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Condensing boilers vs combi boilers 

Condensing boilers vs combi boilers: what’s the difference?

Joe Minihane
Written by  Joe Minihane
Jonathan Leggett
Reviewed by  Jonathan Leggett
Updated: 29 Aug 2023

Understanding how condensing and combi boilers differ is vital when it comes to replacing your existing heating system.    By doing so, you’ll be able to work out exactly what you need and how much you’re likely to have to spend to have your old boiler replaced. 

Are combi boilers and condensing boilers the same thing? 

A combi boiler is a type of condensing boiler. Since 2005, all boilers sold in the UK have had to be condensing boilers.   

Condensing boilers use a special system to capture water vapour produced while the heating is in use, then recycling it to deliver additional heating via a dedicated heat exchanger.  

Known as a flue gas recovery system, it prevents additional heat being lost, improving boiler efficiency. It also lowers bills in the process, as it lessens the demand for gas. 

What is a condensing boiler? 

A condensing boiler covers all of the main kinds of gas boilers now sold in the UK, including combi, system and regular boilers. They have been legally required since 2005. 

While condensing boilers use gas to heat up water and supply it to radiators around the home to deliver heating, they also take water vapour produced in this process and then recycle it through a heat exchanger to deliver additional heat without having to burn more gas. Before condensing boilers, some 30% of the heat a boiler produced was wasted.  

Now, all condensing boilers must be A rated, scoring over 90% for efficiency. 

How does a combi boiler work? 

A combi boiler is a type of condensing boiler. However, while it still recycles water vapour in order to create additional heat and ensure maximum efficiency, it can also supply instant hot water without the need for additional water cylinders.  

That’s because combi boilers get water directly from the mains, rather than drawing water from a cold water tank, before sending it to a warm water cylinder for use around the home. That means you don’t have to wait in order to get hot water - simply turn on the tap or shower and the boiler will fire up, delivering hot water in seconds.  

However, combi boilers do have their downsides. Try using multiple showers or taps at the same time and you often find that the water pressure drops and water may take longer to heat. 

Do other condensing boilers need a water tank? 

Regular and system boilers do require water tanks, yes. That’s because these they receive their water supply from a cold water tank, before feeding hot water into a dedicated hot water cylinder, ready for use around the home.  

That means you’ll need space for both kinds of tanks, with the hot cylinder usually found in an airing cupboard or utility space. While not quite as efficient as combi boilers, which only use the water you need, regular and system boilers have the advantage of letting you run multiple taps, showers and appliances at once, without losing water pressure.  

Choosing a boiler for your home 

Choosing a new boiler can feel daunting, especially as it usually involves a large outlay. If you want a standard gas system, then a combi boiler is often the easiest to install as they are small, efficient and require less time when it comes to getting the work done, especially if you are keeping the boiler in the same place as the old one.  

Regular and system boilers are a good option for larger households. Just be aware that if you need new tanks and cylinders, the work could take some time to complete.  

Alternatively, you may want to opt for a more environmentally friendly system, such as a heat pump. Remember there are numerous private and government grants to help with making the switch.