System Boilers vs Combi Boilers: Should You Change?

Are you wondering whether you should choose a system boiler or combi boiler for your home? Or simply want to know how they compare? 

Choosing any type of boiler is a major decision and since it's going to serve you and your home for many years, it's a decision you want to be confident about. 

PS we fit new boilers with gas installation nationwide. Simply answer these questions, get your fixed price and you could get it installed as quick as tomorrow.

So in this guide, we'll be answering the following: 

• What a system and combi are and how they work

• Pros and cons of system and combi boilers 

• How both types of boilers compare 

• How to know which type of boiler is right for your home 

What are system and combi boilers and how do they compare? 

Combi boilers 

Combi or combination boilers are by far the most common type of domestic boiler installed in UK homes. 

They are typically wall-hung units and take cold water directly from the mains water supply, allowing them to provide both hot water on demand and central heating water from a single unit, hence their name. 

Thanks to their small, compact design and ability to work without a tank, they do not require much room, making them ideal for properties with limited space.  

Modern combi boilers are also extremely efficient and A-rated, meaning you can potentially save money on your heating bills and reduce your carbon footprints, by exchanging your older G-rated model. 

Combined, these qualities make them extremely convenient to use, install and service, hence their popularity. 

System boilers 

System boilers are different to combi boilers as they work in conjunction with a hot water storage cylinder. 

Water is heated in the main boiler and then fed to the hot water storage cylinder where it is stored and kept warm.

This feature allows them to provide hot water to multiple taps and showers, which is where a combi boiler may struggle to maintain adequate water flow pressure.

Is a system boiler the same as a regular boiler? 

No, while both fall into the category of "heat-only" boilers, a system boiler is essentially an upgraded version of a regular boiler.

In system boilers, more components are internalised within the boiler unit, this includes an internal pump, meaning unlike a regular boiler, they do not require a cold water tank in the loft, helping to save space and speed up the installation process. 

For this reason, system boilers are typically preferred over regular boilers, but there are certain circumstances when they can't be installed. 

Pros of Combi Boilers 

Extremely efficient 

Combi boilers are extremely efficient, with modern G-rated models achieving efficiency ratings of 90%+ This not only means you can reduce your heating bills, but compared to older boilers, it will emit less carbon.

In fact, according to the Energy Saving Trust: 

Compact design

Unlike other types of boilers, combi boilers have all their internal parts within a single wall hung unit and do not need a tank.

That means they don't take up as much space, making them ideal for flats, bungalows and smaller properties.  

Some combi boilers are also designed to fit into the average sized kitchen cupboard, meaning they are potentially easier to hide. 

Hot water on-demand

Since combi boilers take their hot water directly from the mains and heat it immediately when it's required e.g. when you turn the tap on, it offers maximum convenience. 

This isn't always the case with boilers that require a tank, as the water store is finite i.e. once you have used all the water within the tank you may have to wait for it to refill and reheat. 

High pressure water delivery

Since combi boilers take water directly from the mains, the water pressure is consistent and typically stronger than other types of boilers e.g. regular boilers, although this depends on the water pressure in your area. 

Cons of combi boilers 

May struggle with high hot water demands

Since combi boilers heat water on demand and do not store water, if there are multiple water outlets being used at once e.g. two showers, it may struggle to maintain water flow pressure. 

Slower flow rates

Since the water is heated on demand, you may have to wait for the water to be heated and be delivered to the outlet e.g. the hot tap when you turn it on. 

This is typically only a few seconds, but you probably wouldn't experience the same delays with a system boiler. 

Relies on mains water pressure

Combi boilers use the water pressure from the mains, therefore if it is weak, the pressure of the water in your home can be weak too. 

Old homes and heating systems may not be compatible

As combi boilers use the water pressure from the mains, this may be too strong for older heating systems and pipework, to the extent that they may be damaged. 

In these circumstances, a regular boiler is typically advised. 

Recommended heating guides:

• How much do system boilers cost?

• Best 40kW system boilers

• What is a boiler filling loop?

• How much does a powerflush cost?

• How to turn off a radiator

• How do you know if your boiler is condensing?

Pros of System Boilers 

Can serve high hot water demands 

Since system boilers work together with a hot water storage tank, they can very easily provide hot water to multiple outlets simultaneously. 

This is because a large volume of hot water is stored and ready to be used, while a combi heats water on demand. 

Requires less space than a regular boiler 

System boilers benefit from more internalised components and take their water directly from the mains, as a result, they do not require a cold water tank in the loft and so do not require as much space to install. 

Compatible with solar 

The hot water storage tank can very easily be hooked up to a solar panel system and water re-heated using the natural renewable thermal energy of the sun. 

As a result, you can potentially reduce your heating bills and carbon emissions by incorporating solar into your system boiler heating setup. 

High pressure, fast hot water flow 

System boilers have an internalised water pump and store hot water in the storage tank. Therefore, hot water is delivered to taps being used almost instantly as you don't have to wait for the water to be heated.

Cons of system boilers 

Needs substantial space to install

System boilers and the tank accompanying it needs to be installed in your home and so an adequate amount of free space will be needed. 

Finite volume of instant hot water

System boilers heat a large amount of hot water (in the cylinder) relatively slowly which means if your boiler has been switched off for a significant amount of time, you’re waiting for that water to be heated from cold which could well test your patience.

Heat loss from the tank is constant

Once hot water is delivered to the tank, it begins to lose heat immediately and so ensuring your hot water storage cylinder is well insulated is essential. 

Repair and maintenance is more complex 

As system boilers have more components than combi boilers, on average any issues that arise can take longer to diagnose and repair, hence costs and downtime can be more significant. 

How both types of boilers compare 

So how do system and combi boilers compare?

If after reading the pros and cons, you are finding it difficult to conclude exactly how they compare, here is a helpful summary of the key points for each:

Combi boiler key points: 

• The compact design makes them ideal for properties with limited space

• Do not need a hot water storage cylinder

• Do not need a cold-water tank

• Typically, not suitable for larger properties with multiple bathrooms/or a high hot water demand

• May not be suitable for areas that suffer from low mains water pressure

System boiler key points: 

• Recommended for properties with multiple bathrooms/hot water outlets 

• Does not require a cistern tank in the loft

• Internal components make installation easier than regular boilers

• May not be suitable for areas that suffer from low water pressure

• May not be suitable for older central heating systems that cannot tolerate high water pressures 

• If you are swapping from a combi, you need to consider where you will store the hot water tank

How to know which type of boiler is right for your home 

As you can see from the pros and cons above and summary, there is no such thing as a perfect boiler and the one that is right for you and your home largely depends on the following: 

• Your hot water demands 

• The size of your property 

• Your present and future hot water requirements 

System boiler vs combi boiler summary

With that said, the biggest factor in determining whether you will need a combi or a system boiler is your home hot water demands. 

Multiple bathrooms going to be used at the same time? The system is advised...

If you have a growing family and are likely to be using multiple bathrooms simultaneously, then a system boiler is likely the way to go. 

However, for most other circumstances, a combi boiler is advised...

Why? 

• They have a range of power options 

• Extremely efficient 

• Easy to repair and maintain 

• No other components at risk of failing 

Read our full article on choosing the right type of boiler for a more detailed explanation.  

Here is a rough guide to the type of boiler required for your home: 

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