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Upgrading your boiler

How old is your central heating boiler? If it’s more than 10 years old, it may be time to think about replacing it with a new high efficiency model

Today’s central heating boilers are much better than the old ones. They can heat water more efficiently, meaning they use less gas or oil to do the same job. This saves energy and will save you money. In fact, if your boiler is over 10 years old, then it’s probably worth investing in a new one. In most cases the savings you make from a more efficient boiler over the years will pay for the outlay.

Building regulations stipulate that if you are replacing an old boiler the new appliance must be A-rated for energy efficiency. This means it must be at least 88% efficient (most new boilers score over 90%), and to reach this level it will almost certainly be one of the new-style condensing boilers.

The technical bit

Condensing boilers are more efficient because they extract the heat from the exhaust gases (or ‘flue gases’) that all boilers produce. What makes this possible is a second heat exchanger where the water vapour in the flue gas condenses into droplets of liquid water, releasing heat as it does so. The condensed liquid water drains away through a pipe installed specifically for this purpose. Non-condensing boilers simply expel the gases into the air and the heat is wasted.

You’ll notice that the vapour that comes out of a condensing boiler’s flue forms a visible plume of ‘steam’ (actually a mixture of water vapour and other gases). This is perfectly normal and is an indication that the gases are cooler than those vented by non-condensing boilers.

‘Combi’ boilers

Condensing boilers come in both combination and regular models. A combination (or ‘combi’) boiler will provide your central heating and produce hot water on demand, firing up when you turn on a hot tap in the kitchen or bathroom. A regular boiler will do your central heating, but rather than produce hot water as and when required, it will heat a quantity of water which is held in a storage cylinder until required.

Some boilers only run the central heating and don’t provide hot water at all, in which case an electric immersion tank or another form of water heating is required.

In a typical semi-detached property that uses mains gas for heating you can reduce your heating bills by around £300 per year if you upgrade from an old G-rated boiler (less than 70% efficient) to a new A-rated condensing boiler with full heating controls (e.g. timer/programmer, room thermostat and thermostatic radiator valves). The savings are even greater for larger properties and for heating systems that run on oil which is more expensive than gas.

Flue gas heat recovery

A flue gas heat recovery system can make boilers more efficient by capturing and re-using heat energy that would otherwise have escaped out of your flue or chimney. All boilers which burn fuel to produce heat create exhaust gases, and these need to be expelled outside. However, as these gases are hot (about 200°C), as much as 35% of the heat being produced by the boiler can be wasted when they are expelled.

This system also uses the hot flue to pre-heat the cold water being fed into the boiler. This means the boiler does not need to work as hard to provide you with hot water. Flue gas heat recovery systems significantly improve the performance of condensing boilers, with many achieving efficiencies of 90% and above. This means they use less gas (or oil), saving you money on your bills. See the downloadable factsheet on the right for more information.

The video below shows the difference a boiler upgrade can make. Mrs Crane from Minehead described her house as an “ice box”. But through CSE's home energy team she was able to get an award for a new boiler and upgraded heating controls to make her home much more comfortable.

So, what do I do now?

If you decide to go for a new boiler, you should get at least three quotes from qualified heating engineers. These should be on the Gas Safe Register or registered with OFTEC for oil-fired systems as it is illegal and also potentially extremely dangerous for someone who is not fully qualified to fit a central heating boiler.

Both Gas Safe and OFTEC have lists of registered installers which you can access by phone or online. An installer registered with a competent person scheme will also be able to self-certify that the work is compliant with the building regulations:

Gas Safe Register is the official list of gas engineers who are registered to work safely and legally on gas appliances. Always check your engineer is on the Gas Safe Register.
0800 408 5500 |

OFTEC represents the interests of homeowners, registered technicians and trade association members, providing advice and information on oil fired heating and cooking.
0845 65 85 080 |

Competent Persons Schemes were introduced to allow individuals and enterprises to self-certify that their work complies with the Building Regulations as an alternative to submitting a building notice or using an approved inspector.

Your new boiler should have a power-flush (or a mains pressure flush for some models) to remove sludge and other deposits from the system which could damage the new boiler. The installer may also add lime scale inhibitors or water softeners to prevent the build up of lime scale. The boiler should be serviced annually to ensure that it stays in good working order and maintains its efficiency.

Plan for replacement

No one wants to go without heating and hot water in the winter, so it makes sense to plan the replacement of your boiler rather than waiting for it to break down. It also means that you can take your time to shop around for quotes rather than having to get an emergency replacement in a rush. And remember, heating engineers tend to be busier in the winter so if your boiler breaks down then, you may have a long wait before someone can come to your home to install a new one.

But before you spend money on a heating system, you should make sure that your home is well insulated. This is because an insulated home keeps the heat in better, so you may be able to meet your heating needs with a smaller (and cheaper) boiler. Measures like loft and cavity wall insulation are extremely cost effective and can pay for themselves in just a few years through savings on your heating bills.

If you live in Bristol or Somerset, we can help you find out if financial assistance is available to help you replace your boiler. Call free on 0800 082 2234 or email

Frequently asked questions

Why are condensing boiler more energy efficient?

Condensing boilers are more energy efficient as they reduce the amount of heat that is wasted through the flue gases. Older boilers release exhaust gases through the flue at a very high temperature and this heat is wasted. However, condensing boilers have a larger or secondary heat exchanger and recover and use the heat from the exhaust gases.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

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Do I have to install a condensing boiler?

Condensing boilers are now specified by the building regulations for new gas and oil fired central heating systems, or if you are replacing your old boiler. This is because they greatly reduce carbon emissions produced by the heating system. They also reduce the amount of fuel need to keep the property warm and therefore give a significant reduction in fuel bills. However if there is no suitable external wall for the flue of the new boiler then it is possible for your heating engineer to apply for an exemption.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234

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Will a condensing boiler be compatible with my existing heating system?

Condensing boilers can be fitted to most new and old systems. Your system may need to have a power-flush to remove sediment from the sysytem that could damage the new boiler. The heating engineer should size the boiler appropriately to meet the heat demand of your home wihtout over-sizing it so that it became inefficient and unnecessarily costly to run.

Need more help?

We can advise you about saving energy, or help you understand what grants and support you're eligible for:

Contact us Or freephone: 0800 082 2234

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Download the PDF

Download this PDF document

This information is available as a freely downloadable PDF from this page.

For more domestic energy advice, view all our advice pages.

Flue gas heat recovery

This system can make boilers more efficient by capturing and re-using heat.

Read this leaflet to find out more.

Warm air units

Some houses are heated by warm air heating systems. If you have this system, you might be able to upgrade to a better unit.

Read this leaflet to find out more.

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