System Boilers

What is a system boiler?

Nowadays, people in the UK are advised to use a combi boiler for their hot water needs. But system boilers can, sometimes, be a better option for certain situations.

Firstly, let’s take a look at what a system boiler means and see how they work. In addition to looking at some of the main reasons why a system boiler might be your best choice. Get a quote for a System Boiler Replacement without delay.

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The difference between system boilers and combi boilers

To begin with, combi boilers have two functions. They provide you with hot water and they heat your home as well ,using a single unit. We recommend people who live in flats or homes with limited space to have a combi boiler. With a combi boiler, you wouldn’t have to worry about making space for the header tank or the hot water cylinder.

However, combi boilers aren’t suitable for homes or flats with high hot water demand. What’s more, they are made to supply only one water outlet at a time. When there is need for hot water, your combi boiler would switch from heating to providing hot water. As a result, your central heating performance might be affected as well, as soon as you turn on your tap. Get a quote for a System Boiler Replacement without delay.

On the other hand, a System boiler consists of a separate cylinder to store hot water. Hot water is kept in the cylinder despite the fact that the main heating and hot water system parts are put within the boiler. As a result, the hot water supply can support more than one taps and showers at the same time. That’s why a system boiler is the perfect option for homes with multiple bathrooms.

The difference between system boilers and regular boilers

The system boiler can be easily mistaken with the regular boiler (also known as a traditional or a conventional boiler) due to the separate hot water storage cylinder. However, there are some primary differences. Furthermore, a regular boiler might need a header tank to draw water from and to keep water levels in the radiator. While a system boiler is a sealed system that only has a cylinder.

That’s why people sometimes prefer to go with a system boiler. Although in some cases a regular boiler can be more suitable to the needs of a certain household. Especially for those who live in areas with low water pressure or homes with old radiator systems which wouldn’t be suitable for high water pressure of a system boiler.

System Boilers FAQs

  • How do system boilers supply hot water?

    A system boiler works together with a hot water cylinder to provide central heating and hot water to our homes. The boiler heats water directly from the mains to warm up our radiators, and fills the cylinder with boiling water that's used for our sinks, showers and more.

  • Do system boilers run out of hot water?

    This factor is determined by the capacity of your hot water cylinder. You should consider how much water you need for taps, baths and showers when choosing the right size. However, you should keep in mind that the size of your cylinder will rely on the space available. A system engineer will help you design the hot water tank and the system to your ultimate satisfaction and your hot water demand. If calculated correctly, you wouldn’t run out of hot water.

  • The efficiency of system boilers

    Due to the use of condensing technology, system boilers are very efficient. They can achieve up to 90% efficiency in ideal conditions. Taps and showers receive immediate hot water at the same pressure. As a result, they can also improve water consumption in your home.

  • How much space is required for a system boiler?

    There are many things to consider but one of the most crucial points is how much space do you need for a system boiler. In general, a system boiler is larger than a regular boiler so you need to make sure that you have sufficient space. What makes a system boiler bigger is the fact that it has a pump and an expansion tank. It’s worth mentioning that the additional parts would be covered by the manufacturer warranty. An airing cupboard is where the hot water cylinder most likely to be installed.

    System boilers are perfect for homes with limited space because they do not need a header tank.

  • How do system boilers function?

    Firstly, a heat exchanger heats the water. Then it moves energy from the gas jets to the water. Once the water is heated, it is moved to the hot water cylinder where it will be stored. Even when you are using multiple taps and showers, a constant supply of hot water is always available.

The Advantages of System Boilers

A large supply of hot water

Generally, with a system boiler you can store large quantities of hot water to use later on. This is very useful when you are using multiple taps, showers and radiators at the same time.

The ability to use solar energy.

Without a doubt, connecting the correct cylinder to the system will enable you to use solar panels. This is not only environmentally friendly but it is also cost-effective and will save you some money.

Needs less space than a regular boiler

Of course, system boilers are easier to install and need less space than regular boilers. Moreover, they do not need an expansion tank or external expansion vessels

Economical and fast

Lastly, system boilers consist of a pump. As a result, they respond fast and have more economic running cost.

Important things to consider in System Boilers

There are also some drawbacks to installing a system boiler that you should keep in mind. Here are a few of them:

System boilers need more space than combi boilers

Although they take less space than regular boilers, they still require more space than combi boilers because of the external cylinder. Overall, this can be a huge problem in smaller homes when it comes to installations.

Busier times means more patience

Firstly, your cylinder will be sized in advance to cover your household need of water (usually 50 litres of hot water stored per person). However, in times when you have more visitors than usual and your use of hot water is doubled, your hot water storage may run dry. As a result, you will have to wait 30-40 minutes while your boiler is reheating the cylinder again.

Finding what you need

In order to find the most suitable boiler for your home and needs you should take the time to compare different models. The question remains as to what to look for and what to compare.

Output rate

The output rate is the power of the boiler measured in kilowatts (kW). Because regular boilers consist of a central heating output rating, you will know how many radiators the unit is able of heating up.


You can know the efficiency rating of a boiler from how much of the converted fuel is used to the maximum. Wthout a doubt, less energy is wasted when the efficiency rating is higher.

Possible Cost

To begin with, there are many options to choose from. Options that will suit your budget that is to say. The prices of a regular boiler can range from £500 to £2,000 (not including the cylinder and the installation cost).


Boilers always come with a warranty by the manufacturer that will cover any problems that may happen after the installation. Each boiler has a different warranty that will cover certain things and not others. They also differ on the time they offer.

Heating controls

It’s important to have good heating control over your boiler in order to keep your energy use low. Now you can use your smartphone or tablets to control your boiler.

Customer reviews

Without a doubt, you would like your boiler to be reliable and to heat your home for years to come. Reading reviews left by homeowner is highly helpful. These kinds of feedbacks will make you familiar with certain problems and issues of certain boilers before buying your own.

Engineer reviews

Of course, heating engineers are experts in boilers. Their insight can help you find what kind of boiler you need. So always make sure to take the time to read what they have to say.

  • What is meant by a Condensing Boilers?

    The terms condensing and non-condensing can be confusing to people shopping for a new boiler.

    Presently, it is stated by law that new boilers must use condensing technology. In short, you don’t have to worry about getting a condensing boiler.

    Condensing is not a type of boiler, but rather a technical feature. In a Condensing boiler you get a much bigger heat exchanger. They also produce additional heat from the fuel. The same fuel that would have been wasted in a non- Condensing boiler. As a result, they are more efficient.

    non-condensing boilers are less efficient by 50% since a lot of heat was lost in the combustion exhaust gases. On the other hand, new boilers would capture this heat and use it to heat water.

    condensing boilers are energy efficient by 90%. Moreover, they use excess energy to heat your home. Not to mention, they are environmentally friendly and reduce your heating bill.

  • The cost of system boilers

    System boilers usually cost between £500 to £2000. Their cost mainly depends on their size, model and manufacturer.

    Homes with high hot water demand might need Premium models. These models can cost upwards of £1500. On the other hand, smaller homes will need boilers that would cost much less.

    Baxi and Worcester Bosch models are available at a low price. Baxi 800 and Worcester Bosch Greenstar i prices range starts at about £900. On the other hand, the Vitodens 200-W range by Viessmann is the most expensive. It can cost around £1,500.

    Here is a look at system boilers prices excluding installation:

    Baxi 800       £900 – £1,000
    Vaillant ecoTEC Plus£1,000 – £1,250
    Ideal Vogue Max£1,000 – £1,250
    Worcester Bosch Greenstar i£900 – £1,100
    Viessmann Vitodens 200-W£1,250 – £1,500

    Keep in mind that replacing or installing a hot water cylinder will add to the cost as well as installation.

  • The cost of installation

    A lot of factors can impact the cost of installation. These might be:

    • Accessibility
    • Installation Complexity
    • How much rates the installer would charge
    • Your place of living (for instance, London can be more expensive).

    Generally speaking, having a boiler installation might cost you between £800 – £1,500. Here is an idea of how much it might cost you.

    ModelBoiler priceApproximate boiler price with installation
    Baxi 800£900 – £1,000£1,400 – £2,000
    Vaillant ecoTEC Plus£1,000 – £1,250£1,500 – £2,250
    Ideal Vogue Max£1,000 – £1,250£1,500 – £2,250
    Worcester Bosch Greenstar i£900 – £1,100£1,400 – £2,100
    Viessmann Vitodens 200-W          £1,250 – £1,500£1,750 – £2,500
  • How long does it take to install system boilers?

    A system boiler can take between 1 to 3 days. The complexity of the installation will determine the duration. A like-for-like boiler replacement would be the quickest to do. However, replacing a different kind of boiler would take longer time to finish. For instance, a combi boiler replaced with a system boiler will need installing a hot water cylinder and re-routing the pipework. Lastly, moving your boiler would also increase the installation time.

  • The right size for you

    When we talk about boiler size, we do not mean its dimensions, but rather its power output. The unit of measurement, used for system boilers, is the kilowatt (kW).

    To clarify, the higher the output of your boiler, the more powerful it will be. Nonetheless, it’s never a great idea to get the boiler with the highest output rating. A too high output rating would increase your energy bills. And a too low output rating will not meet your demand.

    Through counting the number of bathrooms and radiators in your home, you can find the most appropriate output rating. This nformation can also help you have an idea of the level of demand for domestic hot water and central heating.

    With this purpose in mind, we advise you to get a quote from a Gas Safe registered engineer. If you have a brand in mind, then you should check their website and guidelines. A lot of manufacturers, nowadays, offer advice on their websites.

     Here is a table to guide you through what you need to know:

    No. of bathroomsRadiatorsRecommended Output
    1109-15 kW
    2-31518-26 kW
    3+2027-40 kW

    To point out, small homes with one bathroom and little or no space for a cylinder should consider getting a combi boiler.

  • The right hot water cylinder you need

    Choosing the right hot water cylinder is quite important. There are few things to consider like:

    • Vented or unvented
    • Direct or indirect
    • Capacity
  • Vented or unvented?

    Vented cylinders accompany regular boilers because an overhead water tank will supply these boilers with water. Since system boilers do not need a tank, an unvented cylinder is more suitable.

    Unvented cylinders take the water from the mains directly. As a result, this procedure will lower the risk of water contamination and will provide stronger water flow.

  • Direct or indirect?

    The boiler or the solar thermal technology would heat the water within the indirect cylinder. Whereas, an electric immersion heater, without the help of the boiler, would heat the water inside a direct cylinder. In effect, if you wish to install a system boiler, you shall need an indirect cylinder.

  • Capacity

    Of course, models differ in their capacity of hot water. Large homes with multiple bathrooms and a high demand for hot water will need a cylinder with a large capacity.

    Here is a table that shows you the capacity in litres according to the number of bathrooms in your home:

    No. of bathroomsCapacity in litres
    1120 – 150
    2250 – 300

    Always make sure that your new system is in agreement with the unvented cylinder that you’re getting. Your cylinder must be solar compatible if you plan to use solar thermal energy.

  • So why Let’s Heat?

    Let’s Heat Are Quality & Accredited Boiler Installers

    Firstly, Let’s Heat professional engineers and emergency plumbers provide a professional and reliable plumbing and heating service. Accordingly, that includes Boiler Installation in Cardiff and the surrounding CF postcode area in South Wales.

    Secondly, our boiler installation and repair team is fully qualified and ensured to the highest standards. As can be seen, we take pride in producing exemplary work standards which include completing all boiler installation needs. 

    Thirdly, the Let’s Heat team are high-quality professional engineers who have a high 4.9 out of 5 Trustpilot score by our previous happy customers. We are fully accredited by Gas Safe and Corgi registered, as well as FCA approved for all of our financial transactions. 

    Lastly, all of our staff at Let’s Heat are experts in their field and have many years of experience. Above all, we have work on a wide selection of emergency plumbing, heating solutions and services throughout Cardiff, the rest of Wales & the UK.

    Because we care

    Are you still a little confused about where to start or who to use?

    Our easy-to-use online questionnaire will help you with what you need to know. In short, we make sure that you have the correct solution for your needs. We will also ensure that you are completely happy with the solution.

  • Why else?

    • Firstly, we work in Cardiff and the surrounding areas of South Wales.
    • We are reliable, professional and trustworthy.
    • High Trustpilot score of 4.9 out of 5.
    • We can travel anywhere throughout the UK.
    • Gas Safe Accredited (corgi registered).
    • Our finance is FCA accredited.
    • We provide a 12-month snag guarantee.
    • Our prices are very competitive.
    • We make sure your house is kept clean and tidy.
    • Our workmen use floor protection.
    • Lastly, we dispose of your rubbish correctly.