Welcome! If you’ve arrived here, it can’t be for any reason other than you’re wondering “what is a system boiler”!

Not to worry, let’s break it down and make it simple. And if you’re looking for a different type of boiler, check out our other blog posts here

What is a System Boiler?

To begin with, a system boiler is different from others in both appearance and function.

A system boiler consists of the boiler itself (the box on the wall) and additionally a separate water cylinder (which will likely end up hiding in your cupboard). 

The boiler has the major heating and hot water system components built-in. The cylinder is designed for the storage of hot water. 

This overall appliance has a heat exchanger like a regular boiler, but also an expansion vessel and circulating pump. 

Accordingly the design, as a result, does not require a storage tank built in the roof/attic space of your house like a regular boiler. 

How does a System Boiler function?

As mentioned above, all the major components are built directly into the boiler. The only external part is the storage cylinder for the hot water. 

The system boiler heats your home by using water directly from the mains supply. This allows hot water to be vented at numerous sources around your house (such as your radiators).

The water is also heated via the heat exchanger and sent to the storage cylinder. There it is stored until required; it can provide hot water for multiple taps and showers at once due to the large reserve. 

This can be done without losing any water pressure, as the water is coming from the mains. A regular boiler has to rely on the gravity of water coming down from the tank in the loft of your house. This system boiler diagram will visually help:

System Boiler diagram in 2D!

Image Credit: myboilerservice.com

What are the pros of a System Boiler?

A system boiler is an appropriate choice for a variety of homes, particularly as the design provides both heating and hot water. It is important to ensure that a system boiler is going to be cost-efficient for you. 

Let us see all the benefits that a system boiler can bring your home:

  • There is no need for a cold water storage tank at the top of your house. This allows you not only to save on space but have no risk of a leak! 
  • System boilers can be smaller than combi boilers. 
  • The previously mentioned expansion vessel and circulating pump allows fast delivery of water all around the house, which is hugely economical. 
  • The water pressure is always going to be good due to the system boiler using the mains supply instead of relying on gravity like alternatives. 
  • As the main components have been built into the boiler, installation is generally very easy and quick.
  • As a system boiler is a “sealed system” (drawing from the mains), it benefits from condensing technology. Energy is recycled from the exhaust gases to preheat cold water, reducing your CO2 impact. This makes it extremely efficient!
  • Lastly, this type of boiler is compatible with solar water heating systems. The storage cylinder can be altered to heat the water by solar means.

So as we’ve just discovered, there are many pluses on getting a system boiler installed. Let’s take a look at the drawbacks.

What are the cons of a System Boiler?

Although there are lots of good things about system boilers, that doesn’t mean it’s for everyone. 

Let us see what possible reasons you might not get a system boiler installed at your home: 

  • They potentially can take up more space than you’d like. Although more compact than a regular boiler they do require space for the hot water tank. Generally speaking, if you have an airing cupboard this will be the ideal spot for the cylinder.
  • You could run out of hot water depending on the size of the cylinder you opt for. If this happens you’ll have to wait for it to refill then heat again. If the boiler is switched off then the water in the cylinder isn’t heated up either. 
  • System boilers are initially more expensive than combi boilers, but this is the cost of better reliability. 
  • Water is used at the point of demand, so if it has not had time to heat it won’t be hot!

These should not put you off the idea of getting a system boiler, as most of the cons are really a question of time management. Often, a set timer can organise when the water for the cylinder is heated. 

Do I need a System Boiler?

You might be considering upgrading to a system boiler but aren’t entirely sure if you have the right household for it. Price, warranty and reliability aside, let us look at other factors that would make getting a system boiler a smart move. 

If you live in a household that has more than one bathroom, this is already something that a system boiler is well suited to. This is because the storage cylinder can supply hot water to multiple sources at once. It also does this without any loss in water pressure, so you shouldn’t have a problem with trickling taps or showerheads! 

So if you live with quite a large family, who are having spaced out showers or baths across the day, a system boiler would be a shrewd acquisition. In between each person using hot water it would begin to replace what was used by refilling the cylinder. 

Equally, if you are living in a house with little or absolutely no loft/attic space, a system boiler is worth considering. Despite the fact the cylinder will still need space elsewhere, the various sizes can likely ensure it’ll fit someone snugly in your house. If you are replacing an old boiler that space will do fine – it’s just a straight swap!

The majority of system boiler models have achieved 90% efficiency in optimum conditions – that’s a measure of how effectively fuel is converted into heat energy.

What model of system boiler is right for me?

When picking the model of system boiler, the main thing to consider is the size of your home (as you’d probably expect!)

How many bathrooms do you have? And are there a lot of radiators?

On top of this, the number of occupants is also important. If there are going to be multiple people showering in the morning, your boiler needs to be bigger. 

You should think about your kW (kilowatt) output. This is helpful in discovering the demand you put on your boiler to heat your home. On top of heating, there is also the usage of hot water by the people living at your home. 

Here are some boiler size calculations to help: 

12-15 KW1-3 bedrooms, 1 bathroom
18-24 kW3-4 bedrooms, 1-2 bathrooms
30kW +4 or more bedrooms, 2+ bathrooms

Table Credit: homeserve.com

If your home is small enough it can always be worth looking at a combi boiler instead. Check out the blog post on explaining the benefits of a combi boiler here

You should also look at the number of radiators around your house. Here is another boiler size calculator to help with an early estimate:

What size boiler for…Boiler Size CalculationMinimum kW Size
6 Radiators6x 1.5kW + 3kW12kW
8 Radiators8x 1.5kW + 3kW15kW
12 Radiators12x 1.5kW + 3kW21kW
15 Radiators15x 1.5kW + 3kW26kW
20 Radiators20x 1.5kW + 3kW33kW

Table Credit: heatable.co.uk

After you’ve counted your bathrooms and radiators, you can use the boiler size calculators to give you a rough idea of what to look for. 

If you have a lot of both, naturally you’ll have to be looking at the larger end of the scale.

The cylinder size is going to be naturally the most important. There’s little point in having a boiler when you run out of water every time somebody has a shower!

How much does a System Boiler cost?

As with many products, there are a huge number of different brands and models of system boilers, all with different price points.

System boilers are more expensive to install than your standard combi boiler, but over time it’s certainly possible to more than make your money back in terms of the energy efficiency savings, especially if you have a large home and a large family!

Without the cost of installation, a system boiler can cost anywhere in the region of £500 up to £3200. 

System boilers combine a high level of performance with “greener” energy credentials. Lower CO2 emissions make them a greener choice in the boiler market. As a result of this, they are becoming a modern household favourite. 

Hopefully after all of that, we’ve answered “what is a system boiler” for you! If you have any further questions please do not hesitate to get in touch, and why not have a read through more of our blog posts!


Charlie is a content writer at HomeSage. Having left Oxford University he joined HomeSage in 2020 to help create the UK's best resource in the home improvement sector. In his spare time he enjoys walking his dog Zeus and playing cricket.