Combi Boilers vs Conventional Boilers: Pros & Cons

The combination boiler, more commonly known as a ‘combi’, is the most popular type of gas boiler in the UK. They are small and very energy-efficient making them a great choice for any property from studio flats to medium-sized houses. They are easy to install, cost-effective and simple to run. So what are the pros and cons of combi boilers?

If you are considering replacing the heating system in your home they could be a great choice and just what you need. To help you make the switch we’ve put together this complete guide to combi boilers.

So, what is a combi boiler?

In the past, most homes were heated by a boiler and hot water was provided by an immersion heater inside a large water storage tank. This took up a lot of space and required a large amount of pipework running through the property. A modern combi boiler removes the need for these steerage tanks and provides both heating and hot water directly from one single wall-mounted unit. 

System boilers and older conventional systems stored hot water in a separate cylinder or tank. Combi boilers are much smaller and more efficient. There is usually no storage cylinder and the water is heated up as it passes through the heat exchanger of the boiler, making it a great space saver.

Domestic gas and LPG combi boilers are able to heat in the region of 10 to 20 litres of mains water per minute. Though east results depend on their capacity and power rating. They’re linked to your mains water supply and provide heating and hot water as soon you turn on your tap.

So what are the pros and cons of combi boilers?


  • They are cheaper and easier to install.
  • They take up less space as they don’t require a separate tank.
  • You get hot water on demand without preheating.
  • You only heat the water you want to use. This cuts down on energy use and thus lowers your emissions.
  • They are usually cleaner, the lack of water storage helps prevent the build of rust and dirt in your pipes.


  • Combi boilers can occasionally suffer from low pressure. They don’t have a built-in pressure pump and rely entirely on the mains water pressure. If you turn on a tap while using the shower the pressure is shared between the two and can result in a slower flow. Especially on the upper floors of multi-story apartment buildings.
  • Larger houses and properties, especially those with multiple bathrooms, can’t run properly on combi boilers alone. They usually need a system boiler to provide sufficient hot water and heating.

Do all combi boilers use gas?

Combi boilers run on a number of different fuels including:

  • Natural gas
  • Liquid Petroleum (bottled gas)
  • Oil
  • Electric

Gas boilers are most commonly wall-mounted units that are fitted in a cupboard. While they are not very loud they are usually hidden away for aesthetic purposes. After all few people have a desire to include a boiler in their home decor. Domestic gas and LPG combi boilers can heat between 10 and 20 litres per minute depending on their capacity.

Oil powered boilers are mostly floor-mounted and are usually the size of a small fridge. Just like gas-powered models, they heat water instantaneously but, due to the energy or ‘calorific’ value of oil, they regularly depend on a small preheated reservoir to bridge the gap between activation of the boiler and the water reaching its intended temperature. 

Boilers powered by electricity have a much lower heat flow rate than fossil fuel models. They are more expensive to run as gas is commonly cheaper than electric energy. Usually, the only reason to choose an electric boiler is a lack of gas supply to your location. They are usually limited to around 1/3 the power of gas boilers. 

For example a 24 kW gas boiler heat 11 litres of water to 35°C every minute. An electric model is usually limited to about 8kW. This means it can really only provide roughly three to four litres per minute. This may be enough for studio or single bed flats but larger properties or those with multiple residents will need something more powerful to avoid running out of water halfway through the morning showers.

How do they work?

Combi boilers usually have two heat exchangers. The first provides hot water and the other powers your central heating. The heat exchanger is part of the boilers internal combustion chamber and heats your mains water supply by about 35°C as it passes through the boiler.

When you turn on a tap it engages a sensor which turns on the unit and instructs it to begin during your fuel and heating the water. Some units even keep a small 10-15 litre supply preheated to cover the gap between the sensor engaging and the heat exchanger reaching its full temperature. This way you always have hot water when you need it.

This is where the size and power of the boiler itself come into play. Your flow rate, how much water the unit provides, depends on its kW power rating. typically, a 24kW boiler delivers around 11 litres a minute and would suit properties with 2-4 bedrooms.

How much does a combi boiler cost?

The cost of a new combi system depends on several factors:

  • The model/brand you choose
  • What system you are replacing
  • Any extras like smart metres etc
  • The location of the property
  • Where you want to locate the new boiler in your home

Depending on size, power and brand you should expect to pay between £1000 and £2000 for a new combi boiler. This is a little higher than the price for a comparable system boiler due to the need for two heat exchangers rather than one.

Switching to a combi boiler means removing your order system which can increase the overall price to between £2000 and £3000.

However, don’t let that put you off. Installation and maintenance are usually a lot cheaper with a combi boiler. Without the need for separate tanks and a lot less piping, combi boilers are far easier to fit. This gives you a lot more options on where you want to locate the system within your property. While it may seem an expensive change there is the advantage that after it’s fitted you should start to see savings in your monthly energy bills fairly quickly.

Does a combi boiler work with underfloor heating? 

Yes, combination boilers work perfectly well with underfloor heating systems. If you have a lot of underfloor piping it may be advisable to fit a small buffer tank and it is definitely something to mention to your engineer when you’re choosing a unit. They may be able to advise you on your power needs if your underfloor heating covers a larger area. 

Can I get a smart thermostat? 

Yes, in fact, that’s often the best way to improve your overall efficiency and keep costs low. 


So, Combi boilers can be better value for money while still providing the immediate hor water supply you need. If you’re in a larger property you might want a bigger system but most small to medium-sized houses will be fine with a single combi unit.

If you’d like more information on any kind of boiler we can help. Our team are ready to take your call and can help you find the right heating system for your home or business. Get in touch today.

Leave a Comment

Your email address will not be published. Required fields are marked *