Energy Saving
Boiler Efficiency: How Condensing Boilers Save You Money
Reviewed by our Gas Safe registered engineers · Last updated 22 July 2026
Modern condensing boilers are rated over 90% efficient — but many never hit that in practice, usually because they’re set to run too hot. Here’s how boiler efficiency actually works, why flow temperature matters, and how to get yours condensing.
Quick answer: A condensing boiler is most efficient when it can recover heat from its own exhaust gases — and it can only do that when the water returning from your radiators is cool enough (below about 55°C). If the flow temperature is set high (many are at 70–80°C), the return is too hot, the boiler never condenses, and you lose several percent efficiency. Turning the flow temperature down to around 50–60°C fixes it, usually with no loss of comfort.
The Numbers
How efficient is your boiler?
Where your boiler sits depends on its age and, just as much, how it’s set up. These are rough real-world figures.
| Your boiler | Typical efficiency | What it means |
|---|---|---|
| Modern condensing, well set (flow ~55°C) | 90%+ | At its most efficient — where you want to be |
| Modern condensing, set too hot | ~86–88% | Not condensing fully — an easy, free fix |
| Older non-condensing (pre-2005) | 70–80% | A new A-rated boiler usually pays back |
| Back boiler or very old unit | Under 70% | Well worth replacing |
How It Works
Why flow temperature is everything
Why it matters
A condensing boiler recovers heat from its flue gases — but only when the return water is cool enough to make them condense. Run it too hot and it behaves like an old non-condensing boiler, wasting the very feature you paid for.
What we check
We set the flow temperature for your system, check the combustion and flue-gas readings, make sure the condensate is draining, and confirm the boiler is actually condensing — efficient in your home, not just on the label.
Engineer Insight
The free efficiency gain
Almost every combi we visit is set hotter than it needs to be, straight from the installer. Dropping the flow temperature to around 55°C is the single easiest efficiency gain there is — no parts, no cost, and most people never notice their radiators are a few degrees cooler because they simply run a little longer. The only homes where we’re careful are those with undersized radiators, where we balance comfort against the saving. It’s a two-minute change we make at every service.
Costs
Getting your boiler efficient
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
Boiler efficiency — your questions answered
What is boiler efficiency?
It’s how much of the gas your boiler burns actually becomes useful heat. A modern condensing boiler can be over 90% efficient, but only if it’s set up to condense — otherwise it wastes heat up the flue.
What is a condensing boiler?
One designed to recover extra heat from its exhaust gases by condensing the water vapour in them. Nearly all boilers fitted since 2005 are condensing, but they only condense when the return water is cool enough.
Why isn’t my condensing boiler condensing?
Almost always because the flow temperature is set too high, so the water returning from the radiators is too hot for the flue gases to condense. Turning the flow temperature down to around 50–60°C fixes it.
Will turning down the flow temperature leave me cold?
Usually not. The radiators run slightly cooler for a little longer, so the room reaches the same temperature — you just don’t get the initial blast of very hot radiators. Homes with undersized radiators are the exception, and we check for that.
Is it worth replacing an old boiler for efficiency?
If yours is a pre-2005 non-condensing boiler running at 70–80%, a modern A-rated boiler can cut a good chunk off your gas bill and usually pays back over its life. We’ll give you an honest view on repair versus replace.
Related
Related guides
All the savings in one place.
Read guideEnergy SavingLeave Heating On Low All Day?Efficiency and the on-low myth.
Read guideSmart ControlsHow to Set Your Heating SchedulePair efficiency with a good schedule.
Read guideMaintenanceSigns Your Boiler Needs ServicingA service keeps efficiency up.
Read guideCentral HeatingHow Central Heating WorksHow the boiler fits the system.
Read guideEnergy SavingInsulation TipsEfficiency plus insulation.
Read guideBrowse all Energy Saving guides, or head back to the Help & Advice Centre.
Is your boiler actually condensing? Let’s check.
Book a service and we’ll set your flow temperature and confirm the boiler is running efficiently, or call for advice. North London, upfront pricing.
Gas Safe Registered 918817 | North London specialists | Upfront pricing