James Elston

Published February 23, 2024

A combi (combination) boiler is a modern compact boiler that provides hot water for household use and indoor heating. This type of boiler is perfect for smaller homes as it does not require an additional water storage tank like traditional boilers – it heats water directly from the mains. 

Combi boilers offer homeowners a space-saving and energy-efficient heating solution. This means that homeowners can save money on energy bills while reducing their carbon footprint. 

In this guide, we will discuss how combi boilers work, the different types of combi boilers, the advantages these compact boilers offer, as well as what you need to consider when getting a combi boiler for your home. 

How Combi Boilers Work 

Combi boilers are relatively uncomplicated due to not having a separate water storage tank.

1. Cold water input and mains water supply

Cold water enters the central heating system through the inlet pipe to be heated by the heat exchanger.

2. Combustion process and heat transfer

Combi boilers heat water when the burner flame is ignited. This burner charges up the heat exchanger. A heat exchanger is responsible for burning fuel, such as gas or electricity, to produce heat.

3. Thermostat control

The thermostat regulates the temperature of the boiler water. For maximum efficiency, set your thermostat at 60 °C.

4. Diverter valve operation

When you’re not using a hot water tap, the diverter valve redirects hot water to your radiator to keep your house warm. But when you turn hot water taps on, it quickly sends water to the tap instead.

5. Flue gases

Flue gases, or exhaust gases, are released when modern combi boilers generate hot water. A combi boiler captures some of this heat before it escapes through the chimney and reuses it to heat more water. A boiler’s condensate pipe (for wastewater) expels the leftover liquid into drains connected to the sewer.

Types Of Combi Boilers 

There are three main types of combination boilers:

Gas combi boilers

Gas boilers are the most common type of combi boiler in the UK. These boilers run on natural gas supplied to homes via the grid. 

Although natural gas is a fossil fuel, it is considered much cleaner than oil boilers. 

Modern gas combi boilers use condensing technology which prevents heat from being lost via the flue pipe. This makes them highly efficient, with very little heat escaping to the environment. 

Electric combi boilers

Electric combi boilers are even more efficient than gas boilers. This type of boiler boasts efficiency ratings as high as 99%! 

They rely on grid-supplied electricity; however, they can be linked up to renewable energy sources like solar panels to be completely environmentally friendly. 

Oil combi boilers

Oil combi boilers are great for off-grid homes that lack solar power. However, these boilers are much larger than modern gas and electric combi boilers, as you need an external fuel tank.

Although modern oil combi boilers have condensing technology, their emissions are higher than gas boilers and they’re slightly less efficient. 

Advantages Of Combi Boilers 

Let’s take a look at a few advantages of a combi boiler:

  • Energy efficient: The best small combi boilers on the market are 90% more energy efficient than traditional types (50%). This results in lower heating bills and a reduced environmental impact.
  • Compact size: The space-saving design of combi boilers is ideal for smaller homes or properties. Since there’s no need for a cold water storage tank or hot water storage cylinder, combi boiler installation is less cluttered than other boilers.
  • On-demand hot water: A combi boiler eliminates the waiting time for hot water.
  • Installation flexibility: Due to their compact size, these boilers can be installed in kitchens, utility rooms or airing cupboards. This flexibility in fitting a new combi boiler can be beneficial in renovations or new builds.

Considerations For Homeowners

Here are some factors to consider when getting a combi boiler for your home.

Home size

  • Combination boilers are ideal for smaller homes with limited space.
  • They work best in homes where hot water demand is not extremely high.
  • The layout and size of the property can affect the boiler’s efficiency. In larger homes, the distance between the boiler and taps can impact water pressure and temperature.

Mains water pressure

  • Combi boilers rely on water pressure from the mains water supply. If your home has low water pressure, this boiler might not be your best option.
  • It’s important to check the mains water pressure before installation. A qualified Gas Safe engineer or plumber will be able to help.

Boiler size

The size of your combi boiler should match the needs of your home. This includes considering the number of bathrooms and the overall hot water demand.

Here are a few tips for recommended boiler kilowatt (kW) outputs:

  • 1 – 2 bedroom home with 1 bathroom = 24 – 29 kW
  • 2 – 3 bedroom home with 2 bathrooms = 28 – 34 kW
  • 4+ bedroom home with 2+ bathrooms = 35 – 42 kW

If you have a larger home, check out our guide on the best 40 kw combi boilers for the top options on the market.

Comparison with other boilers

If you’ve ever wondered what’s the difference between a condensing boiler vs combi boiler, or how a combi boiler differs from a system boiler, read on.

Combi boilers

  • No need for a separate hot water cylinder or cold water storage tank
  • Ideal for smaller homes
  • More energy-efficient
  • Provide instant hot water
  • Combi boilers are a type of condensing boiler since they use condensing technology

System boilers

  • Require a hot water storage cylinder but no cold water tank – ideal for homes with limited loft space
  • Good for homes with higher hot water demand
  • Require more space than combi boilers
  • More efficient than regular boilers but less so than combi boilers

Regular (conventional) boilers

  • Require both a hot water cylinder and a cold water tank
  • Good for homes with multiple bathrooms
  • Need the most space due to the two tanks
  • Are less efficient than combi and system boilers


Here are average combi boiler costs:

  • Cheapest combi boiler = £500
  • Mid-range combi boiler = £900 – £1,200
  • High end combi boiler = £2,500

You also need to consider combi boiler installation costs.

Combi boiler loans are available as an alternative option:

  • A £2,000 loan is interest-free if you pay it off within 0-4 years, it costs around £83 per month.
  • If you pay the loan over 10 years, a 9.9% interest rate will be added but will only cost about £26 per month.

Maintenance and longevity

  • The typical lifespan of a combi boiler is between 10 and 15 years.
  • Services and maintenance should be done by professional boiler specialists at least once a year. This ensures your combi boiler continues to run smoothly.
  • You can do some DIY maintenance on your boiler, so it is good to know how to drain a combi boiler. However, if big issues arise, it is best to call Eco Happy to get expert advice.

Popular combi boilers

There are plenty of great combi boilers on the market. Knowing about the best 30 kw combi boilers, or being aware of the most energy-efficient combi boilers, can help you make the right choice when it comes to upgrading your system.

  1. Worcester Greenstar 2000
  2. Ideal Logic C30
  3. Viessmann Vitodens 100-W
  4. Baxi 800 Platinum 40 kW
  5. Vaillant ecoTec

Keen to learn more about some top-quality boilers on the market? Make sure to read our guide on the best electric combi boilers that will heat your home at a lower cost.


What are the disadvantages of a combi boiler?

The disadvantages of a combi boiler include their reliance on water pressure from the mains supply, the fact that they’re not ideal for high water demand or multiple-bathroom homes, and their limited flow rate. If your demand is too high for your combi boiler, hot water goes cold pretty quickly, so you might have to upgrade to a different type or larger boiler.

How do I know if my boiler is a combi?

If your boiler doesn’t have a separate water tank, provides hot water on demand, and is compact, it’s most likely a combi boiler.


A combi boiler is more than just a high-efficiency water heater; it’s a step towards sustainable living. A combi boiler is a central heating boiler that provides both heating and hot water all in one go. The advantages of a combi boiler surpass traditional boilers as they are more economical and easier on your wallet.

While they aren’t ideal for high hot water demand, having this boiler installed is perfect for smaller homes.

Now that you’ve certainly found your way out of your “central heating pickle,” you can make a more informed decision regarding your central heating. Chat with the team at Eco Happy today to see how these modern combi boilers work!

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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