System Boiler

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System Boiler

A system boiler is a common type of boiler and a great choice for those looking for something to meet their high demands for hot water. 

System boilers operate like regular boilers, as both systems store hot water for later use. 

System boilers only differ in that they take their water directly from the mains while regular boilers require a cold water tank. 

And though they’re compact compared to regular boilers, they can meet higher hot water demand than regular boilers.

This guide discusses everything to know about system boilers, such as how they work, their differences from other systems, and their pros and cons. 

How Does A System Boiler Work?

A system boiler is a compact system in that everything needed to let it run is built into the boiler. With this, there is no need for an extra component besides the hot water cylinder. 

While a regular boiler would require a water tank in the loft, a system boiler doesn’t need this, as it takes its water directly from the mains. 

The system heats this water directly and uses it to heat the central heating system and also supply it to the hot water cylinder to meet the house’s hot water needs. 

System Boiler Vs Combi Boiler: Any Difference?

The major difference between a system boiler and a combi boiler is the storage of hot water in system boilers. 

A combi boiler doesn’t have a hot water tank, meaning that combi boilers don’t store hot water but rather produce the hot water when needed and supply it. 

In a system boiler, there is a need for a hot water cylinder. This cylinder stores hot water and supplies the water when the need arises. 

With this, a system boiler can provide more hot water volume than a combi boiler. 

Since combi boilers are more compact than system boilers, homes with limited space might jump at a combi boiler. 

However, combi boilers come with their limitations, such that they can’t meet high hot water demands. 

As such, a system boiler would be a great option for homes with multiple bathrooms and hot water demands.

System Boilers and Regular Boilers: Any Differences?

Regular boilers are also known with other names, such as open-vent, heat-only, and conventional boilers. They’re common in older properties since they’re the most traditional heating system around. 

A conventional boiler is different from both system and combi boilers in that it doesn’t take its water from the mains but rather relies on feed and expansion tanks in the loft for water supply. 

Cold water from the mains goes into the feed tank, while the expansion tank ensures consistency in the tank water level. This is necessary to control the expansion of water due to heating. 

A regular boiler is likely suitable for homes that have one installed but that need a replacement. A system boiler is still more straightforward and capable of meeting high water demand.

Is A System Boiler A Good Choice For Your Home?

A system boiler takes after both a combi boiler and a regular boiler in some aspects. 

While a system boiler doesn’t need tanks in the loft and heats water directly from the mains, it stores water in a cylinder, like a conventional boiler, to cover high hot water volume. 

If a system boiler is a suitable choice for your home depends on your home’s size. 

While a combi boiler might suit a home with limited space, a system boiler takes more space but is more effective. 

A regular boiler needs the most space for operation since it requires both cold water and hot water cylinders.

A system boiler is efficient but requires installing a hot water cylinder in the loft. The system is most suitable for large properties. 

A regular system might not be the best for small homes and properties due to the need for a hot water tank. 

Also, since a small home doesn’t have high heating and hot water demands, a system boiler is unlikely to be a great option. Hence, a combi boiler might be a better alternative.

Benefits of System Boilers

Since a system boiler takes great features from combi and regular boilers, it’s not surprising that it comes with a load of benefits. Here’re the pros of a system boiler:

Ease of installation

A system boiler comes equipped with every component needed for its installation. The system is combat and doesn’t require extra components besides the hot water cylinder. 

This makes installing a system boiler easier than installing a regular boiler, which demands a cold water tank and a hot water cylinder. 

In a system boiler, the hot water can be placed in the loft or an airing cupboard.

High hot water supply

A system boiler uses a large tank to store hot water, meaning that it has a high hot water capacity. 

As such, they can meet the hot water needs of large properties. The system can supply multiple taps without the concern of reduced pressure.

High water pressure

One common concern with a boiler is if it has high water pressure that it can supply multiple taps simultaneously. 

A system boiler has high water pressure, thanks to the water coming directly from the mains. 

With this, a system boiler has a higher water pressure than a regular boiler that relies on gravity to supply water through the pipes.


Since a system boiler doesn’t require additional components than a hot water cylinder, it takes less space.

Solar-thermal compatible

System boilers are compatible with solar thermal. Therefore, if you’re considering heating your hot water with your solar thermal system, you can opt for a system boiler.

Cons of a System Boiler

Though it comes with many benefits, a system boiler also has its disadvantages. Here’re the possible cons to watch out for with a system boiler installed:

More space

Though system boilers are a great choice over regular boilers in terms of space-saving, they take more space than installing a combi boiler. 

While a combi boiler doesn’t need cold water and hot water tanks, a system boiler requires a hot water tank to run.

Insulated cylinder

One common challenge faced with any system that stores hot water in a cylinder is heat loss. Hot water usually loses energy while sitting in the cylinder. 

The cylinder is insulated to mitigate this. Though the system will still run and function, insulating the cylinder helps maintain the hot water temperature while in the cylinder. 

This is rather regarded as a consideration than a disadvantage.

Hot water tank size

How much hot water you can get at a time depends on the size of the cylinder. As such, you need to wait for more water to be heated once the one in the cylinder is used up. 

Heating hot water in a system boiler can take up to 30 minutes, depending on the model.

How to Choose the Best Boiler for Your Home

The market is flooded with several models of system boilers. With this, homeowners may have difficulties choosing the best one for their homes. 

As such, when looking for the best system boiler for your home, ensure you go through and compare the different features of the available models. 

Here’re a few features to look out for when choosing a system boiler:

Output rating

This is a measure of the system’s central heating output and is measured in kilowatts (kW). 

This rating gives an insight into how many radiators the system can heat up. A higher output rating means the system will heat up more radiators.


The efficiency of a boiler is the percentage of the consumed fuel the boiler can convert into heat. 

High efficiency means the system will generate more heat from little fuel consumed, meaning less fuel consumption.


The warranty of a boiler tells how long it could be returned should it start malfunctioning. 

The manufacturer puts this in place to relieve customers of the cost of repair during a certain period. 

Warranty varies from manufacturer to manufacture, and a high warranty means longer peace of mind using the system.

Potential cost

While you don’t want to buy a cheap system that might not serve you well, you also don’t want to spend unnecessarily on your system. As such, it’s smart to strike a balance. 

A good system boiler costs between £500 and £2,000. This is usually without a cylinder and the cost of installation. 

Heating controls  

The ease of controlling your system is a great factor to consider when buying a system boiler. 

Many modern boilers come with an ‘app’ to install on your smartphone and use to control your system from anywhere.

Heating engineer reviews 

While you might make your research and have insights into which system is a great choice for your home, heating energies know better. 

They have worked on the boilers before and know what to expect from each one. 

As such, seek heating engineers’ reviews about your choice of system boiler before finally going for it.

Customer reviews

Getting reviews from customers who have used your choice of system boiler before lets you know if they have encountered any problems with it. 

It might be ideal to seek reviews from your neighbor about your particular choice of system boiler.

System Boiler Prices

System boiler prices vary on the market and depend on size and model. While some system boilers can cost as low as £500, some can go for up to £2,000. This is specifically for gas system boilers. 

Note that this doesn’t include the cost of installation. How much an installer would charge installing a system boiler depends on the complexity of the system. 

The complexity of a system also determines how long it takes to install the system. 

Replacing a system boiler for another system boiler is faster and doesn’t attract high installation charges as replacing another boiler with a system boiler. 

However, installing a system boiler should occur between 1 and 3 days.

What Size System Boiler Is Suitable For Your Home?

When discussing the right size of system boiler suitable for a home, it’s not about the physical size, but rather the power output of the system. 

This is a crucial factor to consider when choosing the right system boiler for your home. Again, this value is measured in kilowatts (kW).

A system boiler with a high output rating means a powerful system. 

While the system will dispense a great output to cover high hot water demands, it’s worth noting that the higher the output rating, the more energy the system consumes, hence a rise in energy bills. 

Also, a very low output rating means the system will struggle to meet high heating demands. 

To know what output rating to look for when choosing a system boiler, it’s best to calculate your heating demands. You can do this by estimating the number of radiators, bedrooms, and bathrooms you own. 

For example, a home with 3 bedrooms and 2 bathrooms will need a system boiler with an output rating of 34 kW to meet its heating needs.

What Size Hot Water Cylinder Is Suitable For Your Home?

One common difference between a system boiler and a combi boiler is the need for a hot water cylinder. System boilers need a hot water cylinder to store hot water. 

This means that a system boiler can only supply not more than the hot water in the cylinder. As such, the system will need to heat more water when the one in the tank is used up. 

This necessitates choosing the right hot water cylinder for a system boiler, as waiting for up to 30 minutes to heat up more water can be a delay. 

Though the amount of water a hot water cylinder can hold depends on the manufacturer and the model, it’s equally important to consider certain other factors. 

Here’re two factors to consider when choosing a suitable cylinder for a system boiler:

Vented or unvented?

Vented cylinders are common in old regular boilers. The cylinders have their water delivered to them from the cold water tank. 

An unvented cylinder is a great choice to maximize the efficiency of a system boiler, as vented cylinders take their water directly from the mains and not from a tank. 

This reduces water contamination and increased flow rates.


The size of your home and your hot water needs will determine the right cylinder capacity for your system boiler. 

If you reside in a small home with low hot water demands, a small cylinder might not be a deal-breaker for you.

Also, going for a large-capacity cylinder would be the best choice if you reside in a large home with high hot water demands. 

Note that a cylinder capacity is measured in litres.