How Do System Boilers Work? Re December 15, 2021

How Do System Boilers Work?

Engineer repairing a boiler

How does a system boiler work? This blog discusses how system boilers work, their costs and installation and the current top 5 system boilers.

One of the most common enquiries we get here at WarmZilla HQ is ‘What type of boiler do I have?’, followed by ‘What type of boiler do I need?’. If you haven’t already, head over to our boiler types explained blog, which will help you understand what type of boiler you have installed in your home. 

If you have already read that blog, and have noted that you have a system boiler installed in your home, but would like to learn more about how system boilers work and what system boiler is best for your home, then grab yourself a cuppa and keep reading. 

Don’t forget that you can also convert your system boiler to a combi boiler if it is more convenient for your home. You can learn more about how combi boilers work here. 

If you’re still unsure what type of boiler you have in your home, we can help you identify this whilst getting a quote on a new boiler from the WarmZilla website. Once you have finished this blog, of course 😉, you can head over to our Boiler Comparison tool, which allows you to compare up to three boilers at one time. You can also compare your current boiler against two other boiler models and brands, to help you find the most reliable and most energy-efficient system boiler for your home..

What is a system boiler?

Unlike combination boilers, system boilers directly heat your central heating system and produce hot water which is stored in a separate hot water storage cylinder. System boilers work in a similar way to your conventional boilers, however, system boilers are supplied with cold water directly from the mains. 

The major heating and hot water system components are pre-built into the boiler, however, with a system boiler, the hot water is then stored in a separate hot water storage cylinder. As a result of this, your home will be supplied with a constant supply of hot water that can provide a large number of taps and showers at the same time. System boilers are ideal for larger homes with multiple bathrooms. 

We get asked a lot if a system boiler and regular boiler are the same. Although they both are to be installed with separate hot water storage cylinders, they are not the same. 

A system boiler is a sealed system that requires a boiler and a hot water cylinder only, whereas a regular, or conventional boiler as many may call it, requires a header tank, which is often installed in the attic of your home, which feeds the hot water cylinder.  For this reason, system boilers are often the preferred choice between both a system and a regular boiler.

How do system boilers work?

Cold water from the mains enters the boiler first. The heat exchanger will heat up the water and as it heats up, the water is then transferred into the hot water cylinder. Here is where it will stay until it is required. Because there is a larger amount of hot water available, multiple taps or showers can be used at once – a steady supply of hot water is readily available.

A system boiler has a separate hot water storage cylinder so, naturally, it can sometimes be difficult to tell the difference between this type of boiler and a regular standard boiler. Although, it’s important to remember that a system boiler only requires a boiler and a cylinder. 

This reason alone means that system boilers are the go-to choice when deciding between this type and the traditional type. A regular boiler may be more suitable for those with different needs, such as homes with older radiators or those areas with low water pressure.

Pros of a system boiler

With condensing technology, the boiler itself is extremely efficient, with many models achieving up to 90% efficiency in optimum conditions. Because all taps and showers will receive near-instant hot water at the same pressure, a system boiler can also help optimise your home’s water consumption. The chief advantage of gas system boilers is their ability to supply multiple bathrooms and faucets with hot water at the same time. This is indispensable for large families and is part of the reason why system boilers are one of the most popular types around. Other pros include:

  • Constant hot water
  • Energy-efficient and environmentally friendly
  • There is no need for a cold water tank
  • Flow rates tend to be high and at mains pressure
  • Affordable running costs
  • Higher efficiency than many boilers, helping you save money on your energy bills
  • Can be used with solar thermal systems
  • No need for a cistern in the attic

Cons of a system boiler

However, there are a few disadvantages when it comes to this model type:

  • Hot water may need to be reheated
  • You might need to pre-program your boiler if you want hot water at a specific time
  • Requires a little extra space for the hot water cylinder
  • Cylinder size may dictate how much water you’re able to use
  • May need insulation in order to stop heat loss

What to consider when buying a new system boiler


One of the most important considerations when choosing a system boiler is whether or not you have sufficient space, this is because a system boiler will be a little bigger than a regular boiler as it incorporates an expansion tank and pump. An additional advantage of this is that the additional components will generally be covered by the manufacturer warranty. If your house has been built with an airing cupboard, this is usually where the hot water cylinder would be designed to go. 

As system boilers do not require header tanks to be fitted in the attic, they are suitable for homes with no loft space or where a loft conversion has been done or is planned.


Boilers these days are very efficient. However, it’s worth noting the fact that with a system boiler, you will lose some heat from the hot water cylinder itself. To help prevent this from happening, it’s important to correctly insulate your cylinder to stop that excess heat from escaping.

System boilers are really efficient at providing almost instant hot water for your showers and taps and due to this, it’ll be much easier to help control the water consumption in your home. There is the limitation of hot water you can store because of the cylinder’s capacity, so if you want more hot water after you’ve emptied the cylinder, you’ll have to wait for it to be heated up. 

It’s important to make a note of how much water you use to determine whether a system boiler would make for a more efficient boiler for you.

What size kW system boiler do I need?

We regularly get asked ‘what size boiler is suitable for my home?’. The first thing to clarify is that you mean kW size and not the physical measurements of the boiler. 

You would only really be interested in the physical size of a boiler if it was located in a kitchen cupboard and you wanted to make sure your new boiler would also fit in the same space. All WarmZilla boilers have the physical dimensions in the product description to ensure your new boiler will fit.

The answer to ‘what size kW boiler do I need?’ is a bit more complicated as it depends on a few different factors – but don’t worry, we will answer those questions for you and get you up to speed on the perfect boiler size for you.

It’s worth noting that if you complete the WarmZilla Survey, then you don’t need to know what size kW boiler you need because our clever system calculates your water usage and heating requirements and supplies you with a list of boilers suitable for you.

We also have a blog explaining exactly what kW means when it comes to your boiler, linked here if you want to find out more. 

System Boiler kW Size Guide

No of bedrooms and bathroomsPoorly insulated propertyModerately insulated propertyWell insulated property
1 bed8kW7kW5kW
2 bed8kW7kW6kW
3 bed10kW9kW6kW
4 bed12kW10kW7kW
5 bed15kW12kW9kW
6+ bed18kW15kW12kW

Factors to consider when deciding on the correct kW size boiler for your home:

There are a few other factors that can have a bearing on the kW boiler that would be best suited for your home:

Water Pressure

Different areas can have different water pressure levels and older homes can have lower water pressure than newer homes. If your home has low water pressure, then you will require a higher kW boiler with a good flow rate to ensure you don’t suffer from a drop in pressure when multiple taps are running at the same time.

Alternative Energy

If your boiler is connected to, or you plan to connect it to alternative energy sources such as solar power in the future then you won’t require as powerful a kW boiler as it won’t have to work as hard to heat your home.
Most modern boilers are built to be compatible with alternative energy sources. If you aren’t sure then check with the boiler manufacturer you’re looking at.


Like any piece of machinery, the better condition it’s in, the better it will perform. Giving your boiler an annual service is essential in making sure it keeps running smoothly in much the same way as a car.

Top 5 best system boilers on the market

Worcester Bosch Greenstar i System Gas Boiler

Dimension: H: 710mm | W: 400mm | D: 330mm
Efficiency Rating: 94%
ErP Rating: A Rated
kW Output Options: 9kW | 12kW | 15kW 18kW | 21kW | 24kW | 27kW | 30kW

  • Greenskies solar panels (which can help you save money)
  • A wide range of user-friendly controls 
  • Clear information display screen 
  • Built-in frost protection for extra peace of mind
  • Low energy pump reduces electrical usage
  • Large Condensesure style siphon built into the boiler

Vaillant ecoTEC Plus System Gas Boiler

Dimension: H: 740mm | W: 445mm | D: 330mm
Efficiency Rating: 92%
ErP Rating: A Rated
kW Output Options: 12kW | 15kW | 18kW | 24kW | 30kW | 37kW

  • Advanced modulation to limit energy consumption 
  • A step-by-step full-text installation wizard for quicker setup and installation
  • Compatibility with intelligent controls 
  • Easy to use controls
  • Easy to read display screen 
  • Stylish case design with a drop-down flap for easy access
  • Quiet Mark accredited for quieter operation

Viessmann 200-W System Gas Boiler

Dimension: H: 700mm | W: 450mm | D: 365mm
Efficiency Rating: 98%
ErP Rating: A Rated 
kW Output Options: 11kW | 19kW | 25kW | 32kW

  • Touchscreen display for easy use 
  • Stainless steel Inox-Radial heat exchanger 
  • MatriX Plus burner installed
  • Optional WiFi controls
  • Built-in frost protection 
  • User-friendly controls 
  • Suitable for integration of a solar thermal system
  • Up to 98% of the fuel that is used to convert it into heat

Ideal Logic System Gas Boiler

Ideal Logic with 8 year guarantee

Dimension: H: 700mm | W: 395mm | D: 278mm
Efficiency Rating: 93%
ErP Rating: A Rated
kW Output Options: 15kW | 18kW | 24kW | 30kW

  • Fully backlit digital LCD display screen
  • Easy to read pressure gauge
  • Use-friendly heating control 
  • Scratch-resistant casing
  • Stainless steel heat exchanger
  • Low lift weight 
  • Built-in frost protection 
  • 10-year warranty on heat exchanger 

Baxi 800 System Gas Boiler

Baxi boiler on the wall

Dimension: H: 700mm | W: 390mm | D: 285mm
Efficiency Rating: 93%
ErP Rating: A Rated
kW Output Options: 18kW | 24kW 

  • Temperature control from smartphone/tablet
  • ADEY Micro2 magnetic system filter fitted 
  • Front service access to all components 
  • A brass hydraulics block
  • A central top flue and extensive flue options
  • Easy to use controls
  • Easy to read display screen 
  • Built-in frost protection

System boiler prices

Your new system boiler and installation cost and vary greatly depending on the model of boiler you choose and the manufacturer. In general, system boilers alone tend to cost around £500 to £2,000. 

There are other factors to consider within the price of your boiler package, such as the complexity of your boiler installation, your location and more, we discuss this in detail below. 

When purchasing a new or replacement system boiler with WarmZilla, the costs above don’t just include your boiler and installation, they also include:

  • A flue that is needed for your boiler type and model. 
  • Your Guarantee will be sorted for you by WarmZilla, so you don’t have to worry about sorting that. 
  • Rewiring of your existing thermostat. You have the option to upgrade this if you wanted.
  • A full system cleanse of your heating system to ensure that your new boiler reaches maximum efficiency. 
  • A free magnetic filter for ongoing protection.
  • A free chemical inhibitor.
  • We remove your old boiler and the parts that are no longer needed. 
  • And not to mention, a Gas Safe installation by a friendly, professional WarmInstaller. 

It is worth noting here that when purchasing a system boiler, or any boiler from WarmZilla, we have a £50 Best Price Guarantee. This means that if you manage to find the boiler you are looking for, for cheaper than what we offer, we will beat that price by £50! 

System boiler installation costs

In addition to the system boiler prices, you will also need to remember the installation costs. Many things can have an impact on this, including:

  • The complexity of the installation
  • Accessibility
  • Rates charged by the installer
  • Your location (boiler installations tend to be more expensive in London, for example)

On average, the installation of a new system boiler will cost between £800 – £1,500. In the table below, we reveal the potential cost of the best system boilers including the installation.

However, this can vary depending on the complexity of the installation. Installation of a system boiler can take anywhere between 1 and 3 days.
Your installation time will be faster and more straightforward if it is a like-for-like replacement. Your system boiler installation may take longer when it’s replacing a different type of heating system for example, from a combination (combi) boiler to a system boiler as this would involve re-routing the pipework and installing a hot water cylinder. Installation time will also increase if you’re planning to move the boiler to a new location.

Can I install my own system boiler?

In order to follow building and safety regulations, a new gas system boiler is only to be installed by a Gas Safe Engineer. A Gas Safe Registered heating engineer means that your engineer is certified to work safely and legally on your boiler, ensuring safety and quality.

We hope that this guide has given you more of an idea about how system boilers work and help you decide on the next system boiler model for your home. There are a lot of factors involved in choosing the correct boiler model as you can see from the points discussed above, but you can take the easy choice and take the WarmZilla Survey or use our Boiler Comparison tool, and we’ll work it all out for you. 

If you have any questions, as ever, please don’t hesitate to get in touch with our helpful team by emailing, calling 0333 577 9090 or messaging us on our website live chat.


With a system boiler, the cold water from the mains enters the boiler first. The heat exchanger will heat up the water and as it heats up, the water is then transferred into the hot water cylinder, which is typically installed in an airing cupboard. Here is where it will stay until it is required. Because there is a larger amount of hot water available, multiple taps or showers can be used at once – a steady supply of hot water is readily available.

You should always consider the space you have in your home. As mentioned above, a system boiler is installed alongside a separate hot water cylinder, which is often installed in an airing cupboard or a similar-sized space in your home. For this reason, system boilers are not recommended for smaller homes or flats. You should also consider efficiency. Ensuring that your boiler is efficient enough for your home will ensure that you are not overspending on your energy bills. 

All WarmZilla boiler prices include Gas-Safe installation as standard. Each price also includes a system filter, system cleanse, guarantee, smart thermostat, and any other bits and bobs you need to get your boiler up and running. Not all prices include boiler installation, so make sure your quote does as you’ll be looking at an additional £500 – £1000 to get a boiler installed.

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