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Combi Boiler Guide: Functionality, Prices, Best Models, and Alternatives

A combi boiler (short for “combination boiler”) is a boiler that heats water directly from the mains for central heating and hot water taps. A combi boiler does not require water storage tanks and only produces domestic hot water as needed. Combis are the most popular boiler type in the UK, as they tend to be smaller, cheaper, and more energy-efficient than system or heat-only boilers. However, combis struggle when multiple people use hot water simultaneously.

combi boiler
A combi is the UK’s most popular boiler type

A combi boiler works by heating water via two heat exchangers, one for domestic hot water and another for central heating. The heat exchangers are typically made of aluminium or stainless steel and transfer heat from the burner to the central heating water, and separately to the water that serves the taps.

A combi boiler differs from a system or regular boiler because it does not have a hot water storage tank. Both system and regular boilers work with a hot water cylinder, while a regular boiler uses cold water tanks to maintain water pressure. A combi boiler cannot generate water pressure and relies on good water pressure from the mains. However, storage combi boilers incorporate some of the hot water storing characteristics of system boilers and are thus appropriate for homes with high hot water demand.

The vast majority of combination boilers in the UK are fueled by natural gas. However, oil, LPG, and electric combis are available on the market and make a fitting alternative to gas combis for homes with no access to the gas grid.

A new combi boiler costs between £800 – £1,500, and £2,500 – £4,000+ with installation. Converting from a different boiler type to a combi boiler adds £800 – £1,200 on top of expected boiler installation costs, while combi-for-combi swaps are more economical. The price of a new combi varies depending on the installer, your geographic location, and the boiler make and model you choose.

Below is a list of the five best gas combi boilers in 2024.

  • Worcester Bosch Greenstar 4000
  • Worcester Bosch Greenstar 8000 Life/Style
  • Worcester Bosch Greenstar CDi Compact
  • Viessmann Vitodens 050-W
  • Alpha E-Tec

Combi boilers come in various output sizes and are suited to different properties. Small combis have an output between 24kW and 30kW and are well-suited for 1-2 bedroom apartments. Medium and large combi boilers have respective outputs of 35-40 kW and 45-50 kW and are appropriate in properties with more bedrooms and bathrooms. A compact combi boiler is just a smaller version of a combi boiler designed for apartments with limited boiler storage space.

Below is a thorough overview of a combination boiler’s functionality, prices, best available models in the UK, and answers to common questions about this boiler type.

What is a combi boiler?

A combi boiler is a type of boiler that works by heating domestic hot water (DHW) and central heating water separately, on demand. A combi heats DHW in response to building occupants turning on their taps and showers. Combis get their DHW supply directly from the mains, heat it on the spot, and the heated potable water flows to the tap or shower that’s been opened. Calls for heating cause a combi boiler to shift its operation to the central heating system, whereby the central heating water flows through the boiler’s heat exchanger and achieves the temperature needed to adequately heat the radiators. The diagram below illustrates a gas combination boiler’s functions in a home’s heating and DHW systems.

Combi boiler diagram
Combi boiler functionality in a home’s central heating and domestic hot water systems 

How does a combi boiler work?

A combi boiler works by heating both domestic hot water and central heating water on demand. A typical combi boiler relies on two heat exchangers to meet a home’s heating and hot water needs. The heat exchangers are made with a highly conductive metal, like aluminium or stainless steel, that heats up when your combi boiler burns fuel. Below is an explanation of how a combination boiler’s two heat exchangers raise the temperature of domestic and central heating water.

  • Primary heat exchanger: A combi boiler’s primary heat exchanger heats water for the radiators and towel rails. This central heating water flows through the primary heat exchanger in a closed circuit as the heat exchanger maintains its temperature around 60°C.
  • Secondary plate heat exchanger: A combi boiler’s secondary heat exchanger heats the domestic hot water that you get from your taps and showers. Your call for hot water at a tap or shower initiates a process where the combi boiler pauses central heating and uses the diverter valve to channel heating water through the secondary plate heat exchanger. The clean drinking water then travels through the secondary plate heat exchanger through a sealed pipe and picks up heat from the central heating water. The clean tap water never comes in contact with the dirty central heating water during this process – it only absorbs the heat.
Combi boiler heat exchangers
Combi boilers utilise two heat exchangers for DHW and central heating water

What is a combi storage boiler?

A combi storage boiler is a combi boiler that has an internal hot water storage tank. This internal water tank allows the storage combi to meet higher hot water demands, as such boilers are not limited to the mains water supply. However, a combi storage boiler never runs out of DHW because it heats water directly from the mains. In contrast, system and conventional boiler types may temporarily run out of hot water if you deplete their DHW cylinder completely.

A storage combi boiler is a suitable fit if your property has multiple occupants who use hot water simultaneously but do not have the space for a system or conventional boiler.

Combi boiler vs system boiler

The difference between a combi boiler and a system boiler is that a system boiler requires a hot water cylinder to operate, whilst a combi does not. A system boiler works with a hot water cylinder to store DHW ready for use. A combi boiler does not need a hot water cylinder as it heats water directly from the cold mains. System boilers are a better fit than combis for households that consume lots of hot water simultaneously, whilst combis are more suitable for smaller families with modest hot water demand.

Combi boiler vs conventional boiler

The difference between a combi boiler and a conventional boiler (also known as a “heat-only” and a “regular” boiler) is that a conventional boiler works with a hot water cylinder and cold water tanks. In contrast, a combi boiler has no hot or cold water storage facilities. A regular boiler uses its hot water cylinder to store hot water when the household needs it, whilst the cold water tanks are usually located in the loft and help maintain water pressure for both central heating and domestic hot water. Conventional boilers are more suitable than combis in large homes where the household often runs more than one hot water fixture at the same time. Combis are more appropriate for homes with less floor area and lower DHW needs.

How do I know if I have a combi boiler?

You know you have a combi boiler if the following four statements describe your boiler and heating setup.

  • There is no external pump: Combi boilers’ pumps are located inside the boiler housing, so an external pump is evidence you have a system or heat-only boiler.
  • There is no hot water cylinder: Combi boilers do not work with hot water cylinders. Look inside your airing cupboard, and if you find a hot water cylinder, you have a heat-only or system boiler, not a combi.
  • There are no cold water storage tanks: A combi boiler does not use a cold water storage tank. You have a regular boiler if you find cold water tanks, which are usually in your attic.
  • Seven pipes out of your boiler: A combi boiler typically has seven pipes coming out of the housing. These pipes include the gas supply, flue, mains water supply, DHW outlet, central heating flow and return pipes, and pressure relief pipe. A boiler with fewer pipes exiting the housing is likely of the system or heat-only type.

Are combi boilers gas or electric?

Combi boilers may be gas or electric. Electric combi boilers are considered to be more environmentally friendly than their gas-fired counterparts. However, it’s important to remember that a large portion of electricity in the UK is generated by burning fossil fuels. Whether electric combi boilers are more environmentally friendly than gas combi boilers depends on whether you have a green energy source.

Electric combi boilers are a suitable alternative to gas combis for homes off the gas grid and those with sustainable power generation methods, such as Photovoltaic panels. Oil and Liquefied Petroleum Gas (LPG) are two other fuels commonly used to power combi boilers in areas without access to natural gas.

What kind of fuel does your boiler use?

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Worcester boilers from as little as £1,965.

Can you get electric combi boilers?

Yes, you can get electric combi boilers. There are many more gas combi boilers than electric combi boilers on the market today. However, some heating companies do produce electric boiler models for the UK market. Popular examples of electric combi boilers include the Comet combi boiler, the Elektra Ek.C., and the Mattira MAC15.

How much electricity does a combi boiler use?

A combi boiler uses 150 – 250 watts of electricity. The main power source for a combi boiler is its fuel – either gas or oil. Electricity is primarily used to ignite the combi boiler’s flame and then to power the boiler’s interface and other electrical components.

However, combi boilers that run on electricity rather than gas or oil typically consume 10-15kW of electricity.

Does a combi boiler use gas when not in use?

No, a combi boiler does not use gas when not in use. Combi boilers only use gas to heat domestic and central heating water in accordance with the user’s commands. Combi boilers do not burn any gas when you aren’t actively using them to heat water.

Is a combi boiler more efficient?

A combi boiler is more efficient than a regular or system boiler in most scenarios. Combi boilers are the most efficient type of boiler for smaller properties because they do not keep a tank of water hot throughout the day. Combi boilers only use the energy needed to heat water as you need it. However, system or regular boilers may be more efficient for larger properties because they’re able to heat (and store) large quantities of water using a lower heating output.

How long do combi boilers last?

Combi boilers last between 7-10 years. Most combi boilers come with warranties of this length, and you should expect your boiler to function well for the duration of your warranty. The expected lifespan of each boiler is different and depends on the following four factors.

  • Installation quality: A high-quality installation carried out by an experienced installer like Heatable prevents many issues that may shorten the life of your boilers. The most common cause of boiler problems is improper installation, which additionally invalidates most warranties.
  • Part quality: Boilers with quality components, such as stainless steel heat exchangers and Grundfos pumps, are known to last a long time. Meanwhile, boilers with aluminium and plastic components, as well as those using third-party components from lesser-known manufacturers typically develop faults earlier in their lifecycle.
  • Manufacturing quality: The quality of a boiler’s manufacturing plays a significant role in its longevity. Viessmann, Vaillant, and Worcester are UK market leaders in quality combi boiler manufacturing.
  • Proper maintenance: Combi boilers need regular maintenance to run properly. This maintenance includes annual service, which is able to detect flaws before they progress into significant problems and shorten the boiler’s lifespan.

How much does a combi boiler cost?

A combi boiler costs between £700 and £3,000, excluding installation. The following three factors have the greatest influence over combi boiler prices.

  • The combi boiler brand: A combi boiler’s manufacturer plays a big role in its sticker price, with premium brands like Worcester and Vaillant charging more than lesser-known brands.
  • The combi boiler features: Some combi boilers have additional features, such as WiFi connectivity or weather compensation, and these add to the boiler price.
  • The combi capacity: Higher output combis support more bedrooms, bathrooms, radiators, and occupants. Higher-output boilers are generally (but not always) more expensive than their lower-output counterparts.
  • The combi boiler warranty: Combis that have longer standard warranties cost more than those whose warranty terms are short.

How much is a combi boiler installation?

A combi boiler installation costs between £800 and £1,500, bringing the total boiler costs to the £1,500-£4,500 range. Combi boiler installation costs depend on the following five factors.

  • Geographic location: Where you live greatly affects combi boiler installation costs. It generally costs the most to fit a boiler in a large metropolis, particularly in London, which has the nation’s highest cost of living. Less expensive regions, like the North, have lower combi boiler installation costs.
  • Scope of work: There is a big difference in the cost of replacing a combi boiler with another combi versus replacing a system boiler or regular boiler with a combi. Changing boiler types involves installing or removing water storage tanks and modifying your plumbing and wiring, which adds to the boiler fitting costs.
  • Changing locations: Combi boiler installation costs go up if you move your boiler to a different location in your home. Moving your combi boiler to a new location involves extra costs like a new flue installation and changes to your electrical wiring.
  • Choice of installer: National installers, such as British Gas, typically charge more than local trades or online installation companies, such as Heatable.
  • Extras: Fitting a combi costs more if you add extras, like a new third-party thermostat, or magnetic filter, or get a system flush during installation.

Combi installation costs depend on many factors, so the best way to get an accurate estimate is to ask for a professional quote. Heatable’s online calculator allows you to get a near-instant fixed-price boiler quote on your screen, and all you need to do is input some (anonymous) data about your property.

Heatable boiler fitting cost

How much to change from a water tank to a combi boiler?

It costs £800 – £1200 to change from a water tank to a combi boiler on top of the cost of the boiler and other installation costs. The total costs of converting from a boiler with a water tank (such as a system or a heat-only boiler) to a combi costs between £2,500 – £4,500. Combi boiler conversions are expensive because they involve removing the existing boiler structure, including all the pipework and water tanks and installing a completely different heating and DHW setup.

One of the most expensive boiler conversions is from a back boiler to a combi. A back boiler is an older boiler that attaches to the back of a residential stove or fireplace so that the stove or fireplace provides hot water and central heating. Conversions from back boilers to combination boilers are difficult (and pricey) because back boilers are located behind the fireplace, and removing them requires a great deal of labour. This extra labour involved in taking out an old back boiler translates into higher costs. For example, Heatable charge an extra £395 for all back boiler-to-combi conversions.

What is the cheapest combi boiler?

The cheapest combi boiler is the Alpha E-Tec at £2,000 – £2,300 including the boiler and installation. Most reliable combi boilers are in the £2,800 – £3,500 range when fitted. The Alpha E-Tec stands out compared to models from other boiler brands because the boiler costs considerably less than its counterparts of equal durability, power, and efficiency from other manufacturers. There may be cheaper combi boilers on the market than the Alpha E-Tec. However, we wouldn’t recommend them as they are likely to be of inferior quality, have a short warranty, and cost you more over time in boiler repairs.

Where to find the best new combi boiler deals?

You’re able to find the best new combi boiler deals at Heatable. Heatable offer competitive prices for all the combi boiler models we recommend. Heatable have no physical storefront, a business model that saves money on operating costs. The installer additionally operates across the UK, and the nationwide coverage allows Heatable to source boilers locally at the best prices. Heatable pass all these savings on to you to ensure they offer the best prices on the market.

Heatable offer great value for money because of all the extras they include, such as a new magnetic filter, system flush, and advanced controls, as well as the high quality of installation. The company are so confident of their installations that they offer a 10-year warranty on all their combi boiler installations.

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Which is the best combi boiler?

The following is a list of the 7 best combi boilers.

  • Worcester Greenstar 4000
  • Greenstar 8000 Life/Style
  • Greenstar CDi Compact
  • Viessmann Vitodens 050-W
  • Alpha E-Tec

Below is a more thorough analysis of each of the best combi boiler models.

Worcester Bosch Greenstar 4000

The Worcester Bosch Greenstar 4000 is the best combi boiler on the market for four reasons. First, it has a 94% seasonal space heating efficiency rating. This rating means that for every £1 you spend, 94 pence goes to heating your home. Second, the Greenstar 4000 has a “Quick Tap” feature that saves you money by activating your combi boiler quickly when you turn the tap on. So, you don’t need to leave the tap running while the water heats up. Third, the Greenstar 4000 is able to run on a 20% hydrogen blend. This capacity ensures your boiler remains at the cutting edge as boilers transition to cleaner energy sources (the UK Government is considering a requirement that all boilers be “hydrogen ready” by 2026). Fourth, the Greenstar 4000 connects wirelessly to your smartphone via Bosch EasyControl.

There are two disadvantages of the Greenstar 4000 combi boiler. First, the Greenstar 4000 has an aluminium heat exchanger. Aluminium may be a higher-performance metal than stainless steel, but it is less durable. Second, the Greenstar 4000 combi has limited size options, with only 25kW and 30kW models available. The 25kW and 30kW sizes of the Greensar 4000 make the boiler a good fit for small apartments.

Bosch offer a 10-year guarantee on the Greenstar 4000 when you get it installed by a Worcester-accredited installer. The full cost of a Greenstar 4000 combi is £2,600 – £2,900 (fitted).

Get your own Greenstar 4000 fixed-price quote online by filling in Heatable’s anonymous questionnaire.

Worcester Bosch Greenstar 8000 Life/Style

The Greenstar 8000 Life/Style combination boiler is one of the best combis in the UK for four reasons. First, the Greenstar 8000 Life/Style has advanced features like “Quick Tap”, which boosts its energy efficiency rating to 94%. This rating means that for every £1 you spend on fuel, the Greenstar 8000 uses 94 pence to heat your home and wastes only 6 pence. Second, both Greenstar 8000 combi boiler models come with a wide range of sizes (30kW-50kW) that accommodate small and large households. Third, the Greenstar 8000 has advanced controls, including a full-colour LCD display and wireless connectivity via Greenstar EasyControl. Finally, the Greenstar 8000 comes with Worcester Bosch’s longest guarantee of 12 years when installed by a Worcester-accredited installer.

There are two disadvantages of the Greenstar 8000. First, the Greenstar 8000’s heat exchanger is made from aluminium. Aluminium is less durable than stainless steel but offers heat efficiency advantages. Second, the Greenstar 8000 is one of the most expensive combi boilers on the market, at £3,200 – £3,900 with installation.

Worcester Bosch Greenstar CDi Compact

The Greenstar CDi Compact is one of the best boilers of 2024 in the UK for three reasons. First, the Greenstar CDi Compact is a small boiler you’re able to store in a kitchen cupboard. This size makes it perfect for fitting into apartments or homes with limited storage space. Second, the Greenstar CDi Compact has an energy efficiency rating of 94%. This rating means that for every £1 worth of energy the CDi Compact uses, 94 pence is converted to heat and only 6 pence is wasted. Third, the CDi Compact has built-in frost protection, reducing the risk of potentially costly repairs.

The Greenstar CDi Compact has one disadvantage. The Greenstar CDi Compact lacks advanced features like “Quick Tap” or “Intelligent Filling” included in other Greenstar products.

The Greenstar CDi Compact is a small enough combi to fit into any apartment but powerful enough to heat small and medium-sized homes. The CDi Compact has outputs of 28kW, 32kW, and 36kW. The boiler and installation costs for the Greenstar CDi Compact are between £2,600 – £3,200.

Viessmann Vitodens 050-W

There are three key benefits that make the Vitodens 050-W one of the best-value combis you’re able to get in the UK. First, the Vitodens 050-W features the advanced MatriX-Plus burner and Lambda Pro combustion controller. These devices deliver an impressive energy efficiency rating of 92%, saving you money on your energy bill. Second, the Vitodens 050-W comes with wireless ViCare controls that allow you to adjust central heating settings wherever you are via the ViCare thermostat. Third, the Vitodens 050-W can run on a 20% hydrogen blend, allowing it to use the future’s affordable and environmentally friendly boiler fuel. The UK Government is considering proposals requiring all boilers to be “hydrogen-ready” from 2026. With the Vitodens 050-W, you’ll be ahead of the curve.

The Vitodens 050 has one disadvantage. Vitodens 050-W comes with only a 3-year standard warranty that increases to 7 years when installed by a Viessmann-accredited installer. You’re able to extend this warranty to 10 years for a fee, but this warranty is still far shorter than what many of Viessmann’s competitors offer.

The Vitodens 050-W has some of the most advanced parts on the market, delivers energy efficiency, and is the perfect combi boiler for small to medium-sized properties. The Vitodens 050-W comes in 25kW, 30kW, and 35kW outputs and costs £2,700 – £3,200 to install.

Get a fixed-price online quote on the Vitodens-050 at Heatable.

Alpha E-Tec

There are three advantages that make the Alpha E-Tec one of the best budget combis in the UK. First, the Alpha E-Tec’s high-efficiency Grundfos pump gives it an energy efficiency rating of 93%. That rating means the Alpha E-Tec wastes just 7 pence for every £1 you spend on heating your home. Second, the Alpha E-Tec has a stainless steel heat exchanger, making it more durable. Third, the Alpha E-Tec is economical, with installation costs of £2,000 – £2,700.

There are two disadvantages to the Alpha E-Tec. First, the Alpha E-Tec 7-year warranty is shorter than those offered by competitors like Worcester, who offer a 10-12-year warranty on their combi boilers. Second, the Alpha E-Tec lacks premium features like weather compensation or intelligent filling that other boilers on this list offer.

The Alpha E-Tec is a cost-effective combi boiler that maintains high energy efficiency and performance standards. The Alpha E-Tec has outputs of 28kw and 32kW, making it perfect for medium-sized properties.

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Which is the best electric combi boiler?

The Comet Combi Boiler from the Electric Heating Company is the best electric combi boiler for three reasons. Firstly, the Comet Combi Boiler is 100% energy-efficient, so every penny you spend on electricity gets converted into heat. Secondly, the Comet Combi has a stainless steel heat exchanger. Heat exchangers made out of stainless steel are generally more durable than their aluminium counterparts, as stainless steel is better at resisting corrosion and wear and tear when subjected to heat, moisture, and pressure. Thirdly, the Comet Combi Boiler has an integral magnetic filter, which protects the boiler’s components from the accumulation of sludge. Sludge blockages are a common cause of boiler problems.

Which is the best oil combi boiler?

Which oil combi boiler is best depends on your budget and preference for various boiler features. However, the three brands below produce the best oil boilers of any type in the UK.

  • Warmflow: Warmflow oil combis come with a high fuel efficiency rating and premium components, such as the Grundfos pump and Honeywell diverter valves.
  • Grant UK: Grant UK have a range of internal and external oil boilers, which come with lengthy warranties and high efficiency ratings.
  • Worcester Bosch: Worcester’s Danesmoor and Heatslave II oil boiler ranges are renowned for their long, 7-year warranties and efficient, user-friendly boilers.

Is a combi boiler right for me?

Whether a combi boiler is right for you depends on the following two factors.

  • Property size: Combi boilers are well suited for smaller properties for two reasons. First, combis are more efficient in smaller properties than system or heat-only boilers, as they produce water on demand and have a lower demand for heating. Second, combi boilers are well-suited for smaller homes because they do not require hot or cold water storage tanks, which is a major space-saver.
  • Hot water demand: Combi boilers are ideal for small families with modest hot water demand. Large, thirsty households would find that a combi boiler’s ability to supply domestic hot water is insufficient, especially when multiple hot water taps or showers are on simultaneously.

Knowing the advantages and disadvantages of combination boilers helps you decide whether a combi is right for you. Below is a list of combi boilers’ most prominent pros and cons.

What are the advantages of combi boilers?

The four advantages of combi boilers are listed below.

  • Small footprint: Combi boilers are much smaller than system or regular boilers because they have no storage tank. This small size makes combi boilers perfect for smaller apartments.
  • Energy efficient: Combi boilers only use as much energy as needed to heat the water you’re using at any given moment. A combination boiler does not need to keep a tank of water hot at all times, and thus saves you money on your energy bill.
  • Low cost: Combi boilers are cheaper than system or regular boilers.
  • Instant hot water: Combi boilers heat water as needed, so you don’t have to leave the hot water tap running until it heats up. A combi boiler never runs out of hot water, as occasionally happens with a storage-based system or regular boiler.

What are the disadvantages of combi boilers?

The three disadvantages of combi boilers are listed below.

  • Lower hot water capacity: Combi boilers are unable to simultaneously deal with high energy demands from multiple occupants using hot water. A combi boiler’s lower capacity makes using multiple appliances at once difficult.
  • Reliance on good mains pressure: A combi boiler does not generate water pressure as it only heats water coming from the mains. So, your hot water pressure is the same as the mains water pressure.
  • Immersion heaters not possible: A combi boiler does not accommodate an immersion heater, which means there’s no backup supply of hot water if your combi boiler breaks. You run out of hot water the moment the combi stops working.

What sizes do combi boilers come in?

Combi boilers come in various sizes to suit different properties. A boiler’s size refers to its energy output. A 25kW combi boiler produces 25kW of energy to heat water for showers, central heating, and doing the dishes. The three common size ranges of combi boilers are listed below.

  • 24 – 30kW: This combi boiler size range is perfect for smaller properties with 10 or fewer radiators, 1-2 bedrooms, and a single bathroom.
  • 35 – 40kW: Get this size combi boiler for a medium-sized property with 10- 15 radiators, 2-4 bedrooms, and 2 bathrooms.
  • 45 – 50kW: This size combi boiler works well in large properties with 15 – 20 radiators, 4+ bedrooms, and 2 bathrooms.

Below is a more nuanced explanation of the combi boiler sizes and their suitability for different properties.

24–30kW combi boilers

A small 24-30kW combi boiler supports 10 or fewer radiators. This size combi boiler works well in a 2-bedroom apartment with 1 bathroom. Combi boilers at the 24-30kW output size typically have flow rates of between 11 – 13 l/min. This flow rate range is high enough to accommodate an occupant using the bathroom. However, this flow rate might not be enough if multiple occupants use hot water simultaneously.

There are a lot of options for combi boilers in the 24-30kW range. Viessmann Vitodens 050-W 26kW, the Greenstar 4000, or the Alpha E-Tec 28kW are some of the best combi boilers in the 24-30kW size range.

35–40kW combi boilers

A medium-sized 35-40kW combi boiler supports 10-15 radiators in an apartment or home with 2 bathrooms and 2-4 bedrooms. Combi boilers in the 35 – 40kW range tend to have flow rates between 12.5 l/min and 15 l/min. A combi boiler with a 12.5 l/min – 15 l/min flow rate supports hot water usage by multiple occupants but struggles if hot water is used excessively. Consider the Viessmann Vitodens 100-W 35kW, the Ideal Logic+ 35kW, or the Greenstar 8000 if you’re looking for combi boilers in the 35-40kW range.

45–50kW combi boilers

A large combi boiler with an output of 45 – 50kW supports 15-20 radiators. This size combi boiler works best in a property with 2 or more bathrooms and 4+ bedrooms. A 45 – 50kW combi boiler typically has a flow rate of 15 – 18 l/min. Combi boilers with a flow rate of 15 – 18 l/min support multiple bathrooms with several occupants using hot water simultaneously. However, high-flow combi boilers may underperform when accommodating high hot water demand compared to regular or system boilers.

There are a few options for combi boilers of the 45-50kW size. We recommend the Worcester Greenstar CDi 42kW, the Greenstar 8000 Life 45kW and 50kW, and the Greenstar 8000 Style 45kW. You may find that a system or regular boiler works better for a property of this size if you use hot water regularly.

Use Heatable’s online boiler selection tool to get an idea of what combi size you need.

What is a compact combi boiler?

A compact combi boiler is a combi boiler designed to fit inside a kitchen cabinet, with typical dimensions of <400mm in width, =/<700mm in height, and <300mm in depth. There is no technical difference between a compact combi boiler and a standard combi boiler other than the former’s smaller size. The best compact combi boilers are perfect for the smallest properties with little space to store a boiler. Popular compact combi boilers include the Worcester Greenstar CDi Compact, the Glow-worm Compact, and the Baxi 600.