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Upgrading and replacing your boiler

Close up of a modern gas-fired domestic boiler

Is your boiler more than 10 years old? If so, replacing it with a new high-efficiency model will save you energy and money …

Last updated, January 2023

About half of your annual energy costs go towards heating and hot water, so an efficient boiler is important.

Upgrading your boiler could save you £235 each year on heating bills – using April 2024 prices. This is based on a semi-detached home on mains gas, upgrading from a G-rated boiler to an A-rated condensing boiler with full heating controls. The savings are even greater for larger properties and for heating systems that run on oil.

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.

Building regulations specify that if you’re replacing an old boiler the new one must be A-rated for energy efficiency. This will almost certainly be one of the new-style condensing boilers.

What is a condensing boiler?

Condensing boilers are more efficient because they extract the heat from the hot exhaust gases (or ‘flue gases’) that all boilers produce. Non-condensing boilers simply expel the hot 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’. This is actually a mixture of water vapour and other gases. It’s perfectly normal and is an indication that the gases are cooler than those vented by non-condensing boilers.

Combination 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 on demand, 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 this case an electric immersion tank or another form of water heating is required.

What is flue gas heat recovery

If you have a non-condensing boiler, you could fit a flue gas heat recovery system. This is a device that captures and re-uses heat that would otherwise have escaped out of your flue or chimney, making boilers more efficient. As much as 35% of this heat is wasted when the gases are expelled.

This system uses the hot flue gas to pre-heat the cold water being fed into the boiler. This means the boiler doesn’t need to work as hard to provide you with hot water. Flue gas heat recovery systems significantly improve the performance of boilers. This means they use less gas or oil, saving you money on your bills.

Having a new boiler installed

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. It’s illegal and 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.

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.

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

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 boiler 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 wait for it to break down. It also means that you can shop around for quotes rather than having to get an emergency replacement in a rush. Heating engineers are busier in the winter, so if your boiler breaks down then, you may have a long wait before someone can fix it.

But before you spend money on a heating system, make sure your home is well insulated; an insulated home keeps the heat in better so you may be able to get by 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. If you’d like to replace your boiler with a heat pump, take a look at the Boiler Upgrade Scheme.

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