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Expert help and guidance with central heating


It is important to understand that the boiler is just one component of a central heating system and the boiler alone does not determine efficiency, no matter what it says on the box. In order to have an efficient heating system the whole system needs to be right. We explain the basics of central heating and guide you on each component.

Expert help and guidance with central heating


  1. What is central heating?
  2. Central heating components
  3. How does it work?
  4. On/off heating v continuous heating
  5. Open vented v sealed system
  6. Central heating costs
  7. Condensing
  8. Central heating controls
  9. Radiator balancing and TRVs
  10. Radiator sizing
  11. Central heating maintenance
  12. Cleaning and staying clean


1) What is central heating?

The term ‘central heating’ refers to all ‘wet’ heating systems (wet because water circulates around the radiators or underfloor heating) that have a central heat source such as a gas boiler, oil boiler, electric boiler or renewable energy source such as a heat pump or biomass boiler. For this reason, heating systems that use individual electric heaters or gas fires are not forms of central heating.

2) What are the components of a central heating system?

A central heating system is made up of a boiler (oil, gas, electric, biomass or heat pump), radiators and/or underfloor heating, pipework, heating controls and radiator valves. All central heating systems use a pump which may be integral to the boiler or outside the boiler.

3) How does a central heating system work?

The boiler or heat pump heats the water and pumps it around all the radiators and/or underfloor heating pipes in the home. The boiler/heat pump is told to come on by a thermostat (also referred to as a heating control) and stays on until the set room temperature is met.

4) On/off verses continuous heating

Depending on how the system has been set up, the boiler will operate slightly differently for on/off and continuous heating. This has a big impact on efficiency for some heating systems.

On/off heating, aka intermittent heating

This is when the boiler blasts out heat at high temperatures for a short period and then switches off. Older style gas and oil boilers worked at high temperatures of 70-80 degrees, so radiators got really hot for short periods and then switched off. Many modern gas and oil fired systems are still set up like this, even though modern boilers are more efficient when the radiators run at lower temperatures.

Continuous heating

This is when the boiler or heat pump stays on almost continually but at a much lower temperatures. This is always the case for heat pumps and systems with all underfloor heating as they run a lower temperatures. It can also be the case for radiator systems with out boiler systems. This means the radiators will not get so hot, rather they will be lukewarm but stay on for longer periods.

5) Open vented v sealed systems

Originally all central heating systems were 'open-vented' which meant air was allowed to escape at the highest point of the system - usually in the loft. Today most systems are 'sealed', i.e. air cannot escape on an ongoing basis, so all air must be removed from the system first. Most modern boilers are for sealed systems, although open vented (OV) boilers are still available. If you have an open vented system and are thinking about changing read our Guide to open vented and sealed systems first to consider the pros and cons.

6) Central heating costs

We installers talk about central heating costs they are usually referring to a 'full system' installations, i.e. boiler plus radiators plus pipework. This usually occurs in homes where there has not been a wet central heating system before, for example homes with a warm air unit. We have written a detailed Guide to central heating costs which breaks down the elements of the installation.

7) Condensing heating systems

Some central heating systems need the boiler to condense in order to achieve higher efficiencies.

Gas and oil boilers

Modern gas and oil boiler efficiencies rely on the boiler operating in ‘condensing mode’. Without condensing, efficiencies drop quickly from 92-94% ‘on-the-box’ to closer to 80% in the home. Heating systems that use a condensing gas or oil boiler should ideally be set up to run a lower temperatures for longer in order to maximise efficiencies. For more on condensing read our blog Why condensing boilers do not condense

Electric boilers and biomass boilers

Electric boilers and most biomass boilers do not use condensing technologies. Electric boilers do not need a flue and biomass boilers use a chimney rather than a flue. Electric and biomass heating system can therefore run a higher temperatures, but there could be an argument for increased wear and tear if the boiler if switching on and off all the time rather than running at a lower output for longer.

Heat pumps

Heat pumps must run at a lower output in order to reach their efficiency potential, therefore radiators will never feel hot to the touch. If you have a lower temperature heating system, including a lower temperature gas boiler system, try not to get caught up on whether the radiators feel hot, only whether you are warm enough in the home.

8) Central heating controls

How the boiler is controlled is one of the most important aspects of any heating system, more so for gas and oil boilers as it will play a huge part in how efficiently the boiler can operate. We have extensive guidance on heating controls in our dedicated section.

9) Radiator balancing and TRVs

What does balancing mean? Well for older gas and oil boilers and also for electric, heat pumps and biomass boilers, it just means making sure every radiator gets hot. Water will always take the path of least resistance, so unless it is coaxed round all of the radiators it will just circle around the one closest to the boiler.

In order to balance the system, heating engineers have to restrict the access to radiators nearer to the boiler (using the thermostatic radiator valves or TRVs) and open up the access on the radiators further away. In this way the hot water is circulated around all of the heating system. Radiator valves come in many shapes and styles and we compare the majority of them in our comprehensive Guide to TRVs

For a modern gas or oil fired heating system, the temperature of the water as it goes back to the boiler has to be below 54 degrees (ideally lower) in order for the boiler to operate in ‘condensing mode’. If the system is not properly balanced then the temperature of the water back to the boiler will be over 54 degrees.

The boiler will still work, but it won’t be efficient. Fortunately there are a number of radiator valve products available that will ensure the system stays balanced which increases the efficiency enormously provided they are set up correctly.

10) Radiator sizing

Most of the time radiators are not replaced when a new boiler is fitted. When condensing gas boilers were introduced in 2005, informal guidance suggested that radiators needed to be oversized by 30%. This is because the system needs to run at a lower temperature in order for the boiler to condense.

When systems run at lower temperatures the surface area of the radiators needs to be bigger. This is particularly true for heat pumps that need to run at really low temperatures. This increases the radiator sizes by at least 100%.

If you are fitting a brand new heating system, the heat loss of each room must be calculated and the radiators sized according to the flow temperatures from the boiler (the lower the flow temperature the more efficient the heating system will be). For existing systems it is usually the case that the radiators are already oversized for the room. This is as a result of rough and ready sizing, but it does benefit homes now as it allows for the flow temperatures from the boiler to be lowered and therefore run more efficiently.

11) Central heating maintenance

When central heating water gets dirty it reduces the efficiency of the system. Water gets dirty when air enters the system and causes the pipes to corrode. This causes a build up of ‘sludge’ in the water which in extreme cases can block pipes and radiators.

As the sludge builds up, the boiler has to work harder and harder to heat the home, which uses more gas and increases fuel bills. Loose sludge can also enter the boiler and damage the component parts, in particular the heat exchanger. For more guidance on radiator problems on their causes try our Guide to radiator problems.

12) Cleaning and staying clean

If you have cold spots on your radiators or completely cold radiators, then you will need a powerflush to get rid of all the sludge. A powerflush is a process whereby each radiator is flushed out and refilled with clean water. For more read our Guide to powerflushing.

Once the water is clean, the best method of keeping it clean is by fitting a device called a ‘dearator’. This remove air from the system and stops corrosion at its source.

Magnetic filters are very common now and do a great job of protecting the boiler from any stray bits of sludge entering it. For more on makes and models of magnetic filter, see our Guide to magentic filters. They are not a prevention device however, for that you need a dearator.