How efficient is my gas boiler?

Find out what the energy efficiency of your boiler means, how it’s measured, and some of the different factors that can affect how efficient your boiler really is.
james at job

What does a boiler’s energy efficiency mean?

You rely on your boiler for hot water all year round and especially in the colder months for heating.

Doing all that takes a LOT of energy – over half the average home’s total energy use, in fact. So the efficiency of your boiler has a big impact on your finances as well as your carbon footprint.

A boiler’s efficiency is the amount of energy that actually goes into heating your home, rather than being lost or wasted.

If your boiler is 90% efficient, only 10% of the energy is lost.
But if your boiler’s efficiency is 70%, you’ll lose 30%.

What is ErP?

ErP stands for ‘Energy-related Products’. It’s the European rating system for boilers and other heating appliances.
Every new boiler has an ErP energy label, which ranks its efficiency from A-G. By law, all modern gas boilers in the UK are A-rated, which means they’re highly efficient.

Every new boiler has an ErP energy label, which ranks its efficiency
from A-G. By law, all modern gas boilers in the UK are A-rated, which
means they’re highly efficient.

What is SEDBUK?

There are two different versions of the SEDBUK rating system, which stands for ‘Seasonal Efficiency of a Domestic Boiler in the UK’.

SEDBUK 2009: The 2009 system gives each boiler an exact percentage score. This is what to look for if you really want to get into the detail, but the difference of a few percentage points won’t have a massive impact on your bill. The big savings come from swapping an old boiler for a new one.

SEDBUK 2005: This is the old system still used by lots of manufacturers. Like ErP, it rates boilers from A to G. A-rated boilers score over 90% for efficiency, while G-rated boilers come in under 70%.


At iSafe Gas, we take pride in delivering a world-class service that aligns with the values upheld by Which?, the renowned consumer organization. As an approved Trusted Trader, we prioritize your safety above all else. Our accreditation signifies our commitment to maintaining the highest standards of professionalism, expertise, and customer care.

Why Choose us?

You can rest easy knowing that your boiler, service & repair needs are in safe hands with iSafe gas. We offer a wide range of benefits to ensure you never have any worries about being left out in the cold!

Fixed Price Guarantee

Highly Qualified And Accredited Engineers

Highly Rated Reviews

Free in home survey to get the boiler to suit your needs

Leading boiler manufacturers

0% finance available

Boiler Aftercare Provided

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Watch how we ensure you're getting the right boiler for your home.

Recent Installs

We install boilers from the leading manufacturers including Vaillant, Worcester Bosch, Viessmann, Alpha & Ideal.

Follow our sales manager James as he accompanies our engineer on a recent new boiler fitting for one of our customers.

Hear from one of our happy customers who had a brand new boiler fitted.


Why are new boilers more energy efficient?

In the UK, a 2007 law made it compulsory for all new boilers to be ‘condensing’ boilers. This is a modern design that’s much more efficient than the old style of boiler.
Condensing boilers use the warmth from the hot gases expelled in the flue, rather than just letting all that heat go to waste. A condensing boiler will always be at least 90% efficient, or even more.
So, if you’re wondering how efficient your home’s boiler is, the answer is probably ‘very’ – but only if it was bought and installed recently.

Why should you upgrade an old boiler?

Older boilers are much less efficient than modern condensing boilers.

The energy efficiency of an older boiler can be as low as 60% – which is a G rating. With a G-rated boiler, 40% of the energy you pay for is going to waste rather than heating your home.

That means upgrading an old boiler to a newer, more efficient model is a great way of saving money.

And because more energy efficiency means lower CO2 emissions, it’s good for the planet too.

How much can I save with a new boiler?

This table shows how much you could save in an average year if you swapped an older boiler for a new A-rated model. As you can see, the type of home you live in also makes a difference.

Source: Energy Saving Trust

Figures are calculated using fuel prices as of April 2022. These are estimated figures based on installing a new A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator controls (TRVs) in a gas heated home from an older boiler with a programmer and room thermostat. Savings will vary depending on the size and thermal performance of your home.

What type of boiler is most energy efficient?

If you’re choosing between a conventional, combi or system boiler, none of them is necessarily more energy-efficient than the others. But picking the right one for your property and family size will mean you’re using it in a more energy efficient way.
For example, if you’re in a small property with a small family, a combi boiler might be your best option. A combi boiler makes hot water on demand, so you won’t be storing hot water that never gets used.
But if you have a large property and a big family, a combi boiler won’t be the most energy-efficient choice. A system or conventional boiler is more likely to meet all your needs while helping to keep the bills as low as possible.

Is boiler size important for energy efficiency?

Yes, choosing the right size boiler for your property and your family has a big impact on energy efficiency.

We will do a full heat loss calculation at our survey to get the correct sized boiler for your home. 

When we talk about boiler size, we’re really talking about its heat output. This is measured in kilowatts (kW).

If your boiler isn’t big enough, it will struggle to provide the heat you need. But if it’s too big for what you actually need, you’ll end up wasting a lot of energy in your heating and hot water.

Looking for a new boiler?

If you need advice on anything to do with boilers, our team at iSafe are always here to help.

You can book a free appointment  and one of our heating advisors to come and look at your boiler and central heating system. They’ll talk you through the best options for your home and give you a fixed price quote.

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Frequently Asked Questions

This depends on how complexed the job is so there is no set time but most appointments should only take 30 minutes to 1 hour.
As long as we can see the vital parts of the system such as the boiler, hot water tank and radiators that should be fine. Our surveyor also has ladders if he needs to go in a loft.
We will always aim to have a job completed within that day. We don’t want to inconvenience you for any longer than necessary so we will always strive to put additional engineers on larger and more complexed jobs. If it’s a full system that’s being replaced you should plan for 2 full days.
This depends on what is involved in the install and what boiler is best for you. We will provide you with a free fixed price quote when we come visit you. This gives you additional peace of mind that your price isn’t going to increase if we hit a snag or if more materials are required on the install.
We fit a variety of boiler brands. This varies based on what’s needed for your home and your budget. All of which will be A rated and have good warranties. We have recently fitted Vaillant, Worcester Bosch, Alpha & Ideal. More importantly we will specify the boilers that’s best for you.
We now offer various ways to pay for your install. We offer 0% over 12 and 24 months. Longer finance periods are available up to 10 years. The finance doesn’t require a deposit. In addition to our finance options you can pay upon completion of the job by bank transfer, debit or credit cards.
It’s more than likely that it will be our sales manager James that will visit you for your new boiler. He has almost 20 years experience in the central heating industry. He will be your main point of contact throughout the full process so his email address and telephone number will be left with you on the visit. His aim is your full satisfaction so he will go above and beyond to make sure you’re happy

Absolutely. We have a full section on the website which will show all of this for you. We are very proud of them as nothing beats a happy customer – Reviews. Word of mouth and recommendations is very important to us so we always ask for as much feedback as possible.

We are a Glasgow based company but we carry out work all over the central belt. If it’s within an hours drive of Glasgow we will be used to travelling there so that covers greater Glasgow, Inverclyde, Ayrshire, Lanarkshire and beyond
No, have many contacts that we work with daily. These relationships have been built up over many years so if you need a joiner, roofer, builder or even scaffolding erected to complete your install we will provide the relevant tradesperson to complete your work to the highest standard.