What Is A Combi Boiler And How Does It Work?

Last Updated on January 27, 2025

Combi boilers, short for combination boilers, have become one of the most popular choices for heating systems in homes across the UK. Offering an efficient, compact, and cost-effective solution, these boilers combine central heating and hot water in a single unit, eliminating the need for separate water tanks or cylinders.

Whether you’re upgrading your heating system or exploring options for a new build, understanding how combi boilers work can help you make an informed decision for your home.

In this article, we’ll break down what combi boilers are, how they function, and why they might be the ideal choice for your property.

What Is A Combi Boiler?

A combi boiler is a modern heating system that combines two key functions in one compact unit: providing hot water on demand and heating your home. Unlike traditional systems that rely on hot water storage tanks or immersion heaters, a combi boiler heats water directly from the mains whenever you turn on a tap or switch on the central heating.

This design makes combi boilers highly efficient and space-saving, as there’s no need for bulky tanks or cylinders. This type of boiler is particularly well-suited to smaller properties or homes with limited storage space, but they’re also capable of meeting the demands of larger households if chosen with the right capacity.

Combi boilers are powered by natural gas, electricity, or oil, and they come in a range of sizes to suit different property types. Their all-in-one design not only simplifies installation but also reduces energy waste, as you’re only heating water when you need it.

This means that homeowners can save money on energy bills while reducing their carbon footprint. 

How Does A Combi Boiler Work? 

The magic of a combi boiler lies in its ability to instantly provide hot water and heating without the need for a storage system. Here’s how it works:

Hot Water On Demand

When you turn on a hot water tap or shower, the combi boiler detects the demand and begins heating water directly from the mains. A heat exchanger inside the boiler transfers heat from the burner to the water, ensuring a continuous flow of hot water within seconds. Once the tap is turned off, the heating stops, which saves energy compared to storing hot water that may not be used.

Heating Your Home

For central heating, the combi boiler heats water that is then circulated through radiators or underfloor heating systems. A built-in thermostat and timer allow you to control the temperature and schedule, ensuring your home stays warm when needed.

No Storage Tanks Required

Unlike system boilers that rely on a separate hot water cylinder or cold water storage tank, combi boilers work directly with the mains water supply. This not only reduces installation costs but also frees up space in your home.

Built-In Efficiency

Modern combi boilers are designed to be highly energy-efficient, often achieving an efficiency rating of over 90%. Many models are also equipped with condensing technology, which captures and reuses heat that would otherwise be lost through the flue, further reducing energy consumption and lowering utility bills.

Types Of Combi Boilers 

Combi boilers come in various types to cater to different home setups, energy sources, and heating requirements. Choosing the right type of combi boiler for your property ensures optimal performance, efficiency, and comfort. Here are the main types of combi boilers and what sets them apart:

Gas Combi Boilers

Gas combi boilers are the most common type in the UK, as they use natural gas to provide heating and hot water. They are highly efficient and cost-effective, especially in areas with access to the gas mains. Modern gas combi boilers are equipped with condensing technology, which recycles heat from the flue gases, making them even more energy-efficient.

Best suited for: Homes connected to the gas mains looking for a reliable and economical heating solution.

Electric Combi Boilers

Electric combi boilers use electricity to heat water, making them an excellent choice for properties without access to the gas grid. They are compact, quiet, and environmentally friendly since they don’t produce emissions during operation. However, running costs can be higher compared to gas boilers due to electricity prices.

Best suited for: Smaller homes, apartments, or off-grid properties where gas is not an option.

Oil Combi Boilers

Oil combi boilers are a great alternative for properties in rural areas where mains gas is unavailable. They use oil stored in an external tank to provide heating and hot water. While they offer efficient performance, you’ll need to arrange for regular oil deliveries and ensure sufficient storage space for the tank.

Best suited for: Rural homes or properties with larger heating demands and sufficient outdoor space for an oil tank.

LPG Combi Boilers

LPG (liquefied petroleum gas) combi boilers are similar to gas combi boilers but run on bottled or tanked LPG instead of mains gas. They are an efficient and versatile option for homes without access to the gas grid. Like oil boilers, they require external storage for the fuel.

Best suited for: Homes in off-grid areas seeking a cleaner-burning alternative to oil.

Compact or Slimline Combi Boilers

Designed for homes with limited space, compact or slimline combi boilers are smaller and more lightweight than standard models. Despite their size, they still offer the same functionality, making them ideal for properties where space is at a premium.

Best suited for: Flats, apartments, or homes with limited installation space.

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.

What To Consider When Purchasing a Combi Boiler

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

Pricing

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

FAQs

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.

Conclusion

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!