Installing an energy-efficient boiler

Last updated: May 8, 2024

Most homes in the UK have gas boilers – around 86% in fact – and they currently make up about 80% of a home’s energy bill each year. 

The government plans to phase out boilers powered by gas and oil by 2035. But until then, it’s still possible to purchase a gas boiler, and upgrading to an A-rated model from an older, G-rated version, will help you to reduce your energy bill and household carbon emissions. 

An greener alternative is a heat pump. They’re considered the gold standard when it comes to heating in an energy-efficient way. Instead of using gas, heat pumps use electricity to draw heat from outside and into your home.  

What is an energy-efficient boiler? 

An ‘energy-efficient boiler’ is a catch-all term for a heating system that runs more efficiently than the system that was installed before it. Because so much of a home’s energy usage goes into powering a boiler, having (or upgrading to) an energy-efficient version can make a big difference to your energy bill.  

Boilers are given a rating from A to G, and the grading system works in a similar way to the rating you’ll find on your Energy Performance Certificate. An ‘A’ grade is awarded to the most energy-efficient boilers – so the higher your boiler’s rating, the less energy it will use. 

G-rated boilers tend to be older models, with almost all modern boilers installed after around 2007 falling into the ‘A’ category. A-rated boilers are at least 90% efficient and you can expect them to last for around 15 years. G-rated boilers are usually 70% efficient, or lower.  

What makes older boilers score low on energy efficiency? 

When older boilers burn fuel, they lose heat. Hot gas escapes via the flue (the pipe used to take away fumes, and supply fresh air to the boiler). But modern condensing boilers recover more of this heat from the exhaust flue gas, and use it to heat water for central heating. 

If you’re unsure whether you already have a condensing boiler, there are two key features to look out for: 

• A plastic flue. If it’s made of metal, it’s unlikely to be a condensing boiler 
• A plastic pipe coming out of the bottom of the unit, through a wall, and into a drain 

If your boiler has the above features, you can be almost certain it is a condensing boiler, and therefore among the most energy-efficient. 

How much does a new boiler cost? 

It varies, depending on the type of boiler, your current setup, and the complexity of the installation job. The Energy Saving Trust estimates that the average cost for a routine gas boiler replacement is around £3,700. This should also include the installation of thermostatic radiator valves, which allow you to adjust the temperature of each radiator, rather than having all rooms at the same temperature. 

For oil-fired boilers, the same straightforward replacement will typically cost in the region of £5,500. 

There aren’t any grants available for the installation of new gas boilers as fossil fuel heating is being phased out. But there is a government heat pump grant of £7,500, also known as the Boiler Upgrade Scheme, that’s available to all households, including rented homes.  

Will an energy-efficient boiler reduce my energy bills? 

There is an upfront cost of replacing your boiler, but there are also long-term energy and cost savings. Energy Saving Trust has a range of estimates depending on the type of home and boiler being replaced.  

Potential energy bill savings: replacing a G-rated gas boiler with an A-rated condensing boiler  

Property type	Detached bungalow	Detached house	Flat (mid floor)	Semi-detached house	Mid-terrace house
Annual saving	£285	£475	£120	£310	£260

Savings will vary depending on the size and thermal performance of your home. 

Smaller properties, including semi-detached houses, mid-terrace houses and flats will take longer to recoup the upfront cost, partly because they are naturally insulated by other homes, and require less energy to heat. 

If you’re thinking of replacing your boiler, it could be worth looking at other measures to improve energy efficiency at the same time, such as adding or improving insulation, to maximise long-term savings. You can take a look at more energy-saving steps here. 

Are energy-efficient boilers carbon-free? 

No. Although they could lead to significant energy and cost savings, they still run on fossil fuels, and emit carbon dioxide when they’re in use. So if you’re looking to move to a completely carbon-free heating system, you’ll need to look for an alternative, such as a heat pump. 

A heat pump is the most sustainable carbon-free alternative. A heat pump is not technically a boiler, as they also heat water in a home’s heating system, and use electricity rather than gas. 

The government is also providing heat pump grants. This is being carried out through the Boiler Upgrade Scheme (BUS), which is set to run until 2028. 

The scheme provides grants of £7,500 towards the cost of a heat pump, which can be claimed against either an air source or a ground source version. 

What is the best boiler to install in my home? 

If you’re not in a position to move to a carbon-free heating system right now, but are looking to move to a more energy-efficient heating system, there are still a variety of boilers that can help to reduce your energy consumption, and cut bills. 

Opt for a modern condensing boiler and consider whether a ‘system’ or ‘combi’ boiler is best suited to heating your home. 

System boilers use less energy to heat hot water compared to combi boilers, but are prone to losing heat from the hot water cylinder over time. Combi boilers may be more efficient, but this depends on a range of factors. 

To help you decide which type of boiler suits your home the best, think about your hot water usage. Large households that use lots of hot water may be better off with a system boiler and water cylinder, with smaller households likely to be better suited to combi boilers, as they’ll be able to provide hot water on demand. 

Where you put your boiler is another thing to consider. Because they don’t require water cylinders, combi boilers are better choices for homes with limited installation space. 

If you’re considering installing solar panels, you could link them up with a system boiler – this is because the water cylinder stores the energy produced by the solar panels. Most heat pumps also work with a hot water cylinder.  

Are gas boilers being phased out?  

The government has committed to reducing carbon emissions, with the hope of reaching net zero by 2050. So the intention is to phase out the installation of new gas boilers from 2035.  

Previously, the phase-out was due to begin in 2025, but this has been extended in the hope that over time, the cost of low-carbon alternative heating systems will have come down. Ultimately, costs continue to be a barrier for people thinking of switching to a heat pump.

The government says no one will be forced to remove their existing gas boiler. The ‘boiler ban’, as it’s often referred to, will only apply to the installation of new gas boilers.  

How can I tell if my existing boiler is an A-rated boiler?  

Most boilers should have visible stickers on their casing. For older models without a sticker, check the manual or the model number by looking under the boiler for a brand name and number. You can then go online to the PCDC database to find the rating. 

If you already have an A-rated boiler and your energy bills are still high (you can check what the average household energy bill is here), it may be worth considering other energy-efficiency improvements in your home, such as insulation. 

READ MORE: What’s the average UK energy bill and how can I save money?