What is a Combi Boiler
And how do they work?

Combi boilers are the most common type of boiler in UK households for good reason. Not only are they cost-effective, but they also take up less space than other types of boilers. Read on to find out all you need to know about combi boilers and if they are the right type of boiler for your property and needs.

Written by Kojo Prempeh

Last updated - 08/11/2022

Estimated reading time - 6 minutes

What is a combi boiler?

A combination boiler, or combi boiler, is a combination of a water heater and a central heating boiler in a single unit. When you turn on a hot water outlet, such as a tap or shower, a combi boiler takes water directly from the mains water supply and heats it almost instantly. This means that combi boilers don’t need a separate hot water storage cylinder to provide hot water.

How does a combi boiler work?

Combi boilers work by taking water from the mains water supply and heating it up within a matter of seconds when needed.

When you turn on your hot water or turn up your thermostat, depending on what fuel your combi boiler uses, it will start to burn gas or oil in its combustion chamber. The hot gases emitted from the burning of the fuel move through a series of pipes before entering and heating a coil inside a heat exchanger.

A heat exchanger does exactly what its name suggests, it exchanges heat between two fluids or substances without the substances mixing together, in this case, gas and water.

Water from the mains enters the heat exchanger and surrounds the coil. The coil, which has been heated by hot gas, heats the surrounding water before it gets sent to your radiators. Once the water has circulated through your radiators, it gets sent back to the boiler, where it gets reheated and used again.

The water that circulates through the central heating system is undrinkable. To provide you with drinkable water, when you turn on a hot water tap your boiler pauses the central heating and diverts the water to a secondary plate heat exchanger which heats the sealed pipes that deliver clean water to your hot water outlets.

What are the advantages of a combi boiler?

Hot water on demand


Combi boilers heat water straight from the mains water supply to provide you with hot water on demand, whereas regular and system boilers heat a large amount of water and store it in a hot water cylinder for later use. Once this water cools down, regular and system boilers will have to reheat it to provide some hot water again.

Not only is heating water as and when needed convenient, but it is also the most cost-effective way of providing hot water.

Highly efficient


A new condensing combi boiler is typically between 90% and 94% efficient, whereas many older combi boilers are between 70% and 80% efficient. To put this into money terms, for every £1 of fuel an older 70% efficient combi boiler burns, only 70p worth is used to heat a home, and the other 30p worth is wasted and expelled out of the chimney. A modern combi boiler that is 94% efficient will only lose 6p worth of the fuel it burns to heat a home.

A condensing boiler recovers heat from flue gases and recycles it back into the heating system to pre-heat the cold water entering the boiler. Instead of capturing heat from the gases in the flue, older non-condensing boilers expel it, which means this heat is wasted.

Compact size


A combi boiler is able to deliver the hot water needed for a household’s taps and central heating, all within one single compact unit, whereas a system boiler requires a hot water cylinder to operate, and a regular boiler needs a hot water cylinder and a cold water tank to be able to produce hot water for a home.

The ability to produce hot water in a single unit without the need for external components that take up valuable living space means that a combi boiler can be installed in more places in a property, with some models small enough to fit in a standard kitchen cupboard.

Faster, easier, and cheaper to install

A combi boiler is often easier, cheaper, and faster to install than a system or regular boiler. This is because a combi boiler has the ability to heat water on demand within a single unit, whereas for a system boiler to function, a hot water cylinder needs to be installed, and a hot water cylinder and a cold water tank need to be installed for a regular boiler to operate.

As combi boilers have fewer external parts to install, they are often the fastest, easiest, and cheapest type of boiler to install.

What are the disadvantages of a combi boiler?

Not the best choice for large homes

Although combi boilers come in a range of different power outputs to suit a number of different sized properties, they may not be the best choice for larger properties with very high hot water demands. Combi boilers heat water on demand, and if the demand is too high, the water pressure to each water outlet will be reduced considerably.

If you have high hot water demands, then a regular or system boiler may be a better choice for you, as they store large amounts of hot water for later use.

Needs good mains water pressure

As combi boilers take water directly from the mains water supply, the pressure at which the water comes out of your home’s taps and showers will depend on the pressure from the mains. This means that households with weak mains water pressure will have a weaker flow of water to hot water outlets. Also, if multiple hot water outlets are in use at the same time, the flow rate to each tap or shower will drop.

the water pressure is set by the mains water supply, you also won’t be able to have a power shower if you have a combi boiler.

The options for increasing water pressure in a combi boiler are limited. Some other types of boilers are able to install a pump to increase the water pressure, but that is not possible with combi boilers as they receive water straight from the mains water supply.

To try and achieve the best water pressure available, installing a boiler with a high flow rate is a good option. A boiler’s flow rate is how much hot water it will be able to send to taps and showers in one minute. The mains water flow rate to most UK homes is between 10 and 15 litres per minute, so a combi boiler with a 15l/min flow rate will deliver the best possible water pressure. However, the boiler’s flow rate is limited to the flow rate of the mains, so if the mains flow rate is 12l/min and the boiler’s flow rate is 15l/min, the boiler will only be able to deliver a flow rate of 12l/min.

How much does it cost to buy and install a combi boiler?


The cost of buying and installing a combi boiler depends on the manufacturer, model, size, and complexity of the installation, but you can expect to pay somewhere between £800 and £4000 to buy and install a modern combi boiler.

The cost of installation varies depending on a number of factors, but mainly depends on the type of boiler that it will be replacing. If you are replacing a system or regular boiler with a combi boiler, the boiler installer will have more work to do, such as converting the system to accommodate a combi boiler and removing the water tanks to free up some space, which will cost more than a like for like boiler swap.

A Gas Safe registered engineer will be able to assess your property and give you an estimate of how much it will cost to replace your current boiler with a new combi boiler.

Although the initial cost of buying and installing a combi boiler might seem high, it could save you hundreds of pounds a year on your heating bill, depending on the heating system it is replacing. According to the Energy Saving Trust, replacing an old 70% efficient boiler with a 90%+ efficient boiler could reduce the fuel bills in a gas-heated mid-terraced house by up to £400 a year.

A new combi boiler can last up to 15 years, and with proper maintenance and an annual boiler service, the lifespan of the boiler could be extended further, so the possible long term savings could outweigh the initial purchase price of a new combi boiler.

Get a new boiler.

Has your boiler seen better days? With Smart Plan you can get a new boiler for as little as £30 a month, with a 10 year service plan included.

OR 020 8146 3960