Since 2005, all new gas boilers installed in the UK must be condensing boilers. But what makes a boiler "condensing" and why does it matter?
How a condensing boiler works
A standard (non-condensing) boiler burns gas, extracts heat from the combustion gases, and then exhausts the remaining gases — including a significant amount of unused heat — up the flue at around 180°C.
A condensing boiler has a second, larger heat exchanger that extracts this residual heat from the exhaust gases, cooling them down to around 50–60°C. As the gases cool, the water vapour they contain condenses into liquid — hence "condensing" — releasing additional latent heat that would otherwise be wasted.
The efficiency difference
This recovered heat improves efficiency dramatically:
- Non-condensing boiler: 70–80% efficiency
- Condensing boiler: 88–96% efficiency
For a home spending £1,500 per year on gas, switching from a 75% efficient to a 92% efficient boiler saves approximately £280 per year.
The condensate pipe
The condensation produced (slightly acidic water) must be drained away. This is done via a plastic "condensate pipe" that typically runs to an outside drain. In cold weather, this pipe can freeze — a common cause of winter boiler lockouts.
All modern boilers are condensing
Every new gas boiler installed in the UK since 2005 has been a condensing boiler by law. If your current boiler pre-dates 2005, it's almost certainly non-condensing and significantly less efficient than a modern replacement.