Category - Boilers
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Will Scholfield, Engineer

31 Jan : Updated 5 Dec ● 6 min read

Combi boiler pros & cons: a simple guide

A combination boiler or combi boiler works differently from a conventional boiler in that it is a stand-alone heating system that works as a water heater and central heating - in a single, compact unit that does not require a hot water cylinder or a cold water supply tank.

Combi boilers deliver hot water on demand, without a hot water cylinder found in conventional boilers. This means that you do not have to wait for large volumes of water to heat up when you open the hot water tap - like you sometimes have to do with conventional boilers. You can jump in the shower and have instant hot water!

This article will look at combi boiler pros and cons and why a combi boiler might be the right choice for your home.

The pros and cons of combi boilers

A combi boiler differs significantly from a conventional boiler and comes with its own list of advantages and disadvantages. Let's look at a list of combi boiler pros and cons.


Combi boilers are a popular choice in the UK and are often the preferred boiler type over conventional boilers in apartments and smaller houses. Here are some of the advantages of a combi boiler.

Highly efficient

Compared to a conventional boiler or older heating systems, a combi boiler is incredibly efficient and can help save you a lot of money each year on your water heating bills.

Combi boilers are highly efficient in energy usage, with modern models having a 90+% energy efficiency rating. 

With about 55% of the energy going towards central heating when using conventional boilers, having a combi boiler with high energy efficiency will reduce the amount of energy wasted on your heating system.

This means your house will be warmer while using way less energy and will also help you save money on energy bills.


Combination boilers have fewer parts and pipes to maintain than conventional boilers, meaning that they are fairly affordable to install and maintain. If there are any issues or breakages, it is also easy to find affordable parts to fix any problems so booking a repair with an engineer is no hassle.

Instant hot water when you need it

Running out of hot water is a nightmare for many, especially in winter. A conventional boiler system may sporadically run out of hot water if there is significant demand. The system cannot heat the freezing water temperatures fast enough.

A conventional boiler runs out of hot water because the hot water actually comes from your stored hot water tank or cylinder. Once the cylinder runs out of hot water, it takes a while to refill it and reheat the large volume of stored water.

A combi boiler never runs into this issue, as it does not have a hot water tank that can empty and needs to be refilled. Instead, a combination boiler can provide instant hot water by heating the water as soon as you turn on the hot water tap or set the thermostat.

This means that you are unlikely to run out of hot water unless the boiler breaks.


Because a combination boiler does not require a tank to store hot water like a conventional boiler, these boilers are much smaller and more compact. This makes them the perfect water heating solution for smaller homes and apartments.

There are variations to combi boilers that use a tank, but these are only installed when in homes with high hot water demands. Most combination boilers are small, with no extra tanks, meaning they can be installed in a cupboard or loft space.


Because a combi boiler does not require a separate water storage tank, the water from a combi boiler system is safe for human consumption.

This is because the water flows straight from the mains, whereas a conventional boiler system stores cold water in a tank before it flows through the taps, resulting in bacterial growth if the tank is not cleaned. 

Read: Do you need a water tank with a combi boiler?

Easier to install

A combi boiler is very easy to install - especially when compared to a conventional boiler - because it comes without complicated parts that require separate installation, like a hot water cylinder and multiple pipes.

This makes a combination boiler easier to install, and installation will cost less than a traditional boiler. 


Although combi boilers have a lot of advantages, there are also some disadvantages to keep in mind if you are considering replacing your conventional boiler with a combi boiler.

Needs good mains pressure

Most conventional boilers use gravity to deliver water to the house, so they are installed in loft spaces.

In contrast, combi boilers depend on good mains pressure because water is taken directly from the mains and heated. If your house has poor water pressure because of a lack of pressure in the mains, the combi boiler will not work efficiently.

There are ways around this, such as investing in a booster pump to increase the water pressure in the house.

Use restrictions

Combi boilers are limited in their use. These boilers can only deliver hot water to one faucet at a time, and if you turn on a warm water tap while running a shower, you will lose a lot of pressure or suddenly enjoy a colder shower.

Combi boilers are inefficient for large houses with lots of people that require hot water at the same time.

No immersion heater

Combi boilers do not have immersion heaters like those found in the hot water tank in conventional boilers. If the boiler is not working, you will not have any central heating or hot water until you fix your boiler.

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Is a combi boiler the right choice for you?

Combi boilers work by providing instant hot water, and although this sounds amazing, you need to consider the combi boiler pros and cons to decide whether these boilers are the right choice for you and your home.

Combi boilers are compact and do not take up a lot of space, making them the perfect addition to a smaller home or an apartment with fewer hot water demands.

On the other hand, these boilers require good mains pressure and are limited in the amount of hot water they can provide at the same time, meaning they are not the best choice to heat water in larger homes that require a large volume of hot water supply.

If you are looking into a replacement boiler for your current system, a combi boiler system is the right choice for you if:

  • Your hot water demands are such that you do not require an additional hot water cylinder
  • You have good mains pressure
  • You do not have a power shower

Frequently asked questions

Although the combi boiler pros and cons list is extensive, the advantages definitely outweigh the disadvantages, especially if you live in a smaller household.

These boilers provide hot water on demand, are small in size, and are easy to install and service. Although these boilers might not be the perfect choice for larger homes, there are workarounds like installing a booster pump to increase pressure.

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