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1,570 reviews

How Do Combi Boilers Work?

Nick Geary
By: Nick Geary
Updated: 6th July 2022

How do combi boilers work?

Combi boilers work by taking water directly from the mains supply to provide instant hot water to taps and showers at a strong pressure.

As compact units that don't need any external tanks or cylinders, one of the reasons why combi boilers have become the boiler of choice for homeowners across the UK.

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What is a combi boiler?

There are 3 different boiler types that all work in different ways:

  • Combi boiler
  • System boilers
  • Regular boilers

Both system and regular boilers store hot water in a hot water cylinder to be delivered to taps as and when needed. This is great for delivering hot water to multiple taps at the same time but when the cylinder is empty, you'll have to wait for it to fill up again. Regular boilers also need a cold water tank and feed and expansion tank in the loft which delivers water down to the boiler to be heated. System boilers, like combis, heat water from the mains water supply.

Combi boilers do away with any need for cylinders or tanks altogether, instead heating water directly from the mains and delivering it directly to the tap.

Find out more: What is a Combi Boiler?

How do combi boilers work?

By turning the central heating or a hot water tap on will instantly fire the combi boiler into action. Cold water from the mains will pass through the boiler where it's heated up and then travels through the pipes to either the radiators, tap or shower.

Unlike system and regular boilers, there are no cylinders or tanks. Combi boilers will heat up and deliver hot water instantly as and when needed.

Combi boiler types

Combi boilers can run on different fuels, making it possible for them to be installed in properties connected to the grid as well as off-grid homes.

Natural gas

The most commonly installed type of combi boilers run on natural gas which is supplied directly to the property and doesn't have to be stored onsite.


Oil has been the popular choice for off-grid properties for a number of years. It has to be stored in a tank at the property (which will see the combi system lose some of its space-saving credentials) and need refilling by a supplier as and when it runs out.


Liquefied Petroleum Gas (LPG) is another alternative for off-grid properties. Produced during the extraction of natural gas and oil as well as oil refining, LPG needs to be stored in a tank in the garden.


Electric boilers could well be part of the future of home heating as they don't burn fuel to produce central heating and hot water. By burning fuel such as gas and oil, carbon is released into the atmosphere which is a leading cause of climate change. Burning fossil fuels simply isn't sustainable and electricity offers a solution.

They can't meet high demands for heating and hot water so are best suited to smaller homes and flats with a single bathroom. And while electricity is typically expensive you could save on running costs by Running an Electric Combi Boiler with Solar PV.

Advantages of combi boilers

There are a number of advantages to having a boiler that delivers heating and hot water on demand.


With a combi boiler, you'll be saving a great deal of space around your home over having a system or regular boiler installed. Those bulky tanks and cylinders can take up an awful lot of space but thanks to combi boilers being single-combined units, all you'll need to have installed is the boiler.

Straightforward installation

As all-in-one units that don't need to be installed alongside any cylinders or tanks, the installation of a combi boiler is relatively straightforward.

Mains water pressure

Regular boilers (also known by the names conventional, traditional and heat-only), rely on gravity to deliver water down from the loft, giving relatively weak pressure. By taking water from the mains and delivering it directly to taps and showers, you'll be able to enjoy mains water pressure.

Note: It's worth having a professional check that the mains water pressure at your property is suitable for a combi before having one installed.

Lower energy bills

Combi boilers are well known for their efficiency. The best combi boilers are at least 93% efficient which means that for every £1 spent on heating your home, only 7p is spent on wasted energy (all gas and oil boilers produce waste energy during operation). This could see you make a considerable saving if you're looking to replace an old boiler with an efficiency rating in the region of 70%.

When it comes to system and regular boilers, lots of energy is spent heating up the water in a cylinder. As combi boilers have no need for a cylinder, you'll be saving energy there too.

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Downsides to combi boilers

For all of their advantages, combi boilers aren't the best heating system for all homes. In most cases, they're best suited to smaller homes with one bathroom so if you live in a larger property you're probably better off turning to a system or regular boiler.

Should the mains water pressure at your property be weak then the water that comes out of the taps and showers will be too. If you're set on a combi then a pump can be installed to increase the pressure but this will add to the costs.

Also, as combi boilers are a relatively new type of boiler, they're not always compatible with older pipework as they were designed for weaker water pressure. So if you're replacing an old heating system you will need pipework and radiators that can handle higher pressure. Your installer will be able to advise and recommend the best heating system for your home – get free quotes today.

Which is the best combi boiler?

The best combi boilers with a great reputation for efficiency, reliability and performance are:

  • Baxi 800
  • Ideal Logic Plus
  • Vaillant ecoTEC Plus
  • Viessmann Vitodens 200-W
  • Worcester Bosch Greenstar i

We compared the key features of each in our guide to Which is the Best Combi Boiler to help with your decision-making when it comes to finding a new combi for your property.

How do storage combi boilers work?

One of the biggest advantages of combi boilers is that they save space by having no need for a hot water cylinder. While this is great for smaller homes, properties with a high demand for hot water will need a cylinder to store hot water. This is because the supply from a combi boiler will weaken when being supplied to more than 1 tap at a time.

Storage combi boilers solve this problem by having a hot water cylinder within the unit to store water. So you'll still be saving space while also meeting a higher demand for hot water.

There aren't currently too many storage combi boilers on the market with the most popular being the Vaillant ecoTEC Plus 938, Glow-worm Energy 35 and Viessmann 111-W.

How much do combi boilers cost?

Combi boilers are the most cost-effective heating solution and tend to be priced at around £500 to £1,500 before factoring in the installation. If we go back to our list of the best combi boilers, we'll compare the potential cost of each.

Combi Boiler Potential Cost (Before Installation)
Baxi 800 £900 – £1,200
Ideal Logic Plus £700 – £900
Vaillant ecoTEC Plus £1,000 – £1,400
Viessmann Vitodens 200-W £1,445 – £1,565
Worcester Bosch Greenstar i £800 – £950

Combi boiler installation

On top of the cost of the unit itself, you'll also need to consider the installation. A typical combi boiler installation will be priced at roughly £500 – £700 but depending on the complexity could rise to somewhere in the region of £2,000.

To make sure that you get the most competitive price, we highly recommend comparing quotes from multiple heating engineers. At Boiler Guide, we've made it easier than ever to connect with heating engineers in your area.

Simply take a few moments to pass through our online form and we'll have everything we need to start matching you with combi boiler installers near you. You'll get free quotes from up to 3 fully-qualified engineers with no-obligation to accept them.

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Nick Geary

About the author

Nick Geary

Nick has a wide range of experience writing about conventional and renewable heating solutions, and is always increasing his knowledge by researching any new heating technologies on the horizon.

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