All new and replacement boilers in the UK must, by law, be high-efficiency condensing boilers. These can reduce CO2 emissions by up to 30% and heating bills by 40% compared to old-style conventional boilers.
Condensing boilers are the most efficient available as they convert over 90% of the fuel they use into useful heat, compared to around 60% for an old-style boiler.
A condensing boiler captures some of the heat from the hot gases released from the flue and uses it to heat water returning from your central heating system. It therefore requires less heat from the burner and is more efficient.
You can buy high efficiency condensing boilers in different types. These are:
- Combination or combi boilers, which provide heat for your radiators and domestic hot water on demand. One disadvantage is that you’ll only be able to use hot water from a combi boiler system for one task at a time. For example, one person wouldn’t be able to have a shower while someone else does the washing up using hot running water. Note that many combi boilers are not compatible with solar water heating systems.
- Regular or ‘heat-only’ boilers where the hot water is stored in a dedicated cylinder. You’ll also need a cold-water feed tank in the loft. They are good where several people frequently need to use hot water at the same time, but when the tank runs out you need to wait for the water to heat up again.
- System boilers heat your central heating system directly and produce hot water for your cylinder which usually sits in an airing cupboard. There is no need for a water tank in the loft. They allow hot water to be drawn from lots of taps, but it may not be instantly hot and could run out.
The companies on the score table produce a range of the above.
All condensing boilers have a Seasonal Efficiency of Domestic Boilers in the UK (SEDBUK) rating. The SEDBUK rating is expressed as a percentage. A SEDBUK rating assesses what percentage of the fuel that the boiler consumes is converted into heat.
Since October 2010, only boilers that are at least 88% efficient can be installed in homes.
The SEDBUK rating can be found on the Product Characteristics Database. Annoyingly, it contains obsolete as well as current models so you can’t just use it to search for the most efficient models.
According to the SEDBUK database, Worcester Greenstar was the most efficient brand in this guide. But there are only marginal differences in performance between all the boilers on the market.
EU Energy Label
Additionally, the Energy related Products (ErP) directive came into force in September 2015. ErP has two parts – Ecodesign and Energy Labelling. It means that boilers must meet certain energy efficiency criteria and boilers must display a label showing their energy performance rating from A++ to G.
Nearly all modern gas condensing boilers available in the UK get an A rating on this label so are at least 90% efficient at converting gas into heat (open fires are 30% efficient). The A+ and A++ ratings are usually for the more energy-efficient renewable energy products like heat pumps.
Use the SEDBUK efficiency rating if you are interested in the minutiae of how one A-rated boiler compares to another A-rated boiler.
According to Which?, “you wouldn’t expect to see a difference in your gas bills if you choose a boiler that was 91% efficient over an 88% efficient boiler, or between one A rated boiler and another. The biggest saving comes from making a leap from an old, inefficient boiler to a new one.”
An ‘old’ boiler is classed as one that is 15 years old or more.
According to the Energy Saving Trust: “By replacing your old boiler with a new A-rated condensing boiler and heating controls, you could reduce your home’s carbon emissions and cut your energy bills significantly. In fact, if you upgrade from a G-rated to a new A-rated boiler you could save around £300 and 1,220 kg of CO2 per year.”
Reform of the Energy Label
The European energy label is changing back to an A-G format, saying goodbye to its three pluses (A+, A++, A+++). There are varying timescales for when different product groups will start using the new label. New labels for boilers and heating systems are not likely to come in until 2030.
Stewart Muir from the Energy Saving Trust said: “There is some concern that this will ‘lock in’ fossil fuel condensing boilers until this point, and not allow technologies like heat pumps to break through. A quicker rescale of boilers out of the A class could better drive this transition.”
When the label reverts, gas condensing boilers currently rated as A will probably be downgraded to a C or D, thus leaving the renewable energy heat pumps with the top rating of A. Campaigners argue that this would better incentivise renewable technologies and lead to economies of scale and price reductions.
Score table highlights
There is very little to differentiate the boiler brands with only 4 points between all seven companies.
All of the companies got our worst rating for management of workers’ rights in supply chains. Four of them – Remeha, Groupe Atlantic, Viessmann and Robert Bosch – had inadequate policies whilst Vaillant, Ravenheat and Sime said nothing at all about workers’ rights.
Most companies did better on reporting about their environmental impacts, but none scored best in this category. Of those that scored worst, Ravenheat and Sime did not even mention their environmental impact.
Climate change policies
We looked to see what the companies said about their policies to reduce their climate impact. We checked to see whether they disclosed absolute greenhouse gas (GHG) emissions, not emissions per unit of sales (which rise if sales rise). Of the seven companies, only three had something to say.
Robert Bosch: By 2020, Bosch aims to reduce relative CO2 emissions by 35% over 2007 levels (relative to sales, not absolute). By 2016, the company had already achieved a reduction of 30.67%. It did disclose its absolute emissions of CO2 for 2014-2016 which had increased from 2.5 million metric tons to 3.1million.
Viessmann: For CO2, both absolute emissions and turnover-related emissions had declined between 2005 and 2014: “Turnover-related CO2 emissions have been reduced by more than 50% thanks to efficiency measures and the substitution of fossil fuels with renewable energy sources since 2005.”
Vaillant: It reported that its CO2 emissions had decreased by 25% since 2010 and it was over half way to meeting its 2020 target, which it did not quantify. It did not, however, give a figure for total absolute CO2 or GHG emissions.
It also had a target for reducing the GHG emissions of its products, the only company in this guide to have such a target. Its 2020 target is to reduce the GHG emissions of its products by 15% compared to 2010.
There was no information about GHG emissions from Remeha, Ravenheat, Sime or Groupe Atlantic.
Conflict minerals policies
We rated all the boiler companies for their policy on the sourcing of conflict minerals – tin, tantalum, tungsten and gold – because they use them in their electrical components. There is a risk that revenues generated from the sale of these minerals is being used to fuel conflict in a number of countries in central Africa, most notably the Democratic Republic of Congo. All the boiler companies get a worst rating.
Only Ideal Boilers mentioned conflict minerals in its Modern Slavery statement. It said that: “Collectively they account for approximately 45% of our direct purchases by value.”
Ideal received our worst rating for not having an adequate policy but they at least did better than the following companies which did not even mention conflict minerals – Viessmann, Vaillant, Remeha, Ravenheat, Sime, Worcester Bosch.
Energy Saving Trust endorsed
A number of companies have models that are endorsed by the Energy Saving Trust (EST). EST works with manufacturers, trade associations and stakeholders to agree performance criteria via industry-wide consultation.
A database of endorsed products is known as the EST Register – It currently lists 10 of the brands in this guide:
Ideal, Worcester Bosch, Vaillant, Baxi, Viessmann, Potterton, Sime, Main, Keston, Ravenheat.
We have given these brands a positive half point under Product Sustainability for being EST endorsed.
Which brand to choose?
There are only marginal differences in company performance and the energy efficiency of the products.
On the score table, the differences are largely due to tax avoidance ratings and a middle or worst rating for environmental reporting. Viessmann and Worcester get climate change ratings for involvement in high climate change impact industries – aeroplanes and cars, respectively.
On the other hand, they are two of the three companies who had any climate change policies.
Which? recommended the Worcester Bosch Greenstar boilers and the Vaillant ecoTEC boilers as Best Buys in September 2016 based on reliability and the recommendations of boiler owners and heating engineers.