Can an efficient boiler save you money? Yes (and the environment). In this guide we've done all the jargon busting so you can easily identify an efficient boiler.
Your boiler is probably the hardest-working appliance you own.
It warms your radiators, powers your showers and occasionally flashes a mysterious code at 6am just to remind you who’s really in charge.
But how much of the gas it uses actually becomes useful heat - and how much simply disappears up the flue?
Here’s how to find your boiler’s efficiency rating, what the numbers mean and how to squeeze better performance from the boiler you already have.
PS We fit new combi boilers with gas installation nationwide. Simply answer these questions, get your fixed price and you could get it installed as quick as tomorrow.
The quick answer
Most modern condensing gas boilers are highly efficient.
In England, a new gas boiler installation must achieve a minimum seasonal efficiency of 92% under Boiler Plus.
Older non-condensing boilers can be considerably less efficient, particularly if they were installed before condensing boilers became the norm.
As a general guide:
92% or above: modern and highly efficient
Around 86% to 91%: reasonably efficient, but not top of the class
Around 70% to 85%: showing its age
Below 70%: officially entering boiler-dinosaur territory
These are only broad comparisons. Your boiler’s actual performance will also depend on its condition, controls, settings and the heating system around it.
What does boiler efficiency mean?
Boiler efficiency measures how much of the fuel entering your boiler is converted into useful heat.
In very simple terms, a boiler with a seasonal efficiency of 92% converts roughly 92% of the energy in its fuel into heat for your home. The remaining energy is lost during operation, mainly through the flue.
That doesn’t mean precisely 8p of every £1 on your gas bill is wasted. Your bill also includes hot water use, standing charges and all the wonderfully unpredictable ways people use their heating.
It does, however, give you a useful way to compare boilers.
The higher the percentage, the less fuel a boiler should need to produce the same amount of heat.
How can I check my boiler’s efficiency?
First, find the make and model.
The model number is usually printed:
on the front of the boiler
underneath it
behind the control flap
on the Benchmark commissioning checklist
in the user manual or service paperwork
Once you have it, search for the model’s product data sheet or product fiche. Look for its seasonal space-heating efficiency rather than relying entirely on the letter rating.
No paperwork? No problem. A Gas Safe registered engineer should be able to identify the model and help you find the relevant efficiency data during a service.
Learn how you can save money by trading in your old boiler for a new, efficient model in our YouTube video below:
Check the energy label
Modern boilers are supplied with an energy label showing their rated performance.
A combi boiler may have separate ratings for:
central heating
hot-water production
sound output
boiler output in kilowatts
The heating and hot-water ratings aren’t always identical, so make sure you’re reading the right one. Boiler labels: surprisingly detailed for something that spends its life inside a cupboard.
UK suppliers and retailers must provide regulated energy information for products covered by the labelling rules. You can read the official UK energy-labelling guidance on GOV.UK.
Is an old boiler automatically inefficient?
Not automatically.
A boiler doesn’t receive a G rating simply because it celebrated its tenth birthday.
Some older condensing boilers remain reasonably efficient, particularly when they have been properly maintained and paired with decent heating controls.
Age is still a useful clue, though.
From April 2005, boiler-installation rules in England and Wales generally required new gas boilers to use condensing technology, except in limited circumstances. Similar requirements were introduced across the rest of the UK around the same period.
That means a gas boiler installed before 2005 is more likely to be an older non-condensing model.
A boiler installed after 2005 is more likely to be condensing - but the model number is still the best way to know for certain.
If your boiler is 10 to 15 years old, consider more than its efficiency rating. Reliability, repair history, parts availability and the quality of its controls all matter too.
A 12-year-old boiler that behaves itself may have life left in it. A 7-year-old boiler that has your engineer on speed dial may be making a stronger case for retirement.
What is a condensing boiler?
A condensing boiler recovers heat from its exhaust gases before they leave through the flue.
Older non-condensing boilers allow more of that heat to escape. A condensing boiler cools the exhaust enough to recover additional energy from the water vapour created during combustion.
The resulting liquid leaves through a condensate pipe - usually a plastic pipe connected to the boiler.
Despite the name, “condensing” isn’t a separate type of heating system. Combi, system and regular boilers can all use condensing technology.
It’s less a boiler type and more a very sensible refusal to throw perfectly good heat outside.
What is ErP?
ErP stands for Energy-related Products.
The ErP framework introduced standardised efficiency information for products including boilers, heat pumps and water heaters. It helps consumers compare products using consistent laboratory testing and energy labels.
For boilers, the important information includes:
seasonal space-heating efficiency
energy-efficiency class
rated heat output
hot-water efficiency, where applicable
sound-power level
The percentage is normally more useful than the letter alone because two A-rated boilers can still have slightly different seasonal efficiencies.
And yes, the label can go beyond A. Apparently ordinary excellence was no longer enough.
Recommended heating guides:
What is SEDBUK?
The SEDBUK system (or Seasonal Efficiency of Domestic Boilers) was largely replaced by the ErP system, but some manufacturers still display it on their products, as some believe it provides more useful and accurate information.
Similar to the ErP system the SEDBUK system also rates boilers from A (highest efficiency) down to G (lowest efficiency).
Boiler Plus Legislation (Implemented in 2018)
On the 6th of April 2018, the Department for Business, Energy and Industrial Strategy (BEIS) implemented Boiler Plus Legislation. And here you thought the legislation was always boring.
The aim of this new legislation was to drastically increase the energy efficiency of residential boilers, helping to reduce carbon emissions, while decreasing heating costs. As a result, all new boilers installed in UK homes must meet these new standards.
A major requirement of this new legislation was that all gas boilers must now have an energy efficiency rating of at least 92%, as well as a programmable timer and smart thermostat installed if they are not already installed and in working order.
In addition, it’s also a requirement to have one of the following four fitted at the same time as the new combi boiler:
Load compensation - this will adjust the heat output from the boiler based on the room temperature
Weather compensation - this detects the outside temperature and weather and adjusts the boiler heat output in response
Smart controls e.g. a smart thermostat and or smart radiator valves allow further optimization and control of the system
PFGHRD (Passive Flue Gas Heat Recovery Device) - the most hideous name of all functions, but this allows a boiler to recycle heat, increasing efficiency. New boilers may already have this installed
Boiler Flue Regulations 2018
Post-April 2018, there are now boiler flue standards that dictate where it must be installed.
The main reason for this is safety and the new rules help to reduce the hazard of coming into contact with the toxic exhaust gases. If only you could install a flue on some people.
These new standards mean that the risk of anything or anyone coming into contact with the gases is reduced.
The standards dictate that a boiler should be positioned at least 30cm from any window or door or 60cm for larger boiler models.
In certain cases, this isn’t always possible, in which case the boiler should be a height of over 2.1 metres.
How Efficient is Your Boiler?
The ErP energy efficiency labelling system makes it incredibly easy to check the efficiency rating of your boiler, as long as you can identify colours and the first seven letters of the alphabet - qualifying most children aged two and up.
In order to ensure all boilers met new energy efficiency standards, new regulations were introduced in 2005 dictating that all new boilers had to be condensing rather than non-condensing, this change drove major improvements.
Essentially, condensing boilers are built with a new technology called a Flue Gas Heat Recovery system that recycles heat from the flue gases, ensuring that the majority of the heat produced is put to use.
This prevents energy waste and means you are getting more for the same amount of money.
Therefore, if your boiler was installed after 2005, it should be a condensing boiler, which is more efficient than the old non-condensing boilers. In all fairness, the old boilers were an improvement over the kettle of boiling water over an open fire.
As mentioned, if your boiler is a brand new model or was installed after April 2018 it should also be at least 92% efficient in order to fall in line with the Boiler Plus Legislation that came into force after this period.
How can I make my boiler more efficient?
Before marching your boiler towards the nearest skip, check whether your existing system can be improved.
1. Lower the central-heating flow temperature
Your boiler’s flow temperature is the temperature of the water it sends to your radiators.
Condensing boilers perform better when the water returning to the boiler is cooler. Reducing the flow temperature can help the boiler spend more time in condensing mode, recovering heat that would otherwise escape.
Official government guidance has recommended trying a central-heating flow temperature of around 60°C for suitable combi boilers. Some homes can run lower, while others may need a higher setting during cold weather.
Reduce the temperature gradually and make sure your home still reaches a comfortable temperature.
Importantly, the radiator flow-temperature setting is not the same as the temperature of the hot water coming from your taps.
If you have a hot-water cylinder, don’t start changing temperatures enthusiastically. Stored water must remain at a safe temperature. Check your boiler manual or speak to a heating engineer first.
Ofgem also recommends lowering the flow temperature of suitable combi boilers as one way to reduce energy use. See its energy-bill guidance.
2. Use your thermostat properly
Turning your thermostat to 30°C won’t heat the room faster.
It simply tells the boiler to keep heating until your home resembles a moderately priced sauna.
Choose the lowest temperature that keeps your household comfortable and use a schedule that matches when you are actually at home.
3. Turn down unused rooms
Thermostatic radiator valves let you reduce the temperature in rooms you use less often.
You don’t necessarily need to switch those radiators off completely. Very cold rooms can become more vulnerable to condensation and damp.
4. Check for short cycling
Short cycling happens when a boiler repeatedly fires up and switches off over short periods.
This can occur when the boiler is oversized, its minimum output is too high or the heating system can’t disperse heat effectively. Frequent cycling can reduce efficiency and increase wear.
If your boiler seems to be auditioning as a light switch, ask an engineer to investigate.
5. Balance and bleed the radiators
Cold patches, noisy radiators and unevenly heated rooms can indicate trapped air, poor balancing or sludge.
Bleeding and balancing the radiators can improve comfort and help the system distribute heat more effectively. Persistent cold patches may require professional attention.
6. Use compatible controls
A modern modulation control can communicate with a compatible boiler and adjust its output more precisely.
Compatibility matters. A very clever thermostat connected incorrectly can become a very expensive on/off switch.
7. Have the boiler serviced
A regular service can identify safety, combustion and performance problems before they become larger - and usually more expensive - problems.
Any work involving a gas boiler must be completed by a suitably qualified engineer. You can verify an engineer or business using the official Gas Safe Register.
Signs your boiler may be wasting energy
Your boiler may need attention if:
it is an old non-condensing model
your gas use has risen without an obvious explanation
your home takes much longer to warm up
the boiler repeatedly switches on and off
radiators heat unevenly
your controls are basic or difficult to use
breakdowns are becoming more frequent
replacement parts are difficult to obtain
the boiler makes noises normally associated with submarines
Not every symptom means you need a replacement. The problem could be a faulty control, poor system balance, trapped air, sludge or another repairable issue.
Get the system checked before holding a formal retirement ceremony.
Is it worth replacing my old boiler?
Replacement may make sense when a boiler is inefficient, unreliable, expensive to repair or no longer supported by the manufacturer.
A new boiler could give you:
higher seasonal efficiency
improved heating controls
quieter operation
better hot-water performance
a new manufacturer warranty
fewer surprise repair bills
However, replacing one modern condensing boiler with another may not dramatically reduce your gas use.
The biggest efficiency improvement is normally achieved when replacing a much older non-condensing model or upgrading a poorly controlled system.
It is also worth considering whether another heating system, such as a heat pump, could suit your home before committing to another gas boiler.
The correct answer depends on your property, budget and heating needs. Boilers are annoyingly resistant to one-size-fits-all advice.
Find the right boiler for your home
If your current boiler is inefficient, unreliable or simply making noises that feel personally threatening, Heatable can help.
Answer a few simple questions about your home, current heating system and hot-water needs. We’ll match you with suitable boilers and show you a fixed online price.
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