How Do Combi Boilers Work? Re February 22, 2021

How Do Combi Boilers Work?

Vaillant Ecotec Plus (Heat Only) Boiler

How do combi boilers work?

A combination boiler or ‘combi boiler’ as it’s known to many of us, is a very popular choice when it comes to buying a new boiler for your home. Due to its compact and lightweight size, it can be installed neatly into your kitchen cupboard, out of sight, out of mind. Because combi boilers don’t have a hot water tank or a cold water tank, they are generally the preferred choice for providing heat and hot water in the UK.

What is a combi boiler?

combi boiler combines a water heater and central heating into one single unit, removing the need for any additional hot water cylinders or cold water cisterns like you would need with a system or regular boiler. 

Moreover, combi boilers can also run on different fuels, meaning that they can also be installed in properties connected to the grid and off-grid homes. 

Natural gas 
Natural gas is the most commonly installed boiler. The boiler is run on natural gas which is then supplied directly to the property. 

Oil combi boilers are a popular choice of combi boiler for off-grid properties. However, you would need to accommodate an additional tank on the property to store the oil, which will need to be refilled by a supplier as it runs out. 

LPG, meaning ‘Liquefied Petroleum Gas’, is an alternative fuel type for off-grid properties. LPG is produced during the extraction of natural gas and oil. Again, you will need to have an additional storage tank in your garden for this.

Electric boilers don’t burn fuel to produce hot water or central heating. Electric boilers help reduce climate change as a result of not producing carbon. However, the factories that produce electricity do create carbon, so electric boilers are not always as environmentally friendly as they first appear.

Electrical boilers are not suitable for larger homes or homes with more than one bathroom as they can’t meet the demands for heating and hot water.

How does a combi boiler work?

To put it very simply, with a combi boiler, water is supplied directly from the mains, a gas burner is ignited when the tap is turned on, the heat exchanger transfers the energy to the water which heats it up, ready for you to use. 

Combi boilers are the best boilers for smaller homes with limited space and for those who need hot water and central heating on demand. Modern boilers are designed with a built-in pre-heat system, which helps to deliver hot water even more quickly. This is a small reservoir of water that is kept at a certain temperature, meaning you will have instant hot water as soon as you turn the tap on. 

If the temperature falls below a set point, a combi boiler’s room thermostat will send a signal to the boiler to fire-up so that heat can be pumped into the central heating system. Once the optimum temperature, that you have set, is reached again, the boiler will then modulate (posh word for turn itself down). This helps to maintain the correct temperature and also helps you save energy. 

What is a condensing boiler?

Just to add to the confusion about what is the best combi boiler to buy, there is also the question, what is a condensing and non-condensing boiler? 

Don’t panic, we’ve got your back. 

The law today states that all new boilers must use condensing technology, this means that the worry around whether or not you need a condensing boiler can be removed from your frail shoulders. A condensing boiler is not a type of boiler, it’s technology within the boiler that makes the boiler more efficient.

Condensing boilers have larger heat exchangers and produce additional heat from the fuel, making them more efficient. Non-condensing boilers are 50% less efficient due to the fact that a lot of the heat produced would be lost via the combustion exhaust gases.

Condensing boilers have an efficiency rating of at least 90% making them better for the environment, helping you reduce your carbon footprint, but not only that but also your heating bills!

To put it into context for you, when you spend £1 on fuel, with a condensing boiler, only 10p of it is wasted. However, older, non-condensing boilers, were only around 70% efficient, meaning a lot more would be wasted, costing you more. 

Many of you might be thinking from reading that; what is a heat exchanger? Maybe not, but we’ll go through it anyway. 

What is a heat exchanger?

A heat exchanger does precisely what its name describes. It allows heat to be exchanged between two fluids or substances, usually water or gas, without letting the substances mix together.

When the heating is turned on in your home, with a combi boiler, water is supplied from the mains and circulated around the radiators.

Combi gas boilers burn gas, the heated gas rises towards the heat exchanger that cold water passes through. As the water circulates, the heat is transferred from the gas to the water, which then heats up to effectively warm your radiators or heat water for your tank. 

In simple terms, you start with hot gas and cold water supplied from the mains. Thanks to the heat exchanger, you end up with a cooler gas and hotter water, without the two separate fluids having to meet.

Pros of a combi boiler

  • Cost-effective | Very cost-effective as the water is heated instantly.
  • Compact in size | Perfect for smaller homes with limited space.
  • Less pipework in the home | Cheaper installation due to the fewer pipes being used during fitted. 
  • Simple time and temperature controls | The ability to programme your heating to come on at certain times at your chosen temperature.
  • On-demand hot water | Hot water whenever needed.
  • All-in-one | All necessary elements to deliver hot water to your taps and heat your radiators is included in the boiler casing.
  • Safer & cleaner water | Water delivered straight from the mains so is argued that it is fresher than stored water.
  • Flow rate | Combi boilers can supply up to 18 litres of water a minute at 35 degrees, making them great for flats and smaller homes.

Cons of a combi boiler

  • Reduced water pressure | If more than one outlet needs hot water at any one time, then the water pressure can decline. 
  • No backup | Combi boilers have no back-up immersion heaters to heat the hot water if your boiler breaks down.

What to consider when buying a new combi boiler

Now that you have an idea about how combi boilers work and their benefits, let’s talk about what boiler is best to choose for your home. It is important to consider the following when purchasing a new or replacement combi gas boiler, to ensure that it will meet the demands of your home. 

Available Space

It is important to take into consideration the space available in your home. One of the best benefits of a combi boiler is that it can be installed neatly into a standard kitchen cupboard. A combi boiler also removes the need for any additional storage tanks or cold water cisterns.

Hot water

Hot water usage is an important aspect to consider. Make sure you make note of how much hot water you use that you can ensure that your new combi boiler will meet these demands. 

For most households with a single bath and shower, a 24-30 kW boiler is generally recommended. Where there is an additional en-suite bathroom, it may be advisable to choose a larger 30-35 kW boiler.

Size of property

As well as providing your home with hot water, your boiler will also be your main source for heating, which is why it is essential to choose a boiler that is sufficient enough to provide the heat needed. 

However, it is also important not to purchase a new combi boiler that exceeds the needs of your home, as this can increase your energy bills. Try and avoid choosing a boiler that is too big big for your home, for example, installing a boiler that meets the demands of a home with up to 20 radiators when you only have 8. This will only result in a more costly energy bill and is a waste of gas and electricity.

What size kW boiler do I need?

The general rule of kW is that the more heat and hot water you require for your home, the higher the kW boiler you will need. We will discuss this in more depth throughout the article. 

Households with a higher demand for hot water, such as homes with a bath, will need a boiler with a more powerful kW size than smaller homes. 

Not sure what kW means? Well, we have a blog that will explain exactly that. Click here for the whole blog.

Many of us are not professionals when it comes to boilers, and don’t know what size kW boiler we need to meet the demands of our homes. We have written a whole blog explaining what size kW boiler are best suited to all homes. If you want to read our article on the ‘What size kW boiler do I need to heat my home?’ blog, click here.

However, here’s a little table for reference, just for now. 


1 Bath / Shower 2 Baths / Showers
2 bedrooms26kW


3 bedrooms29kW


4 bedrooms


5 bedrooms


Combi boilers are suitable for most homes!

It would be best to replace your combi every 10-15 years, as a new combi will help you heat your home faster and more efficiently. That’s because boilers, in general, tend to lose energy efficiency over time, costing you more to run. And let’s be honest, after 10-15 years, you’ve got your money’s worth!

But why should I get a new boiler?

In addition to having a brand new boiler that makes your home warm in minutes, there are other benefits to replacing your old boiler. 

According to the Energy Saving Trust, you could save up to £305 per year by upgrading your old boiler to an energy-efficient A-rated boiler, which typically operates at 90% and over energy efficiency – and it is much better for the environment too! 

Use this tool to calculate the energy efficiency of your current boiler and see the savings you could be having!

Top 5 best combi boilers on the market

Now we know exactly how combi boilers work, we have decided to share the top 3 rated combi boilers that are not only reliable but also cost-effective and budget-friendly. 

If you are interested in what the best boilers to buy in 2021 are, we have covered it in a blog, click here to read to our ‘Best boilers to buy in 2021 blog’

Viessmann Vitodens 100-W Combi Gas Boiler

Viessmann Vitodens boiler
  • Quiet operation
  • Up to 94% of the fuel used is converted into heat 
  • High-quality stainless steel Inox-Radial heat exchanger 
  • Low CO2 emissions 
  • Easy to use controls 
  • LCD touch screen display
  • Optional ViCare thermostat 
  • WiFi interface included

Worcester Bosch Greenstar 4000 Combi Gas Boiler

  • Wide power range to help reduce gas consumption and lower energy bills 
  • Quick Tap water savings function
  • Modern, sleek design
  • Wireless connectivity 
  • Instant hot water 
  • Ideal for small homes with limited space
  • Compact design
  • 100% planet-friendly packaging 
  • Operates well in low water pressure areas

Ideal Logic 24kW Combi Gas Boiler

Ideal Logic Boiler
  • Ideal for homes with up to 15 radiators
  • Low-lift weight of 28.9kg meaning easy installation
  • Easy to see pressure gauge
  • Built-in filling loop 
  • Simple diverter valve and two-stage flame sensor to help maximize reliability 
  • Built-in frost protection 
  • Large display 
  • Easy to use controls 
  • Automatic bypass 
  • The latest generation of Honeywell gas valve

Worcester Bosch 2000 25kW Combi Gas Boiler

WB 2000
  • Ideal for homes with up to 15 radiators
  • 36kg lift weight 
  • Worcester Bosch’s quietest boiler to date
  • Optional flue pipe location
  • Return copper tails included 
  • Easy access due to removable panels
  • Quick installation
  • Easy to use controls 
  • Easy to read display screen

Worcester Bosch Style 8000 30kW Combi Gas Boiler

  • 0% interest-free for 2 years if bought from WarmZilla
  • Longest guarantee of 10 years
  • Unique intelligent filling system
  • Wireless connectivity 
  • Colour options (Black or White)
  • Easy to use and read boiler display
  • Easy access due to removable side and top panels
  • Ideals for homes with up to 15 radiators.
Worcester Bosch Lifestyle boiler

Top 5 Combi Boiler Comparison Table


Viessmann Vitodens

100-W 25kW

Worcester Bosch Greenstar 4000 Combi 25kWIdeal Logic Combi 24kW        Worcester Bosch 2000 25kWWorcester Bosch 8000 Style 30kW
Efficiency Rating 94%94%94%93%94%
Band RatingA-RatedA-RatedA-RatedA-RatedA-Rated
Flow Rate12 l/min9 l/min9.9 l/min9 l/min10.7 l/min 
ErP RatingA Heat 
A Water 
A Heat 
A Water 

A Heat 
A Water 

A Heat 
A Water 
A Heat 
A Water 
Guarantee 12 years 10 years8 years 5 years10 years 

Here at WarmZilla, we offer different finance solutions if needed. For a boiler quote in 90 seconds, click here! 

How long do combi boilers last?

The longevity of your boiler really depends on how well you maintain them. However, the quality of your boiler will vary so it is important to consider the efficiency rating when looking for a new or replacement boiler.

Generally, a combi boiler’s lifespan is between 10 and 15 years, but again, it depends on how well you maintain it. Getting an annual service and regular maintenance when necessary is essential when keeping your boiler in good nick.

If you are looking for more information on how to to keep your boiler in good condition, we have a blog called ‘7 Ways to increase boiler lifespan’.

Why should you buy a new boiler from WarmZilla?

When you purchase a new or replacement boiler with WarmZilla, your boiler package will also include;

  • Your chosen boiler, along with the necessary flue kit needed for your central heating system
  • Installation by a qualified Gas Safe Engineer 
  • Removal of your old boiler and parts
  • A FREE system cleanse of your central heating system
  • Chemical Inhibitor for added protection for your new boiler
  • 10-years guarantee, which will be registered by WarmZilla after installation
  • A FREE magnetic system filter for ongoing protection
  • The Neomitis RF Digital Wireless Room Thermostat

At WarmZilla, we’re here to help you find the best boiler for your home at the best price, that’s why we’ve written blogs about the best combi boilersbest system boilers and best conventional boilers in 2022. So make sure to check them out!

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