Upgrading boilers and radiators can reduce heating demand. Upgrades are important for buildings unsuitable for more comprehensive changes to their existing heating system.

Boilers & Hot Water Cylinders

Where a new heating system is not yet practical or affordable, upgrading the current boiler and hot water tank to make it more efficient is often the most advisable way forward.

Enables: Changing from a combi (combination) boiler to system with a hot water cylinder tank enables Solar Water Heating and immersion heating of water with excess renewable electricity generation.

Condensing boilers are the most efficient boiler technology. They have larger heat exchangers than other types of boilers which recovers more heat and send cooler air up the flue. If the building has a gas or oil boiler installed after 2005, or has a plastic flue, or a plastic pipe coming out of the bottom through the wall and into a drain, it is a condensing boiler. (1) If it is not a condensing boiler, consider upgrading to one. It is worth considering the generally higher maintenance costs of boilers as they get older; the longer an old boiler is kept the more it will cost compared to in previous years.

Combi (combination) boilers provide hot water directly, whereas system boilers heat water only when their programmers tell them to, then store this water in hot water cylinders. Combi boilers can be a more efficient choice than system boilers. This is because hot water cylinders are not perfectly insulated, so hot water in them cools slowly. If there is a long enough gap between heating and demand, the water will need reheating. (1)

Which boiler to upgrade to may depend on several factors, most notably:

  • Compatibility with Solar Water Heating. Most combi boilers are either incompatible or less efficient with solar water heating, A system boiler should be chosen if the installation of a solar water heating system is a possibility.
  • If the installation of renewable generation such as Photovoltaics, Wind Turbines, or Hydroelectric is being considered then a system boiler might be chosen to provide the opportunity to use excess generated electricity for immersion heating utilising the energy on site instead of exporting it.
  • A building’s hot water usage. Generally, the higher the usage the more sensible choice a system boiler is. This is because system boilers are more efficient when hot water is used often as the hot water in the tank does not get as much time to cool down before use, so less reheating is needed.
  • Space for hot water cylinders. Combi boilers do need a hot water cylinder, so these are best suited if boiler room space is an issue. (2)

If a building has and is retaining a hot water tank, install cylinder jackets with 80mm of insulation, and foam tube pipe insulation. These reduce heat loss from the cylinder. They are cheap, simple, and easy ways to get hot water up to temperature faster and keep it hotter for longer, saving money and emissions. Alternatively, new hot water cylinders are factory insulated and extremely efficient compared to older ones. Depending on the age of the existing cylinder it could be worth replacing it with a new better insulated one. (2, 4)


Replacing old radiators is another way to increase the efficiency and effectiveness of a heating system.

Radiators installed before 2000 can be up to 50% less efficient than the best new designs. These heat up faster due to lower water content, lose less heat through the back panel, and have a larger surface area.

A cheaper, if less effective, efficiency improvement for old heating systems would be the use of chemical inhibitors to reduce corrosion and the build-up of scale deposits. (2)

Radiator reflector panels are another low-cost solution. They are installed behind radiators on external walls, improving radiator efficiency by reducing the amount of heat radiated through outside walls instead of into a building. The better any installed wall insulation is, the less necessary reflector panels are. (3)

(1) Energy Saving Trust, Heating Your Home, Heating and Hot Water. Available from: https://energysavingtrust.org.uk/advice/heating-and-hot-water [Accessed 26th January 2021].

(2) Energy Saving Trust, Is Renewable Heat Right for Your Home? Available from: https://energysavingtrust.org.uk/renewable-heat-right-your-home/ [Accessed 26th January 2021].

(3) Energy Saving Trust, Heating Your Home, Boilers. Available from: https://energysavingtrust.org.uk/advice/boilers/ [Accessed 26th January 2021].

(4) Energy Saving Trust, Reducing Home Heat Loss, Insulating, Tanks, Pipes and Radiators. Available from: https://energysavingtrust.org.uk/advice/insulating-tanks-pipes-and-radiators/ [Accessed 26th January 2021].