combi boiler example on a blue background

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Combi boilers, or combination boilers, are a high-efficiency central heating and water heating option for small to medium homes. When you turn on a tap, the unit takes water directly from the mains and heats it, so you don’t need a water storage tank or cylinder that takes up space in your loft or requires its own room. Combi boilers are also very energy-efficient and can deliver tap and shower water at mains pressure. 

Because of their compact size, these boilers are ideal for properties with fewer than two bathrooms. The systems are also generally easy to install and maintain, can work with various fuels and are incredibly cost-effective because you only pay for what you use. 

So, let’s go into a little more detail about combi boilers, how they work, whether they make a good replacement boiler, the most popular brands and how much they cost on average. 

What’s the difference between a combi boiler and a conventional or system boiler?

The main difference between a combination boiler and a conventional boiler has to do with the way the water is stored. Combi boilers don’t need a storage tank or cylinder and instead heat the water directly from the mains and only when it’s needed. This is why many smaller properties choose this type of system. 

Conventional boilers require two water tanks. One draws the cold water supply from the mains, and the other feeds the system and manages the distribution of heated water. The cold water storage tank is usually kept in your property’s loft, so this is a more popular option for larger properties with extra space. Conventional boilers are also a good idea for areas where water pressure is low.

Workman mounting water heating radiator near the window in the white renovated living room

New radiators increase the cost of installing a combi boiler, but increase your heating’s efficiency. (Image credit: Adobe)

System boilers don’t require a second tank for cold water and instead heat the water in the boiler itself. This hot water is then stored in a cylinder that is insulated to maintain the temperature. You can choose to have your boiler top up the cylinder at set times during the day or on demand. Cylinders come in various capacities depending on the estimated demand for hot water in a house, but they can usually fit in a typical airing cupboard. Many system boilers are compatible with solar water heating systems. In general, these boilers are economical to run and can provide a constant supply of hot water to numerous taps at the same time.

How do combi boilers work and what are their main advantages?

The inner-workings of a combination boiler are relatively complex, but the result is a simple-looking unit that is both powerful and efficient. Once the boiler is ignited (when you turn on a tap or switch the heating on), the fuel produces a flame inside it to create heat. This heat is absorbed by heat exchangers, which have pipes to distribute it through your home. As the water travels through them, the temperature continues to rise. 

Combi boilers also have a set of internal control valves that allow or restrict the water from going in certain directions. For example, to the correct tap when operated. Once it has circulated, the water is sent back around the circuit to be re-heated and re-used. 

Combination boilers can provide both central heating and hot water from the same unit. Because they don’t require storage tanks or cylinders and instead get water from the mains, they are usually pretty compact (they can be wall-mounted easily or hidden in a kitchen cupboard). They also don’t require extra pipework as conventional boilers do.

Double-circuit gas boiler in a new house for winter heating and water heating.

Combi boilers also have a set of internal control valves that allow or restrict the water from going in certain directions. (Image credit: Adobe)


The different types of combi boilers 

The different models of combination boilers tend to look similar from the outside, but they can use four main types of fuel to operate: Gas, LPG (Liquified Petroleum Gas), oil or electricity. 

Gas combi boilers use natural gas that is sent to residential properties through the national gas grid network. This is the most common type of combi boiler fuel, as most UK homes are connected to the grid. Gas is a fossil fuel (and therefore it causes damage to the environment), but it burns cleaner than other fossil fuels. 

LPG boilers use a combined natural gas and oil product called Liquified Petroleum Gas. This is a much cheaper fuel option and slightly cleaner than natural gas overall because it produces fewer carbon emissions in the extraction and production process, and more combi boilers are being designed to use with LPG. These boilers require large fuel storage tanks, rather than being connected to a mains supply like gas boilers.

Combi boilers that work on oil are a cost-effective alternative for those not connected to the gas grid. If you can access gas, however, oil is not recommended, as it can create more difficulties. Oil needs to be stored on your property, usually in a large tank sunk into the ground. You will also need to make sure you refill it regularly to maintain access to hot water and heating. 

The most energy-efficient combi boilers in the UK market work with electric power. The electricity heats the water, so there is no waste – all energy is transferred from the fuel to heat instantly. There is, however, a downside: electricity is more expensive than all the other types of fuel mentioned.

Are combi boilers energy-efficient?

Modern combination boilers are some of the most energy-efficient systems you can find to date – up to 92% efficient. This means that only 8p is lost in the heating process for every £1 you spend on energy. But for older boilers, this number can be as low as 40–60%. 

Combi boilers use built-in condensing technology, which burns fuel to produce water vapour to contribute to warming up the central heating. Before this advance was made, boilers got rid of this heat through the flue pipe. This meant that 30% of the heat produced was lost. With condensing technology, the heat from the steam is recycled and fed back into the system, keeping energy usage down. 

If your existing hot water and heating system is getting old and you are considering buying a new boiler, combi boilers are an excellent replacement option if circumstances don’t permit installing an air source heat pump or ground source heat pumps

These systems tend to be more energy-efficient, reliable and quieter, however this is only true if the boiler is installed in a well insulated house. If you live in an older house, it’s worth considering other improvements – such as insulation, double glazing and replacing doors to reduce draughts – before switching to a combi boiler. 

Combi boilers also have many features that make them easy to operate and maintain.

Many combi boilers are compact, so can be installed in small kitchens or cupboards. (Image credit: FMB)

It’s worth keeping in mind, however, that combi boilers are better suited for smaller homes because they might not be powerful enough to heat multiple taps at once. 

Disadvantages of combi boilers

Combi boilers offer many benefits, but they are not a universal solution. There are some cases in which a combination boiler might not be a good idea. 

If you have a large home, a combi boiler might not be powerful enough to provide you with a large amount of water. For example, if you have more than two two or more bathrooms that can be used simultaneously, you might want to look into a different boiler (such as a system boiler). If you’re planning to extend the floorspace of your house through a loft conversion or an extension, you will need to find out if your combi boiler is powerful enough to heat a home that is bigger than it was when the system was installed. Your builder will usually be able to advise you on this. 

Combi boilers are also not always compatible with old pipework. If you have an older system and want a replacement boiler installed, you will need to evaluate if your existing pipes will be compatible with a more efficient and newer design. For example, you need to consider whether the lines you already have can handle the mains water pressure. 

Homes with weak mains pressure are also not an ideal set-up for combi boilers. If your water supply is inadequate, your new boiler might struggle to achieve the flow of water your taps require. However, if this is the case and you don’t mind a higher overall cost, you can consider installing an additional pump to strengthen the flow.

The most reliable combi boiler brands

The combi boiler market is quite competitive, with several brands offering efficient options at various affordability levels. 

One popular combination boiler and winner of the Which? Best Buy award 2020 is the Worcester Bosch Greenstar 4000. This system is one of the latest models of this top manufacturer in the UK, and replaces its former star product Greenstar i. Like many new combi boilers, the Worcester Bosch Greenstar 4000 is hydrogen-ready, meaning that it can be amended to run on 100% hydrogen by changing just a few parts. Hydrogen has been tapped by the UK Government as a low-carbon alternative to natural gas and oil for heating homes in the future. 

Another ultra-efficient combi boiler is the Viessman Vitodens 100-W, a gas-fuel system featuring powerful and innovative technologies. The Vitodens 100-W has an Inox-radial heat exchanger, which reduces its carbon footprint to a minimum. 

If you’re looking for something more economical, a low-cost combi boiler that has made waves is the Alpha E-Tec from Alpha Heating Innovations. This system is their smallest combi boiler to date and has a stainless steel heat exchanger and two outputs available. 

Read our guide to the best combi boilers.

How much do combi boilers cost, and how long do they last?

Combi boilers are easy to install and cost less than traditional or system boilers. Due to their ability to reduce energy waste, these systems can help you save money on your energy bills. 

The cost of a boiler replacement depends on the model, the type of boiler, and extras such as smart controls and filters. A new combi boiler costs, on average, between £600–£900 for a budget model, £900–£1,200 for a mid-range option and £1,000–£2,000 for a premium alternative. The installation of a combi boiler costs between £1,500–£3,000. Converting from a regular boiler into a combi is usually in the range of £2,500–£4,500. 

A combination boiler should last you between 10 and 15 years, provided it’s serviced and maintained regularly. Most big manufacturers offer guarantees of about five years, which can be extended in most cases to 10 years. The length of the boiler guarantee can generally be used as an indicator of the boiler’s reliability. If you buy a gas combi boiler, you will need to use a Gas Safe Installer. 

What are the advantages of combi boilers?

Combi boilers are quite distinct from other boiler types. Their design is relatively straightforward; you get hot water directly to your taps and radiators without needing to store it in a tank. But what are the benefits of a combi boiler compared to other systems?

No need for a storage tank or cylinder

Combination boilers don’t require a cylinder or water storage tank to function because they receive cold water directly from the mains and heat it on demand only when you turn on a tap. Because of this, they are also generally easier to install and maintain and very convenient if you want hot water immediately.

Energy-efficient and cost-effective

Combi boilers are highly efficient, especially when compared to conventional boilers. Replacing your older system can, in fact, help you save money on your energy bills. These systems are also cheaper to buy, and there is a large selection of available models and outputs to choose from.  

Water at mains pressure

Because combi boilers take the cold water straight from the mains, there is less chance of airlocks and low pressure. Showers and taps also have better pressure overall, provided you don’t use many at the same time (the system is better suited for smaller homes with less than two bathrooms, though there are a number of factors that can impact its efficiency, such as water having to travel up pipework to higher floors).  


Combi boilers tend to be very small. They usually fit in a standard kitchen cupboard or can be easily set up on walls, which makes them an excellent option for smaller properties that cannot fit water storage tanks. 

Easy to install and service

Since combi boilers are so popular, a majority of boiler engineers are trained to repair and service them. Their compact size and lack of storage tanks or cylinders also make them easy to set up. Lastly, combination boilers have less sludge build-up because there is less of this mix of rust and dirt particles that break off from the pipework and joints over time when compared to conventional boilers.