-only boilers are the heart of most old-fashioned heating systems. They provide heat for radiators directly and produce hot water which is stored in a separate hot water cylinder. The cold water is normally supplied by a cold water cistern, usually in the loft above the boiler (a gravity feed) and there is also a separate expansion cistern.

These arrangements take up more space than a combi boiler system, but because there is a large tank of stored hot water you can use several taps at once. This makes this type of system more suitable for larger houses.

Unless a cylinder is very well insulated it usually loses heat into its surroundings, thereby wasting energy. And the amount of hot water you can use is limited by the size of your cylinder.

For instance you might run a bath, then need to wait for the boiler to heat up the water in the cylinder again before you can run another.

System heat-only boilers

A system boiler, just like an open-vent boiler, works on the principle of stored hot water. However, many of the major individual components of the heating and hot-water system are built into a system boiler. The system boiler removes the need for a feed and expansion cistern, but there is still a cold water tank.

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. 

It is the ideal choice for the modern family home which needs every inch of space for storage and has a high demand for hot water.

Boiler size

A boiler is sized to suit your home's hot water demands

Most manufacturers make each of their boilers in a range of different heat outputs, typically ranging from 12 to 40kW.

A higher number means a boiler can heat water more quickly, so it can supply hot water to more radiators or taps. The greater the number of radiators or taps you're likely to use at a time, the higher the boiler kilo-wattage you're likely to need. 

As homes have vastly different numbers of radiators and bathrooms these days, it's difficult to generalise about what size boiler is needed for a flat, cottage or larger house. Your boiler will be sized according to the amount of heat and, in the case of a combi boiler, hot water you require.

Your installer will size the boiler according to how many rooms and radiators you have, as well as the number of hot water taps or devices that use hot water. They should also take factors such as wall and loft insulation and double glazing into account.

When buying a new boiler you'll need to get your hot water demand assessed individually by your plumber or installer. 

Boiler heating controls

Effective heating controls can increase the efficiency of a boiler-powered central-heating system. They can also help you minimise energy consumption by ensuring each room is at the right temperature for comfort, while avoiding overheating.

Timer or programmer

An electronic timer or programmer decides when the boiler runs. A seven-day timer makes it possible to set a different heating pattern for weekdays and weekends.

Some timers allow different patterns for each day of the week; this can be useful for people working part-time or on shifts.

Intelligent heating controllers

These combine several of the basic controls and can also learn how long it takes for a house to heat up in different weather conditions. They also often allow for different temperatures to be set between day and night.

Weather compensators

These measure the temperature, either internally or externally, and delay switching on the central heating on milder days.

Full zone control

Most homes have a single heating zone – the only controls in the rooms are by thermostatic radiator valves (TRVs - see below).

When a new central-heating system is installed, it's possible to fit a full zone control that has different pipe loops and separate thermostats for two (or more) areas. This can save significant amounts of fuel in larger houses

Cut heating costs by turning your thermostat down

Thermostats

Room thermostat

This is best located in a living room or bathroom, which you'll probably want to be the warmest room in the house. The thermostat records the room's temperature, and if this is at or above the set level (20°C is usually adequate), stops the boiler from operating the central heating.

A programmable room thermostat lets you choose what times you want the heating to be on, and what temperature it should reach while it's on. Some allow you to select different temperatures in your home at different times of the day and days of the week.

Thermostatic radiator control valves (TRVs)

These detect the local air temperature and switch individual radiators on or off, depending on how hot the room is. It's not a good idea to have a TRV on the radiator in the same room as the main thermostat as, if you alter the TRV, the thermostat will adjust itself to be in line with this local radiator.

They usually have a fat valve at one end, marked with a * and numbers from one to five. The * setting is to protect against frost.

Cylinder thermostat

This thermostat is strapped to the outside of a hot water cylinder near the bottom. It works by sensing the temperature of the water inside the cylinder, switching on the water heating when the temperature falls below the thermostat setting.

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