Your guide to combi boilers.
What is a combi boiler?
Combination boilers, more commonly known as combis, are an all-in-one heating system for your home. They provide instant hot water on demand, without the need for a hot water cylinder or cold water tank.
Combi boilers explained.
Check out this quick video guide to combi boilers and find out if they’re the right choice for your home.
How do combi boilers work?
They heat water on demand directly from your mains water supply. When you turn on your heating or hot water it will trigger your boiler to start burning fuel, which is used to heat the cold water using a heat exchanger. This is then transported around your home to your taps, showers, and radiators.
Many new combi boilers will also have a small reservoir of hot water stored within to help provide hot water immediately.
What is the difference between a combi boiler and a regular or system boiler?
Choose the right boiler type for your home with our quick comparison.
What are the advantages of a combi boiler?
As they heat water on demand instead of storing pre-heated water on a timer, your hot water supply will be instant and won’t run out. Plus, you won’t be heating water you don’t need, helping to save energy at home.
Better yet, combi boilers are highly compact, meaning they won’t use up valuable storage space in your loft or airing cupboard.
How much do combi boilers cost to install?
At E.ON Next you can get a brand new combi boiler from £2,495 - including professional installation by E.ON at no extra cost. Complete your boiler bundle with helpful add-ons, such as CO detectors and smart thermostats, for a safer, smarter home.
How much does a combi boiler cost to run?
Combi boilers are generally cheaper to run than comparable regular or system boilers, as they only heat what you need.
The exact running costs of your combi boiler depend on its energy efficiency and how much you use your heating and hot water. The better your boiler efficiency the less energy it will waste - saving you money on your heating bills and reducing your carbon emissions. However, the more you use your heating and hot water, the more often your boiler will need to be on, raising the running costs.
Getting the right power boiler for your needs will help to make sure it is cost-effective. More powerful boilers will cost more to run, but if it is not powerful enough then your boiler may struggle to meet your hot water demand.
Why choose a Worcester Bosch combi boiler?
The UK’s favourite boilers.
As the UK’s market-leading domestic boiler brand, Worcester Bosch has been heating our homes for more than 50 years.
Powerful and efficient.
Our range of Worcester Bosch Greenstar condensing boilers are A-rated and up to 94% efficient - helping you waste less fuel and save more on your energy bills.
Awarded Best Buy by Which?
The award winning Greenstar 4000 combi is our most popular model. It’s one of the quietest boilers in our range, with easy to adjust settings, user friendly controls, and the water saving ‘Quick Tap’ function.
Your boiler installation in four simple steps.
1. Find the right model for you.
Choose from our range of energy efficient boilers to find the right model for your home, and get a no obligation quote online.
2. Book your boiler installation.
Pick a time that suits you, and get a professional boiler installation by E.ON at no extra cost.
3. Get your boiler installed.
Leave it to the pros. During your installation, we’ll remove your old boiler and flush your system. Then, once we’re done, we’ll show you how it all works.
4. Post-installation support.
Once you're up and running, we're always here to help with repairs or boiler parts. For some extra peace of mind, try adding HomeServe boiler cover.
Explore the Worcester Bosch Greenstar range.
Find the right boiler for your home with our Worcester Bosch condensing combis - explore our full range.
Greenstar 4000
Key features:
One of the quietest combi boilers available.
Suitable for small to medium sized homes with one bathroom.
Innovative 'Quick Tap' water saving function.
Compact dimensions (mm): 724 x 400 x 310.
Fuel type: Gas boiler.
Heating output: 24kW.
Energy efficiency rating: A.
Greenstar i
Key features:
Our most popular combi boiler.
Suitable for small to medium sized homes.
Compact dimensions (mm): 710 x 400 x 330.
Fuel type: Gas boiler.
Heating output: 24kW.
Energy efficiency rating: A.
Greenstar Si Compact
Key features:
Our space saving combi boiler.
Fits within a standard kitchen cupboard.
Suitable for small to medium sized homes.
Compact dimensions (mm): 690 x 390 x 280.
Fuel type: Gas boiler.
Heating output: 24kW.
Energy efficiency rating: A.
Greenstar CDi Compact
Key features:
Our space saving combi boiler.
Fits within a standard kitchen cupboard.
Suitable for homes with more than one bathroom.
Compact dimensions (mm): 690 x 390 x 280.
Fuel type: Gas boiler.
Heating output: 24kW.
Energy efficiency rating: A.
Greenstar CDi Highflow
Key features:
Our ultra-high performance floor standing combi boiler.
Designed for larger two bathroom homes.
Compact dimensions (mm): 850 x 600 x 600.
Fuel type: Gas boiler.
Heating output: 29.2 - 30.6kW.
Energy efficiency rating: A/B.
Greenstar 8000 Life Combi
Key features:
Modern new design.
Part of our most powerful wall hung boiler range.
Suitable for larger homes.
Wireless compatibility with the Bosch EasyControl.
Compact dimensions (mm): 780 x 440 x 365.
Fuel type: Gas boiler.
Heating output: 30 – 35kW.
Energy efficiency rating: A.
Greenstar 8000 Style Combi
Key features:
Stunning design-led boiler available in a high gloss black or white.
Part of our most powerful wall hung boiler range.
Effortlessly heating larger homes.
Wireless compatibility with the Bosch EasyControl.
Compact dimensions (mm): 780 x 440 x 365.
Fuel type: Gas boiler.
Heating output: 30 – 35kW.
Energy efficiency rating: A.
What size combi boiler do I need?
The right boiler size is ultimately determined by your heating and hot water demand, which usually increases with house size.
1-2 bedroom homes need roughly 24-27kWs.
3-4 bedroom homes need roughly 28-34kWs.
5+ bedroom homes usually need 35kWs or more.
Another rule of thumb is that you will need 1-2kWs of power for each radiator in your home.
Where can combi boilers be fitted?
You can opt to have your combination boiler fitted in a number of locations, so you can decide what best suits your home. Most are wall-mounted, but you can choose a freestanding option.
Some popular locations include:
Inside a kitchen cupboard - ideal for keeping your boiler out of view but could take up valuable kitchen storage.
Utility room - often a great choice for minimising the noise of your boiler.
Airing cupboard - often chosen if switching from a system or regular boiler, but be aware they are often close to bedrooms that could be disturbed by the noise.
Bathrooms - popular choice but must comply with additional safety regulations.
What are the disadvantages of a combi boiler?
Check if they are the right choice for your home.
They usually struggle with high demand for hot water, such as from multiple hot water taps at once, causing low water flow rate.
Low water pressure areas may be better off with a regular boiler, as combis rely on mains water pressure.
As they don’t have immersion heaters, there is no back up if they break down.
They are not compatible with power showers as they do not use a hot water cylinder.
Should I change to a combi boiler?
If you don’t currently have one, it could be worth changing if:
You want to save space.
You have good mains water pressure.
You do not often use multiple taps or showers at once.
You use hot water for long periods and often run out.
You have replaced old pipework that might have previously been damaged by high mains water pressure.
Looking for more answers?
How do I know if I have a combi boiler?
The simplest way to find out is to check if you have a hot or cold water tank in your home. Combis do not require any storage tanks, and so if you do find one you do not have a combi boiler. Water tanks are most often located in your attic or airing cupboard.
Is a combi boiler a dual purpose boiler?
Yes, they provide both hot water and central heating.
Will a combi boiler increase water pressure?
No, they use your mains pressure. It is also worth bearing in mind that as they do not have a water cylinder they are incompatible with power showers and most solutions for increasing water pressure.
Will combi boilers be banned?
The government is not banning combi boilers or gas boilers. From 2025 the government has set energy saving targets on newly built houses that will mean most new builds will need to use renewable solutions, such as heat pumps. This will have no impact on existing houses and heating systems though.
Are combi boilers gas or electric?
They are most commonly fuelled by gas, as most homes are connected to the mains gas supply. However, they can be powered by alternative fuels, such as oil, or you can get a completely electric combi boiler.
Are combi boilers energy efficient?
They are generally more energy efficient than regular and system boilers, as they heat water on demand to precisely meet your needs. Modern boilers in general are all high efficiency as they use condensing technology to reduce fuel waste.
Get £150 off a new boiler.
Did you know? Heating and hot water make up over half of your annual energy bill. Upgrading to a new high efficiency boiler could save you up to £540 on your fuel bill each year.^
Get more out of your energy. T&Cs apply.
Get a new boiler^ Savings information for boilers in the UK provided by the Energy Saving Trust, based on a semi-detached house with a boiler rating of G.