Best System Boilers in 2023

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Last updated: October 31, 2023

Combi, regular, system – there is a domestic boiler for all types of households and their need for heat and hot water. And while a large proportion of UK homes have a combi boiler installed, for some – particularly households that have a high demand for hot water – a system boiler could be the better option.

We take a look at how system boilers work and when they are the best choice as well as review some of the best system boilers currently on the market, so you can decide whether ‘going system’ is right for you.

What Is A System Boiler?

Also known as closed vent boilers, system boilers are most commonly found in larger homes where there are more residents or multiple bathrooms and so a greater demand for hot water.

A system boiler will directly heat your central heating system as well as produce hot water, which is then stored in a separate water cylinder. This makes a system boiler similar to a regular boiler, in that it needs to store its hot water. But unlike the older regular boiler style, a system boiler takes its water supply directly from your water mains. And this makes the boiler much more compact yet still able to provide large volumes of hot water for your home.  A system boiler also doesn’t require a hot water storage tank in the loft and so will take up less space and can still provide a constant supply of hot water at the same pressure for your home.

How Does A System Boiler Work?

Thanks to the use of condensing technology, a system boiler is very efficient, but will need proper insulation to work at its optimum. And because all the taps and showers in your home will receive almost instant hot water at the same pressure, a system boiler will also make the most of your water consumption.

So, exactly how does a system boiler work? Well, with a system boiler, most of the heating components are built into the boiler, rather than externally so there is no need for a separate water tank. Instead, cold water from your mains enters the boiler and passes over the internal stainless steel heat exchanger to heat up, before being transferred either to your central heating system or for storage in the separate hot water storage cylinder. And, as the size of the hot water cylinder will be appropriate to the number of bathrooms you have and the size of the house, you will have plenty of lovely hot water on hand, ready to be used.

As a system boiler is a sealed system – that is, it has all the heating components inside the boiler – it can also make the most of condensing technology by recycling the energy that is created to heat up the cold water. And, as an energy-efficient method of heating your home and hot water, the best system boilers can help to reduce your carbon footprint too.

Best UK System Boilers for 2021:

To give you an idea of what to look for when it comes to an effective and energy efficient boiler, we take a look at some of the best system boilers you can currently buy in the UK.

Viessmann Vitodens 100-W System

  • Brand: Viessmann
  • Output available: 3.2 to 32kW
  • Warranty: 5 years as standard (can be extended up to 12 years)

The Viessmann Vitodens 100-W is this leading German manufacturer’s best-selling gas condensing boiler and for the price, you get a lot of the features of its premium model, 200-W, at a mid-budget price. Ideal for smaller homes that are well-insulated, the 100-W is Wi-Fi enabled for greater on the go control and is class A rated for energy efficiency. Plus, with 94% of its fuel converted into heat, the Viessmann Vitodens 100-W boiler system will help to keep those heating bills down.

Worcester Bosch Greenstar 24i System Boiler

  • Brand: Worcester Bosch
  • Output available: 9 to 24kW
  • Warranty: 10 years

Best suited to medium sized homes, the ever-popular Worcester Bosch Greenstar 24i is this leading boiler manufacturer’s best-seller and with its easy-to-use functionality and energy efficiency, it is clear to see why. At the lower end of the system boiler budget, the Worcester Bosch Greenstar is great value for money, has frost protection and, as it is also compatible with solar energy, can save you money on your bills. The compact size of this Worcester Bosch boiler makes it a versatile system boiler for the family and, if you are off the gas mains grid, there is also an LPG model available.

Vaillant ecoTEC Plus System Boiler

  • Brand: Vaillant
  • Output available: 12-37kW
  • Warranty:  5 years as standard (can be extended to 10 years)

One of the most efficient system boilers, the Vaillant ecoTec Plus is also versatile, and can be used in most sizes of homes. It is careful with its energy use too, precision managing its maximum power to bring you the best return on your bills. And with LPG conversion kits available for some models, is another good option if you don’t have a mains gas supply. Add in smart connectivity for finger-tip control, and this mid-priced system boiler has all the plus points you need to make the switch.

Baxi 800 Combi

  • Brand: Baxi
  • Output available: 25-36kW
  • Warranty:  10 years

Compact, lightweight and uncomplicated to install, the affordable Baxi 800 system boiler is a great introduction to the many benefits of system heating for your home. The unit is small enough to fit in a standard kitchen cupboard, so a good choice if space is tight, and comes complete with a protective magnetic filter, as standard. And you get the energy efficiency you would expect from one of the world’s leading boiler brands.

Best UK System Boilers for 2021

System Boiler Prices & Installation Costs

The price of a system boiler will depend on the make, model and size, but you can expect to pay anywhere between £500-£2,500, with installation costs averaging around £1,000. But this does depend on the size of your home and the complexity of the installation. This means that a straightforward like-for-like system replacement will be cheaper than if you are removing a different boiler type to install a new system boiler set up.

Taking a range of system boilers from the leading brands into consideration, we have put together a rough guide to how much system boilers cost, from entry level to premium.

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The Pros and Cons of a System Boiler

As with any heating system, there are advantages as well as disadvantages to installing a system boiler in your home. To help you decide which type of boiler is right for you, here are the main pros and cons to a system model:


  • Uses less space than a regular boiler

A system boiler doesn’t need a separate feed/expansion or cold-water tank and so takes up less space than a regular boiler. Instead, they use a pressurised hot water cylinder which gives you more options when installing as the cylinder doesn’t have to go in the loft.

  • Good for high hot water demand

As it uses a separate cylinder to store the hot water and you can size the cylinder to suit the size of your home, a system boiler is better equipped to cope with times of high hot water demand, all without a loss in water pressure. This makes a system boiler a good option for busy or large households that have more than one bathroom in use at the same time.

  • Decent hot water pressure

As a system boiler doesn’t rely on a cold-water feeder tank in the loft, but uses water directly from the mains, there will be no loss of pressure due to gravity. This means that the water coming out of the taps or shower will be at a higher pressure than found with regular boilers.

  • Solar energy compatible

As long as you use the right cylinder, system boilers are easily used alongside solar panels to heat the water and make your home heating even more environmentally friendly as well as cost-effective when it comes to your energy bills.

  • Easy to install

As it is a sealed unit with the key heating components inside and no need for a separate cold-water tank, a system boiler is typically easier to install than a regular boiler system. And, as it is compact with just the need for a hot water cylinder, a system boiler can be installed in a smaller space, such as an airing cupboard, rather than taking up a large proportion of your attic.

  • Economical

As long as it is sufficiently insulated, a system boiler is fast to heat up and store your domestic hot water, meaning it is more economical to run than some other types of boilers.


  • Requires more space than a combi

While more space efficient than a regular boiler, a system boiler does need more room than the widely popular combi. It requires a separate hot water cylinder which a combi system doesn’t, so a system boiler can be a little tricky to install in smaller homes where space is a premium.

  • Needs to be well insulated

For a highly efficient system boiler to work at its optimum, and so save you on your heating energy costs, it needs its hot water cylinder to be well insulated so as to avoid any drop in the stored water’s temperature. Fortunately insulating a hot water cylinder is straightforward and should be relatively cheap to do.

  • You may need to wait for your water

Although you will choose a hot water cylinder size to suit your home’s user needs, there may be times when your water usage may be higher than normal, meaning you run out of hot water and have to wait. So, unlike a combi boiler, a system is not truly instant and when supplies run low, it can be another 30 minutes or so of boiler use, before freshly heated hot water comes back out of your taps.

Frequently Asked Questions

How do I Know if a System Boiler is Right for my Home?

A system boiler is not right for every home set up so it’s important to assess whether it is the best option for you. And that means taking into account the following:

  • They may not work in small houses and flats – you will need enough space for the boiler and its separate hot water cylinder – an airing cupboard could work well, and it will save you using up room in your attic as it doesn’t need to be located in the highest art of the house. However, you will need to expect to lose the use of the cupboard for anything else and need to ensure the space can also accommodate the cylinder’s all-important insulation.
  • You have large family – the extra capacity provided by the hot water cylinder means that it can serve a busy household, which can put an increased demand on the need for a constant hot water supply.
  • Your home has multiple bathrooms – once of the key selling points for a system boiler is that it can supply a large amount of hot water to multiple taps and showers at the same time, without losing any pressure.

What’s the Difference Between a System Boiler and a Combi Boiler?

Both combi boilers and the system design are sealed units – that is, their internal heating and mechanical components are built inside the boiler itself. And both boiler types are effective at providing pretty instant hot water than can hold its pressure. However, when it comes to choosing between a system and combi boiler, there are some key differences which can be the dealbreaker.

The main difference is that unlike Combi boilers, which are fed directly by the mains cold water which it then heats on demand before sending either the taps or radiators, system boilers have a separate cylinder where it stores all the hot water it has heated. This cylinder then keeps the water hot, so it is ready to be used, when it is needed. And this difference means that a system boiler is better than a combi at meeting high demands for heating and hot water at any one time, so are ideal for larger properties with more than one bathroom.

What Size System Boiler do I Need?

The size of a system boiler will be determined by its power output and is measured in kilowatts (kW). The higher the kW, the more powerful the boiler but going for the most powerful boiler can actually cost you as can going for a boiler that has too low a power output to heat your home. The system boiler size needs to be appropriate to the size of your home, the number of bathrooms you have and the number of people who live there.

As a guide, you should refer to the following when choosing a system boiler, or get advice from a Gas Safe registered heating engineer:

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What Temperature Should a System Boiler be Set At?

As the water from your system boiler will be stored once heated, you need to balance energy efficiency with water quality. If the stored temperature is too low, it could allow for bacteria that can cause Legionnaire’s disease, to grow. The ideal stored water temperature should be between 60°C and 65°C. Any lower than 60°C and the water can be at risk of Legionnaire’s contamination.

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