Why boiler efficiency matters

Every new gas boiler installed in the UK today has to be a high-efficiency condensing boiler, and has done since building regulations changed in 2005. If your home still has an older, non-condensing boiler, you're very likely paying more than you need to for your heating.

How a condensing boiler works

A standard boiler sends a lot of heat straight up the flue and out of the building. A condensing boiler uses a second heat exchanger to recover much of that heat before it escapes, using it to pre-warm the water coming back into the system. The water vapour in the exhaust gases condenses back into liquid as it gives up this "latent heat", which is where the name comes from. You'll sometimes see a wisp of steam (pluming) from the flue outside, and a small amount of mildly acidic condensate is piped away to a drain or soakaway. Both are entirely normal.

A condensing boiler doesn't run in condensing mode all the time: it depends on how cool the water returning to the boiler is, which is why the right heating controls matter almost as much as the boiler itself. Even outside condensing mode, though, it's still significantly more efficient than an old cast-iron or back boiler.

Efficiency ratings explained

Boiler efficiency is shown as a percentage: roughly, how much of the fuel's energy actually ends up as usable heat rather than being lost. Modern A-rated condensing boilers run at 90%+ efficiency. Many boilers still in service around Devon are 15–20 years old and were never required to hit that standard.

• A 90% and above
• B 86% – 90%
• C 82% – 86%
• D 78% – 82%
• E 74% – 78%
• F 70% – 74%
• G Below 70%

Replacing an old G-rated boiler with a new A-rated condensing boiler can make a real difference to a typical Devon home's annual heating bill, and cuts the associated CO₂ emissions by a similar margin, since a boiler alone can account for close to a third of a home's total carbon output.

Controls matter as much as the boiler

A high-efficiency boiler paired with poor controls won't perform as well as it should. Two upgrades make the biggest difference:

• Thermostatic radiator valves (TRVs): let each room find its own temperature by closing down as it warms from sunlight, a fire or a full room of people, rather than heating every room to the same level. Don't fit one to the radiator in the room with your main thermostat.
• Programmable room thermostats: give you proper control over when and how the heating runs, rather than an on/off schedule that doesn't match how the house is actually used.

What this means when you buy or sell a home

Since 2007, records of a property's heating installation and maintenance history have formed part of the paperwork buyers expect to see, and the industry's Benchmark scheme exists specifically to make sure new appliances are installed and commissioned correctly, and that a maintenance log is kept with the boiler from day one. If you're buying a home, it's worth having any gas or oil appliance checked before you complete. See our home buyers pack inspections.