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The most common way to power central heating within your home is through a combi (or combination) boiler. A combi boiler system takes up very little space in your home compared to conventional boilers, without a bulky storage tank. Because combination boilers receive their supply of cold water directly from the mains, making them easier to install than traditional boilers. 

Combi boilers are extremely economical as they supply hot water on demand, which means that homeowners only pay for what they use. The best combi boilers are up to 90% energy-efficient and affordable. And if you pick the right company, you shouldn’t have to worry about buying a new boiler for at least 10-20 years. Energy efficiency and cost savings are top of mind for many households these days. Upgrading your boiler – along with double glazing, roof and floor insulation, and using energy-efficient light bulbs – to make your home as energy efficient as possible, can reduce your energy bills dramatically.

To help customers make a decision about their next combi boiler purchase, we’ve conducted hours of in-depth industry research into hundreds of combi boilers, including analysing more than ten thousand customer reviews from trusted third-party sites like Trustpilot, Which.co.uk and The Boiler Guide, studying the marketing material of the companies, and evaluating various independent data sources to help us identify the top combi boilers in the industry.

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What are the best combi boilers?

  • Best value for small homes: Worcester Bosch Combi Greenster 4000
  • Most powerful: Viessman Vitodens 111-W B1LD-35 Storage Combi Boiler
  • Most energy efficient: ​​Vaillant ecoTEC exclusive Green iQ 835 Combi
  • Most affordable: Ideal Logic+ Combination Boiler
  • Most flexible installation: Baxi Platinum Combi Boiler

Our recommended installers

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Worcester Bosch Combi Greenstar 4000: best value for small homes

Worcester Bosch Combi Greenstar 4000 in a kitchen

Image credit: Worcester Bosch

  • Guarantee options: 10-year guarantee on parts and labour 
  • Price range: between £1,200 – £2,500
  • Efficiency rating: A
  • Output rating: 25kW – 30kW

Worcester Bosch (sometimes simply called Worcester) is a UK-based heating and hot water product manufacturing company founded in Worcester in 1962. 

Worcester Bosch has a number of combination boilers in its range for larger and smaller homes, as well as outdoor boilers compatible with gas, oil and LPG fuel. All are standard with a 10-year guarantee. According to Worcester Bosch, their most popular boiler type by far is the Greenstar 4000 combi boiler. There are several features that make this dynamic little boiler perfect for small and medium homes, aside from its compact size.

It can run on a 20% hydrogen blend, which means it can efficiently run on gas for its entire lifetime, and it comes with a wide power range that can reduce gas consumption and energy bills. It’s also a solid choice for buyers who want to keep the environment in mind, but don’t want to spend the extra cash on the Vaillant ecoTec range, which we’ll cover later on. It’s energy efficient, with an A-rating, and comes with an innovative Quick Tap water saving function that will cut down on water wastage. It’s also a better buy than the Viessman 111-W for buyers in low water pressure areas, as the Combi Greenstar 4000 performs well and guarantees powerful showers in all conditions. We also love how quiet it is, which is a real benefit if you live in a smaller home. 

The Greenstar 4000 is more compact, quieter and far more attractive than its predecessor and is available in multiple sizes, including the 25kW and 30kW. It’s compatible with all Worcester Bosch controls and third-party Wi-Fi smart controllers (e.g. Hive, Nest), holds an ErP-rating of 94% A-rated, and comes in at a great price. 

Cost

The Worcester Bosch Combi Greenstar 4000’s compact size makes it easy (and cheap) to install without compromising on quality. 

In terms of costs, buyers can expect to pay the following:

Worcester Bosch Combi Greenstar 4000 25kW

Typical Price (incl VAT)

£1,200

Typical Installation Cost (incl VAT)

£500 – £1,000

Sundries (filters, flues)

£300

Total Cost

£2,000 – £2,500

Our recommendation

The Worcester Bosch Greenstar 4000 Combi Boiler is small but packs a powerful punch. It’s not only a consumer favourite but Worcester Bosch has held a Which? Best Buy Award for more than 10 years and counting. It’s not the cheapest boiler, but it sits perfectly between the 2000 and 8000 series in terms of functionality and price. 

The Greenstar 4000 is a low-maintenance and hassle-free combi boiler that comes with a generous guarantee and boasts easy access through removable side panels. As a Worcester Bosch brand, parts are readily available in the UK, and customer service is guaranteed. With dimensions of (h) 724mm x (w) 400mm x (d) 310mm, the 25kW combi boiler has a flow rate of nine litres per minute, making it suitable for flats and small properties. The 4000 range can also be employed as a system boiler, and used to heat multiple bathrooms and water outlets. 

The Greenstar 4000’s exterior is also sleek, featuring a curved-edge casing and a digital display that uses physical push control buttons. It comes complete with wireless connectivity, which enables you to connect your boiler to the Bosch EasyControl system (sold separately) to control the boiler remotely from your smart device. At a retail price starting at £1,200, this boiler offers real value for money with enough power to heat your small house or flat while fitting neatly into a small cupboard. It’s quieter, sleeker and more efficient than the Ideal Logic+ or Baxi Platinum and performs really well considering the price tag. 

Coupled with Worcester Bosch’s stellar customer ratings, lengthy warranty and price-match options, this is the best value for money for small homes by far. 

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Viessmann 111-W Storage Combi Boiler: Most powerful

Viessmann 111-W Storage Combi Boiler product photo

Image credit: Viessmann

  • Guarantee options:  10-Year on parts and labour 
  • Price range: £1,575 – £3,000 (excluding installation)
  • Efficiency rating: A
  • Output rating:  26kW – 35kW

Viessman started as a family locksmithing business in 1917 – but things have changed. Today, the brand is synonymous with high-quality, German-engineered boilers, and their popularity and acclaim in the UK has skyrocketed in recent years. 

If you need a real powerhouse of a boiler at a fair price, the Viessman Vitodens 111-W is a robust, high-capacity storage combi hybrid that can meet the toughest hot water demands of any modern home without purchasing a separate hot water storage cylinder. Viessman Vitodens 111-W might still be much bigger than the likes of the Worcester Bosch Greenstar 4000, but it’s still a significant space saver. With a flow rate of 18 litres per minute, it’s suitable for most spaces with slightly higher than average hot water needs, filling a bath with hot water in eight minutes while still supplying a washbasin or sink at the same time. 

At (h) 950mm x (w) 600mm x (d) 500mm , it’s not quite compact, but it’s still small enough to fit unobtrusively in a cupboard or storage space. The Viessman Vitodens 111-W has extremely positive reviews on the likes of Trustpilot and other consumer review sites, with an average rating of 4.5 / 5 stars. 

The Viessman Vitodens 111-W is available in either a 25kW or 32kW option, which is slightly larger and more suitable for larger homes with multiple bathrooms and radiators. Its 46-litre integrated water storage cylinder heats water separately, which ensures that pressure and water supply remain constant during usage. It’s also compatible with smart thermostats like Hive, tado and Nest, which means that you can control your boiler remotely via your phone. 

Cost

Viessman offers extremely competitive pricing on their boilers, considering the quality and engineering. Most homeowners should expect to pay upwards of £3,000 for their Viessman Vitodens 111-W combi boiler, but a conversion could cost as much as £5,000. It’s best to obtain a thorough quote before making a decision.

Viessmann Vitodens 111-W B1LD-35 storage combi gas boiler

Typical Price (incl VAT)

£1,700 – £1,800

Typical Installation Cost (incl VAT)

£500 – £1,000

Sundries (filters, flues)

£300

Total Cost

£2,500 – £3,000

Our recommendation

The Vitodens 111-W is a great solution for homeowners that need more power than a conventional, compact combi boiler can offer. Despite being more powerful, the boiler is highly efficient, which can decrease your energy bills dramatically (especially if you are replacing an older model). 

The 35kW model can meet the heating demands of up to 15 radiators, with a domestic hot water output of 34kW, whereas the lower heating output model can still reach a hot water output of 29.6kW. If your property has a high demand for hot water but limited space, this combi boiler is your best bet. 

The warranty on the boiler is a 5-year standard warranty, extended to 10 years if installed by a Viessmann Trained Installer. It’s highly reliable, with a modulating ratio of 1:6 lower energy consumption, constant temperature control and even weather compensated operation (if installed with outside temperature sensors). 

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Vaillant ​​ecoTEC exclusive Green iQ 835 Combi: Most energy efficient

Vaillant ​​ecoTEC exclusive Green iQ 835 Combi installed in a kitchen

Image credit: Vaillant

  • Guarantee options: 5-year standard, 10-year if installed through improved installer 
  • Price range: From £1,525 (excl installation)
  • Efficiency rating: A rated with 94% efficiency
  • Output rating:  26.4kW to ​​35.3 kW

Vaillant is the second-largest boiler manufacturer in Europe. It was founded in Germany in 1874 and is well-known for its environmentally friendly, innovative designs. The company produces combi boilers, system boilers for larger or older homes, and regular open vent boilers. Vaillant boilers are often compared to UK market leader Worcester Bosch, but they have a slight edge when it comes to eco-friendly advances. 

The ecoTec Boiler range was the first to be built using 85% recyclable, environmentally friendly materials. It’s a premium, reliable, high-performance combination boiler that can heat large properties with up to 20 radiators with ease. 

The Vaillant ​​ecoTEC Green iQ 835 Combi boiler also offers a fair 14.5 litre per minute flow rate, but there is a caveat that your home needs adequate water pressure to work optimally, so homeowners should get their pressure tested before choosing this model. 

The output is significant for a combi boiler at 35kW, and there is a 43kW version for homes with even higher hot water demands (although it is much less compact). It’s most similar to the Worcester Bosch Greenstar 34CDi Classic, but the central output is higher, and it’s much more eco-friendly. 

Cost

The Vaillant ecoTEC Exclusive Green IQ 835 Combi Boiler is reasonably priced, although installation costs have to be taken into account before making a purchasing decision. Most buyers can expect to pay less than £3,000 for the boiler and installation. 

Vaillant ecoTEC Exclusive Green IQ 835 combi gas boiler

Typical Price (incl VAT)

£1,525

Typical Installation Cost (incl VAT)

£500 – £1,000

Sundries (filters, flues)

£300

Total Cost

£2,225 – £2,725

Our recommendation

The Vaillant ecoTEC exclusive Green iQ 835 Combi is a great option for homeowners with larger homes and higher hot water demand that still want to prioritise eco-friendliness and energy efficiency. 

The green IQ technology and 94% efficiency rating, ErP A-rating for heat and hot water, and its extra-condense stainless steel heat exchanger make this a green and sustainable option for eco-conscious consumers. Other eco-friendly options include weather compensation features that alter the water temperature based on the weather outside. 

The load compensation feature alters the temperature of the hot water produced according to the temperature of the room, while the smart controls optimise and automate your central heating system. These features ensure that boilers are always working at peak performance, with programs that include a holiday program to keep your heating and hot water turned off while you are away from home and flexible time programs. 

The Vaillant ecoTEC 835 combi boiler is compatible with the Vaillant vSMART smart thermostat and mobile app, which uses machine learning to adapt to your home heating preferences and manage your needs with even greater efficiency. Like most combi boilers, the Vaillant ecoTEC 835 is more compact than regular boilers, but it’s not as small as some of the other combi boilers on the market. 

At 720mm (h) x 440mm (w) x 406mm (d), it’s too big to fit in a standard kitchen cupboard but can fit neatly into a utility room. (The ecoFIT pure combi in the same range is much lighter and more compact and also comes with a 35kW output. The flow rate is much lower, however). 

The Vaillant ecoTEC 835 combi boiler comes standard with a five-year warranty, but if you use an approved Vaillant installer, this can increase to a 10-year warranty. It offers great hot water output, flow and central heating without environmental trade-offs associated with other boilers of its size and output. The reviews we’ve studied during our research show that it’s popular with larger homes with multiple occupants and bathrooms and that service levels are excellent. 

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Ideal Logic+ Combination Boiler: Most affordable

Ideal Logic+ Combination Boiler product photo

Image credit: Ideal

  • Guarantee options: 2-7 Year Standard Guarantee
  • Price range: Between £848 – £1,011
  • Efficiency rating: A
  • Output rating: 30.3kW

Sometimes the only consideration when it comes to buying a boiler is the price, but cheap doesn’t have to mean poor quality. Even if you are buying the Ideal Logic+ combi boiler as a budget grudge purchase, you can rest assured that you’ll get better-than-expected value. 

The Ideal Logic+ Combi is manufactured in the UK, holds a Queen’s Award and a Good Housekeeping award, and has extremely favourable customer reviews online, despite a much lower than average price tag. You definitely get more than you pay for when you choose an Ideal Logic+ boiler. 

Even the installation is much cheaper than the norm. Of course, there are some trade-offs to going the cheaper route, primarily a lower warranty period. Ideal Logic+ does come with a lower warranty, but they are also market leaders in terms of technical details and performance. 

If you’re looking for a sturdy entry-level boiler, you could do worse than the Logic 24kW boiler, which has a flow rate of 9.9 litres per minute – perfect for a flat or smaller house. Installation is simple, and it comes with a standard control panel that remains in full view at all times – which makes it easy to spot faults within the boiler if they crop up. 

The Ideal Logic+ Combi has several useful features that can cut down your energy bills as well, including the Ideal Weather Compensation kit and digital thermostats. It also holds a Quiet Mark accreditation, so you don’t need to worry about a noisy boiler disturbing you in the mornings. 

Cost

The Ideal Logic+ Combi Boiler series is extremely affordable. Even the more premium models in the range (eg the 38kW Logic Code Combi) don’t exceed the £1,500-mark. 

Ideal Logic+ 24 combi boiler

Typical Price (incl VAT)

£848 – £1,011

Typical Installation Cost (incl VAT)

£500 – £1,000

Sundries (filters, flues)

£300

Total Cost

£1,648+

Our recommendation 

The Ideal Logic+ 24kW combi boilers provide good value for money. Ideal Logic is the standard range. It’s cheaper and comes with a 2-year warranty. 

From this range, we would strongly recommend opting for the Ideal Logic+ combi boiler instead as it comes with an extended 7-year warranty and additional features, despite a relatively small £200 price difference. (Think of it as paying £40 per year for the warranty). But whichever combi boiler you choose, there is a lot to like about the Ideal Logic combi boiler range. 

Its compact size at (h) 700mm x (w) 395mm x (d) 278 mm makes it a breeze squeezing the unit into a standard kitchen cupboard, and the Logic+ range’s seven-year warranty, which covers parts and labour, is comparable to that of some of the more premium brands on the market. It’s also energy-efficient, with a 94% A-grade ErP rating. The Logic+ 24 perfectly demonstrates that Ideal can produce high-quality boilers at low prices. That being said, it’s not an especially powerful combi boiler, but it would suffice for properties with 1-3 bedrooms and a single bathroom. 

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Baxi Platinum Combi Boiler: Most flexible installation

Baxi Platinum Combi Boiler product photo

Image credit: Baxi

Image credit: Baxi

  • Guarantee options: 10-year guarantee
  • Price range: Average £850-£1,250
  • Efficiency rating: A
  • Output rating: 24kW- 40kW

Baxi is part of the BDR Thermea Group in the Netherlands but has been manufacturing in the UK since 1866 in places like Preston, Norwich and Milton Keynes. It’s active in more than 70 countries around the world, employing more than 6,000 people and with annual sales of €1.7 billion. Baxi has also acquired many of the best-known brands in the United Kingdom and Ireland, including Potterton, Main Heating, Megaflo and others.

In terms of quality, Baxi is comparable to Worcester Bosch boilers but with even better flue options (and better pricing). Baxi boilers come with a variety of parts and labour warranties. The Baxi 200 Combi has a 3-year warranty, for example, whereas the Platinum and 800 Combi range has a 10-year warranty. Many of the boilers in the range are also Energy Saving Trust endorsed. 

Cost

Baxi Platinum is an affordable brand. A typical Baxi Platinum combi boiler will cost an average of £200-£300 less than a Worcester Bosch equivalent (if materials like flue kits, control systems, and system filters are included in the installation cost). Prices start at £850 for the 24kW unit and go up to £1,250 for the 40kW boiler.

Baxi Platinum combi range

Typical Price (incl VAT)

£850 – £1,250

Typical Installation Cost (incl VAT)

£300 – £1,000

Sundries (filters, flues)

£300

Total Cost

£1,450 – £2,550

Our recommendation

We recommend the Baxi Platinum Combi Gas boiler. The 40kW model has an impressive 16.4 litre per minute flow, high outputs, impressive warranties and a fairly low price compared to others in the market (from £1,250). It can power homes with up to 15 radiators and two showers simultaneously. 

Baxi Platinum Combi Gas boilers are suitable for a number of different households, with good flow rates, energy efficiency, and reliable components. Unfortunately, the Platinum range was designed with heat output in mind, not size, which makes it slightly bulky if you were hoping to place it in your kitchen. For smaller properties, the Baxi 624 or 630 are suitable alternatives. 

Baxi’s Platinum range comes equipped with a frost thermostat, which means you have greater installation flexibility. It can be installed in low-temperature environments, including garages, lofts and other parts of the house with subzero temperatures without the risk of the water inside the hydraulics system freezing. (You do need to ensure that the space has sufficient side clearance to fit the boiler and that you are adhering to Building Regulations requirements). Baxi compares well to some of the other brands on this list in terms of pricing and output, and it definitely has an edge over the others when it comes to versatility. For straightforward installations, the Worcester Bosch Greenstar is a better bet, however, as they offer a lot more features for the price tag. 

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How to find the best combi boilers 

Choosing the best combi boiler can be confusing. The most important factor to consider will be the size of your home and your consumption needs. Large or older homes may require a system boiler (or regular boiler), but most homes can get away with installing a much more compact and affordable combi boiler. These are the factors that need to be taken into consideration:

1. Heating output

Combi boilers are rated by their kilowatt (kW) rating or heating output. As a general rule of thumb, you need at least 2kW per radiator; e.g. a house with 10 radiators will be able to use a 24kW combi boiler. 

However, the lower the heating out, the fewer litres that can be heated per minute. If you have several bathrooms and several people living on the property, you may want to buy a combi boiler with a higher output to avoid issues down the line. 

A four-bedroom house with more than three bathrooms and up to 20 radiators, for example, shouldn’t settle for a combi boiler with a wattage lower than 35 or even higher. If you live in a multi-storey home, you may want to consider a more powerful boiler – the longer the pipework between the tap and the boiler, the longer you’ll have to wait for hot water. 

2. The number of occupants

The number of bedrooms and occupants can have an impact on the performance of your boiler. The larger the home, the greater the demand placed on your central heating system. 

If several occupants share multiple bathrooms, you will need a bigger boiler as the likelihood of simultaneous hot water demands (e.g. bathing) is much higher. If you have more than one bath or shower in the home, using them simultaneously will be tough with a smaller combi boiler. 

3. Type of fuel

Most households in the UK use mains or natural gas, but a few isolated buildings aren’t connected to the gas network and use LPG bottles or oil. This may influence the type of gas boiler you can use. LPG has lower carbon emissions and is the most efficient fuel type. 

4. Energy efficiency 

Environmentally-conscious consumers must consider the energy efficiency of their boiler as it can account for more than half of the emissions generated by the home. If your boiler isn’t efficient, it wastes energy and drives up the heating costs of your home. An A-rated, high-efficiency boiler can have hundreds of pounds a year, especially if you opt for a boiler with a good heating control system (which allows you to choose which rooms are heated and when). This can prevent you from wasting energy heating unused rooms. 

5. Cost

Combi boilers are far more affordable than regular boilers, but they aren’t cheap. The cost of the boiler can be as low as £575 but always take installation cost (and the cost of venting, accessories and components) into account. These costs are often as high (or higher) than the cost of the boiler itself. 

6. Warranty

Most boilers are built to last at least 10-15 years. A faulty boiler is expensive to repair or replace, and any disruption to your central heating and hot water supply is unpleasant to deal with, especially in winter. Look for lengthy and comprehensive warranties when you make a purchasing decision. That way, if anything goes wrong due to a faulty boiler or shoddy workmanship, you have some recourse to resolve the issue quickly and affordably. 

7. Experience 

The longer a company has been in business, the better. Several years (or even better, decades) of experience shows that the company has the longevity and experience to install and manufacture a quality boiler that will give you years of use. Most of the companies we reviewed have at least 20 years of experience or more backing their business. 


Why should you install a combi boiler?

1. Combi boilers are more compact

Compared to conventional boilers, combination boilers are very small. There’s no need to store hot water (although hybrid models exist), which saves space and decreases the risk of your pipes freezing during winter. The average combi boiler can fit easily into an airing cupboard or kitchen, freeing up your roof space. Their size makes them much easier to install and maintain, as well.

2. Cost savings

Combi boilers are highly efficient. Most of the combi boilers listed in this article are more than 90% A+ efficient, making it extremely affordable to heat your home. Traditional boilers heat vast quantities of water, which means that they expend a huge amount of energy, whether you are taking a shower or simply washing your hands. Combi boilers have to heat much smaller quantities of water. Most modern combi boilers are also fitted with smart temperature controls that allow you to choose which rooms to heat and when. 

3. Fast access to hot water

Combi-boilers heat water according to demand – there’s no need to wait for the water to be heated. The experience of using a combi boiler (and queuing to use the bathroom) is much more pleasant because there’s no risk of running out of hot water. Some of the more powerful combi boilers on the market can heat homes with several bathrooms and occupants, solving hot water issues for good. 


Conclusion

Choosing a combi boiler for your home shouldn’t be a guessing game. We’ve spent hundreds of hours gathering research to determine the best combi boilers on the market according to consumer needs.

Market leader Worcester Bosch has a number of excellent, reliable combination boilers to choose from, at fair prices, with long guarantees. The company has a stellar reputation, good customer ratings, and a wide range of products to choose from. It certainly has the most attractive compact boiler for smaller homes, notably the Worcester Bosch Combi Greenster 4000. While Worcester Bosch doesn’t have the cheapest products on the market, it offers true value for money, and buyers will have years of hassle-free enjoyment using their Worcester Bosch boiler. The outputs and waterflow are great for a small boiler, and yet the model remains highly energy efficient. 

For those with larger homes, multiple occupants and several bathrooms that will be used simultaneously, the Viessman Vitodens 111-W Storage Combi Boiler is a better option. This hybrid combi boiler comes with its own separate hot water storage, powerful wattage and water flow, and an impressive energy efficiency rating. These German-engineered products are known to be high-quality, backed by lengthy warranties. 

Consumers that are concerned with their carbon footprint and energy efficiencies would do well to consider the Vaillant ecoTec exclusive Green iQ Combi Boiler. Vaillant has focused on environmental friendliness in both design, features and the materials used. Homeowners who choose this brand can rest assured that their boiler will be durable, silent, economical and nearly full recyclable at the end of its lifecycle. 

If affordability is the primary driver, the Ideal Logic+ Combination Boiler offers great quality, a solid 7-year warranty and a low price for the cost-conscious consumer. These boilers are also energy efficient enough to cut down your power bills considerably, extending your cost savings for years to come. And if you need to install your boiler in a small space subject to subzero temperatures, the Baxi Platinum Combi Boiler offers the flexibility you need. 

While every single one of these leading brands offers quality and value for money, it’s important to consult with an expert installer before making a final decision. Even the best-reviewed combi boiler may fall short if the wrong wattage, size or type is selected. 


Frequently Asked Questions

Are combi boilers cheaper to run than regular boilers?

Combination boilers are cheaper to run than regular boilers, particularly if you own a smaller home. Most consumers switch from conventional boilers to combi boilers due to energy savings. The Energy Savings Trust has said that homeowners can save up to 40% on their energy bills by replacing a 10-year-old boiler with a new, A-rated combination boiler, especially considering how badly the efficiency of a boiler can go down over the years. 

Conventional boilers also store a large amount of water in the tank, which cools down. As it heats up all of the water again when it’s in use, it wastes a lot of extra energy, even if you are just washing your hands. Combi boilers have to heat a smaller amount of water and work on demand. Installation is generally cheaper, too, and most modern boilers come with added energy-saving apps and features. 

Can you run several showers at once using a combi boiler?

Whether or not you can run multiple taps and showers at the same time with a combi boiler depends on the flow rate and wattage of your combi boiler. A smaller boiler with a 24kW output will not be able to run two showers simultaneously, but a 40kW and higher unit will be able to (depending on the water pressure of your home). 

If you need to run multiple showers at the same time in the evenings, you may want to consider purchasing a higher output boiler or purchasing two cheaper boilers to carry the extra load. Depending on the type of home, you may even want to consider a boiler system or regular boiler instead. 

What size combi boiler is right for my house?

The size of the boiler you’ll need largely depends on the type of house, number of residents and number of bathrooms and radiators. As a rule of thumb, there should be 2kW of wattage for every radiator in the home. A small flat or house with 10 average-sized radiators and a single bathroom can easily get away with using a 24kW combi boiler. 

A medium home with 15 average radiators and two bathrooms should use a 34kW combi boiler, and large homes should look at 40kW combi boilers or an entirely different boiler system altogether. The fuel type used (gas, oil) may also impact the boiler type used. 

What is the difference between a combi boiler and conventional boiler? 

Conventional boilers have two tanks – a cold-water supply tank and a hot water storage tank. The cold water is heated and then stored in the hot water tank and pumped around the central heating system. This way, the entire house is constantly supplied with hot water. 

On the downside, these systems are complex, harder to maintain, less energy-efficient and harder to install. The tanks require additional storage space, which is why they are best suited to larger homes. 

Combi boilers do not have external tanks but heat water and the home through radiators. They provide instant hot water on demand. The heat exchanger inside the combi boiler transfers the heat from the burning gas inside the boiler to the cold water and delivers it promptly. Control valves direct water through the heating system or to the hot water tap as required. Combi boilers are more compact, faster to install and perfect for small homes with limited storage space. 

Are there disadvantages to a combi boiler?

Combi boilers are not ideally suited to homes with significant water demands, eg if several people turn on the taps in different rooms at the same time. A high-wattage combi boiler can meet the level of demand, but the water pressure may drop as the combi boiler doesn’t have the backup water tank that a conventional boiler has. A hybrid combi boiler can overcome this issue, but it depends on the size of the home and the number of bathrooms used simultaneously. 


Our methodology

This research was conducted and then analysed using a scoring system based on the most important considerations consumers would take into account when choosing a boiler based on their needs. 

Each combi boiler was rated on its ability to deliver against these factors. We also analysed hundreds of independent consumer reviews found on third-party review sites, testimonials, marketing materials and independent journals and media coverage. Each combi boiler was given a score out of 100.

Companies were rated according to:

  • The power outputs of each boiler with a total score of 12; 
  • The energy grade or efficiency of each boiler, with a total score of 12;
  • The sound power of each boiler with a total score of 12;
  • The weight of the boiler with a total score of 12;
  • The guarantee/warranty period for a total score of 5;
  • The years of experience of the boiler manufacturer for a total score of 7 – 10; 
  • The average price of the combi boiler with a total score of 3;
  • The reputation of the company on review sites like Trustpilot, with one point given to each star; 
  • Additional benefits include whether or not hot water storage is included in the boiler (2 points), whether a controller is present (3 points), whether or not the boiler is a ‘smart boiler’ and works with Alexa, Google etc. (2 points) and whether or not 24/7 customer support is available (4 points).