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Last updated: 25th November, 2023


What Is a Combination (Combi) Boiler?

What Is a Combination (Combi) Boiler?

Key takeaways

  • Combi boilers are the most popular type of boiler in the UK.
  • Combi boilers 'combine' a home's central heating and hot water into one compact unit.
  • Viessmann, Vaillant, Ideal , Alpha and Worcester Bosch are among the best combi boiler brands in the Uk

Is a combi boiler worth it? Would your home benefit from a combi boiler? We will find out..

You've probably heard of a combi boiler before, and there is quite a good chance that you have one in your home or business premises already, but not everybody will know what they are.

If you will be using one, familiarising yourself with what they are, how they work, and how they differ from other boiler types is a good idea.

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What's the difference between a combi boiler and a normal boiler?

What is a combi boiler? Combi boilers (short for 'combination boiler') combine a water heating system and a central heating unit into a single body, acting as a unified way to handle heating and hot water without needing two separate boilers.

Combi boiler designs can vary, but they almost always take it directly from the mains and can provide hot water almost instantly as long as they're properly connected.

Because of this, there's also no need for a separate water tank system: boilers like this generally have one tank they use to heat water, regardless of where it's going.

There are various sub-types of combi boilers based on different features, designs, and extra functions, such as a condensing combination boiler focused on recycling already-used water. The basic design is almost always the same regarding how hot water storage and the heating system are managed.

What are the benefits of a combi boiler?

Combi boilers have plenty of useful benefits, some of which are more important than others. Since they all have the same kind of structure and layout in how they work, most of these benefits are constant across all types of combi boilers, but they might not be obvious straight away if you've just gotten a new boiler.

To read iHeat's best combi boilers of 2022, read our latest article!


One of the most notable benefits of a combi boiler is the amount of space they take up. Since you're combining two different boilers into one body, it cuts down on the amount of room they use in total and gives you more room for other things in your home.

Even if the boiler would be inside an attic or cupboard anyway, needing less space is always good, especially in a smaller home where you might not have the necessary room to fit multiple appliances into a single area.

Since a combi boiler acts as a single appliance, it only needs one set of pipes, so installation is cheaper and faster. Not only that, but some new boiler designs have even been able to use the reduced size as an excuse to add larger water tanks - this isn't always that useful, but it can still be helpful in certain situations.


Another major benefit is the overall efficiency of the heating system. Some older water heater boilers are barely over 50% efficient - for every £1 you spend trying to heat water for your home, you're wasting almost £0.50 for no reason.

Not only are combi boilers generally the more efficient type of boiler, but since they're two units combined into a single new boiler, water is always ready to go into the heating system as soon as needed.

Some can go as high as 92% or even higher, dramatically reducing the amount you're paying for your water heater compared to a standalone gas boiler.


A combi boiler almost always takes its water directly from the mains, meaning it retains much of the mains pressure as normal.

Unlike a regular boiler, which often uses gravity to collect the water, you get a much stronger flow that matches the original mains pressure more closely.

Aside from helping you get stronger jets of water, this also means that your boiler won't end up with as many areas where water can get stuck or pool in a curved pipe since there's more force behind it to keep it moving through the system.

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Many people overlook it, but a good combi boiler can be a great way to ensure you get hot water nearly instantly and at any time. Unlike regular boilers, a combi boiler can produce it on demand since it's already a water heating system, so there can always be some ready-to-use.

You don't need to heat it separately ahead of time or wait for it to warm up as long as it's already running properly, so your home can get a much more consistent supply of hot water as a result.

What is a combi boiler bad for?

Combi boilers still have flaws, but they're much less of a problem than you might think. Like all boiler types, there are some things they can't do quite as well as, say, a normal gas boiler, so you'll want to be aware of their limitations or shortfalls too.

Water Storage

A combi boiler system doesn't usually include storage tanks. Very few boiler designs even try to include a water storage tank, but a combi hot water system boiler will rarely include any hot water tank.

Since the water cylinder is just a chamber for water heating, boiler designs like this rely on a constant flow from the mains, which means that you will need some other storage method if you want your boiler to keep running without a regular water supply.

There are ways around this, like installing your tanks in your home, but this is a separate appliance that will take up more room.

Not all homes need a storage system like this. If you don't, or if your building already has one installed anyway, this downside doesn't matter (apart from very specific situations that most people will never actually encounter).

Poor Pipe Systems

Since a combi boiler gets its pressure from the mains water pressure, weaker pipe systems will lead to weaker pressure overall.

This isn't a problem with the combi boiler itself but with the water heating systems and pipes: a bad flow rate and low pressure in the pipes will lead to the same issues in the boiler, which will cause taps, showers and other water-related systems to get weaker as well.

This makes a combi boiler much less useful for older pipe systems that might have been built around some other high-pressure source.

Alongside this is the issue of old pipework. An older system can lead to lower energy efficiency and an increased chance of the combi boiler not being compatible with the connections.

Some boilers won't accept particular connections or pressure levels, and the only way to fix it is to either adapt the existing system or replace part of the pipe structure.

Is a combi boiler cheaper to run?

A combi boiler is one of the most efficient types of boilers you can install in your home, although each combi boiler model will have its efficiency level.

As mentioned earlier, a lot of older boilers barely reach 60% efficiency, meaning that half of the money spent on heating is wasted each time you use it.

However, combi boilers have a higher efficiency level, meaning it costs less to heat any water.

That's not the only improvement, though. Think about it: what exactly is a combi boiler? Since it combines two different types of boiler-related systems, you're often using half the power and resources to run it (at least in theory), meaning that most combi boilers are flat-out more efficient in terms of energy usage even if they have the same level of heating efficiency on paper.

This can save you plenty of money and make the installation cost worth it, even if the installation costs are higher than you expected to pay initially.

Is a combi boiler better?

While combi boilers reduce your heating bills and take up less airing cupboard, kitchen cupboard or loft space, that doesn't necessarily mean that they'll always be the best in every single situation.

Always choose one that suits your home and your situation since that's what matters most: every home is different and has different levels of heating usage, and even the smallest detail down to directions of certain pipes can affect the way that it performs.

No boiler design is perfect for all home types and living situations, so you must plan accordingly.

That being said, if you can get a combi boiler, it's often much more reliable and efficient for home use than a normal boiler as long as your house isn't too large.

The lower running costs and combined boiler controls can help, especially if you use your central heating system a lot. If your boiler systems can support it properly and you have a direct connection to the mains, it's worth at least considering a new combination boiler, even if you'd only be saving a small amount of money and power every month.

Exploring Top Combi Boiler Brands and Their Key Models

Combi boilers have become a popular choice for modern heating solutions, offering efficiency, convenience, and space-saving benefits. Among the plethora of options available in the market, brands like Vaillant, Viessmann, Worcester Bosch, Alpha, and Ideal stand out for their innovation and reliability. Let's delve into some of their key models and explore what makes each range unique.

Vaillant ecoTEC Combi Range

The Vaillant ecoTEC combi boiler range is renowned for its efficiency and performance. Among its standout models is the ecoTEC Plus, which is highly efficient and available in various outputs to suit different home sizes. These boilers are known for their quiet operation and compact design, making them an excellent choice for homes where space and noise are concerns. Vaillant's ecoTEC range also includes features like an advanced modulation system for more efficient fuel use and a stainless steel heat exchanger for durability.

Viessmann Vitodens Combi Range

Viessmann's Vitodens combi boilers are at the forefront of heating technology, offering high efficiency and low emissions. The Vitodens 100-W and 200-W models are particularly popular, known for their energy efficiency, user-friendly controls, and compact, sleek design. These boilers use a stainless steel Inox-Radial heat exchanger and a modulating MatriX cylinder burner for optimum efficiency and longevity. Viessmann Vitodens boilers are a great fit for environmentally conscious homeowners looking for cutting-edge technology in their heating system.

Worcester Bosch Greenstar Combi Range

Worcester Bosch is a trusted name in the heating industry, and their Worcester Bosch Greenstar combi boilers are among the most popular in the UK. The Greenstar i, Si, and CDi models are known for their reliability, efficiency, and user-friendliness. These boilers are suitable for a wide range of homes and come with features like simple boiler controls, low energy pumps, and the ability to link with smart thermostats for better energy management. The Greenstar range is a testament to Worcester Bosch's commitment to quality and sustainability.

Alpha E-Tec Combi Range

Alpha's E-Tec range offers a solid balance of performance and affordability. The E-Tec models, including the E-Tec Plus, feature a stainless steel heat exchanger for efficient heat transfer and longevity. They are compact in size, making them suitable for smaller spaces, and come with a high-efficiency Grundfos pump and hydroblock assembly for improved energy savings. Alpha E-Tec boilers are a great option for those seeking a reliable and cost-effective heating solution.

Ideal Logic Plus Range

The Ideal Logic Plus range is designed for ease of installation and maintenance, making it a favourite among installers and homeowners alike. These boilers are compact, efficient, and come with a user-friendly control panel. The Logic Plus range boasts a low lift weight, making it a practical choice for installation in various properties. With its quiet operation and comprehensive warranty, the Ideal Logic Plus range offers peace of mind and comfort.

Comparative Analysis of Key Models

When comparing these models, it's essential to consider factors like energy efficiency, ease of installation, maintenance requirements, and warranty offerings. For instance:

Energy Efficiency

Models like the Viessmann Vitodens and Vaillant ecoTEC are at the top end of energy efficiency, making them ideal for those looking to minimise their carbon footprint and reduce heating bills.

Ease of Installation and Maintenance

The Ideal Logic Plus and Alpha E-Tec ranges are noted for their ease of installation and straightforward maintenance, a key consideration for properties with limited space or for quicker installation requirements.

Innovative Features

The Worcester Bosch Greenstar range, known for its smart compatibility and innovative features, is well-suited for tech-savvy homeowners who value modern conveniences like smart thermostat integration.

Choosing the Right Combi Boiler for Your Home

Selecting the right combi boiler involves assessing your home's size, hot water demand, and your preferences regarding efficiency, technology, and budget. For larger homes with higher hot water needs, models like the Vaillant ecoTEC Plus or Worcester Bosch Greenstar CDi might be more suitable due to their higher output options. Conversely, for smaller homes or apartments, the compact design of the Alpha E-Tec or Ideal Logic Plus could be more appropriate.

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Frequently Asked Questions

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

Combi (combination) boilers differ from regular boilers in several key ways. Combi boilers combine both water heating and central heating in one unit, providing hot water on demand without the need for a separate hot water tank. Regular boilers, on the other hand, require a hot water cylinder and often a cold water storage tank. Combi boilers save space and tend to be more energy-efficient, as they only heat water when it’s needed.

What are the disadvantages of a combi boiler?

While combi boilers offer many benefits, they have some disadvantages. They may not be suitable for homes with multiple bathrooms, as the water flow can decrease when multiple taps are used simultaneously. Additionally, since they provide hot water on demand, there can be a brief delay in water heating. Combi boilers also rely heavily on mains water pressure, so homes with low pressure might experience weaker performance.

What is a combi boiler and how does it work?

A combi boiler is a high-efficiency water heater and a central heating boiler, combined within one compact unit. It works by drawing water directly from the mains supply and heating it as needed, meaning there's no need for a separate hot water storage tank. When a hot water tap is turned on, the boiler heats water instantly, providing a continuous supply of hot water.

Does a combi boiler use gas or electricity?

Combi boilers can be powered by gas or electricity, although gas combi boilers are more common in the UK. Gas combi boilers use natural gas to heat water, while electric combi boilers use electricity. The choice between gas and electric usually depends on the availability of gas supply in the area and personal preference regarding energy sources.

What is cheaper to run, a system boiler or combi?

Generally, combi boilers tend to be cheaper to run compared to system boilers. This is because combi boilers only heat the water that is being used, reducing energy wastage. System boilers, which store hot water in a cylinder, can lead to some energy loss due to heat dissipation from the tank.

Should I change my system boiler to a combi?

Whether you should switch from a system boiler to a combi depends on your specific needs and circumstances. Combi boilers are more suitable for smaller homes with limited space and fewer bathrooms, as they provide hot water on demand and do not require a storage tank. However, for larger homes with higher hot water demands, a system boiler may be more appropriate.

Why are combi boilers being phased out?

Combi boilers are not being universally phased out, but there is a growing emphasis on more sustainable and energy-efficient heating solutions. As part of efforts to reduce carbon emissions and improve energy efficiency, there is a shift towards alternative heating technologies, such as heat pumps. However, combi boilers remain a popular choice, especially energy-efficient models.

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Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.