What is a system boiler?

February 15, 2022

When you’re looking for a new boiler - whether it’s a planned upgrade or because your current one is on its last legs, it can be daunting to understand what the difference is between them all. Before you can even start looking at manufacturers; models; and sizes of boiler; you need to know what type of boiler best suits your needs. Whilst a lot of people have heard of combi boilers, we find that there’s a lot of confusion about what a system boiler is. So, here’s a handy guide to everything you need to know about system boilers, and whether they’re right for you.

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If you’re looking for a new boiler; but are wondering what a system boiler is – then this blog’s for you!

When it comes to boilers, there’s a lot of jargon out there. Before you can even start looking at manufacturers; models; and sizes of boiler; you need to know what type of boiler best suits your needs.

And it’s not a decision to be taken lightly; because a new boiler is a big investment for you and your family.

But don’t worry, The Heating People are here to help!

There are three main types of boilers available – conventional or ‘heat-only’ boilers; system boilers; and combi boilers.

We find that people have often heard of combis, but are often confused about what a system boiler is.

So, here’s a handy guide to everything you need to know about system boilers, and whether they’re right for you.

You might like our related blog: 6 top tips for buying a new boiler.

How do system boilers compare to the other types of boilers?

Before we dive in, it’s useful to see how system boilers compare to heat-only and combi boilers…

Heat-only or ‘conventional’ boilers

Heat-only boilers are sometimes called ‘conventional’ or ‘regular’ boilers. This is because they’re often thought of as being the traditional, or ‘conventional’ way of heating a home.

Heat-only boilers provide heating and hot-water with the use of an additional cold-water tank and hot-water cylinder.

The cold-water tank is usually stored in an attic or loft. It feeds water down to the boiler when needed to be heated up. After the water is heated, it’s stored inside the hot-water cylinder until needed for your taps and radiators.

Since heat-only boilers store hot water, they work well in large households with multiple bathrooms. Unlike combi boilers, they can supply large amounts of hot water to multiple outlets at the same time.

Heat-only boilers don’t rely on pressure from the mains. They’re gravity fed cold water from the tank in the loft. This makes them a good choice for homes in areas that suffer from low pressure.

System boilers

System boilers are sometimes confused with conventional boilers because they also store hot water in a cylinder for later use.

As with heat-only boilers, system boilers are a good choice for larger homes with a high demand for hot water. They can supply multiple outlets with hot-water at the same time. This is great in homes where family members want to shower at the same time.

But unlike heat-only boilers, system boilers don’t need a cold-water tank in the attic. Insteaad, they take their water supply directly from the mains (like a combi boiler).

So, system boilers share some similarities with both heat-only and combi boilers – hence the understandable confusion surrounding them!

Combination or ‘combi’ boilers

A combi or ‘combination’ boiler provides both heating and hot water from one all-in-one (combined) unit.

Combi boilers are the most popular type of boiler sold in the UK.

Unlike heat-only or system boilers, combis don’t need any additional tanks or cylinders – everything it needs is inside the unit.

A combi gets its cold-water supply directly from the mains (like a system boiler). This means that it doesn’t need a cold-water storage tank in the loft.

A combi doesn’t need a hot-water cylinder either because it heats water instantly on demand. When you turn on a hot tap, a combi heats the cold mains water instantaneously.

But whilst combi boilers provide both heating and hot water; they can only perform one of these tasks at a time. And they always prioritise hot water.

Since combis don’t need external water tanks or cylinders, they’re space saving. Many models are compact enough to fit in a small cupboard.

Another advantage is that with no water tank to heat up; combi boilers can provide an unlimited supply of hot water on demand. This is ideal in busy homes where showers may be used back-to-back each morning.

But since combi boilers don’t store hot-water; they’re best suited to homes with relatively small hot water demands. If you’re a household where everyone wants showers at the same time, then a combi won’t be suitable.

And this is where a system boiler might come in for you…

How does a system boiler work?

Now let’s take a closer look at how system boilers actually work.

System boilers receive cold water directly from the incoming mains supply and pass it over a heat exchanger.

As the temperature of the heat exchanger rises; energy is transferred from the gas jets to the incoming mains water that meets it.

Once the water is hot, the in-built pump diverts some of it to the hot water tank for later use. This will then be used to supply hot water to taps, showers, and baths around the home. The rest of the hot water travels to the radiators, powering the central heating.

Since there is a large store of hot water always available; a steady supply can be maintained even if multiple taps or showers are in use at the same time. This is perfect if you want to have multiple hot water outlets running at the same time.

Are system boilers condensing boilers?

Efficient boilers are definitely (and quite rightly) on the political agenda. So you may have encountered the term ‘condensing boiler’ and are wondering what it means.

There’s a common misconception that a ‘condensing boiler’ is a type of boiler, when in fact, it isn’t.

The terms ‘condensing boilers’ and ‘non-condensing boilers’ are misleading. They actually describe the technology within the boiler itself, rather than what sort of boiler it is.

A condensing boiler works by recycling the wasted exhaust gas that is created during a boiler’s normal operation. By re-using the exhausted gas, water vapour and steam back through the system; some of the heat is retained making the boiler’s operation more efficient.

But don’t worry, you won’t have to choose between condensing and non-condensing technology. The Boiler Plus Regulations require all new boilers to use condensing technology as standard.

So whatever type of heating system you opt for, it will be condensing.

This is because condensing boilers are more energy efficient than their non-condensing counterparts. They can typically extract 90-92% of heat from fuel; compared to 50-80% that a non-condensing boiler can.

How efficient are system boilers?

As we’ve mentioned, system boilers have condensing technology which makes them extremely efficient. New models are ErP A rated.

But it’s important to ensure that your hot water cylinder is properly insulated to prevent heat loss.

Another great thing about system boilers, is that they can are compatible with renewable technologies, such as solar thermal. And they can be installed to high efficiency, low temperature heating systems. This can help to save you money on your energy bills, as well as reducing your carbon footprint.

Can a system boiler run out of hot water?

The only limitation to the amount of hot water a system boiler can supply – is the capacity of the hot water cylinder.

If you use up all the hot water in the cylinder, you will have to wait for more to be heated.

During your free survey, The Heating People will ask you about your hot water usage. This helps with the correct sizing of your cylinder. This is limited by the space available in your home.

What are the advantages of a system boiler?

We’ve dedicated a whole blog to the pros and cons of system boilers, which you can read her. But in summary, here are some of the pros to opting for a system boiler:

System boilers don’t need as much space as heat-only boilers

Because there’s no need for a cold-water tank in the attic, system boilers take up less space than heat-only boilers. This is good if you’re short on space. But remember that you will still need space for a hot water cylinder.

System boilers work well in large households with multiple bathrooms

System boilers store large amounts of hot water in a cylinder. So they can supply multiple outlets at the same time, without there being a drop in flow or temperature.

This is an advantage over having a combi boiler, which will struggle to supply several outlets at the same time.

So, if you have a larger family, with people wanting to have a shower; a bath; or do the dishes; all at the same time; a system boiler could be a good option for you.

System boilers can use an additional electrical immersion heater

With a system boiler, you can install an electric immersion heater to the hot water cylinder. This heats water instead of the boiler in the event of a breakdown. This is great because you won’t be left in the dreaded no hot water situation!

System boilers are easy to integrate with renewable heating systems such as solar

Another advantage of system boilers is that they are easier to integrate with renewable heating systems.

Solar thermal heating systems use free (and environmentally friendly) heat from the sun to heat your water. This means you won’t have to burn as much expensive gas!

This can save you money on your energy bills as well as helping to reduce your carbon footprint.

System boilers have a wider range of control options available, compared to heat-only boilers

Boiler controls make a big difference to your heating system. They can boost your system’s overall performance; whilst reducing your energy bills.

System boilers have a much wider range of control options available, compared to heat-only boilers.

System boilers can run low temperature heating systems

Low temperature heating (LTH) systems are becoming more popular as our awareness of climate change continues to grow. LTH doesn’t mean that your house is cold.

It means that you can be comfortable whilst running a more efficient system at a lower temperature. And a system boiler can be part of that.

To find out more about this, check out Heat Geek’s article (listed at the end of this blog). You can also ask expert engineer, Liam, who is passionate about greener heating!

What are the disadvantages of a system boiler?

But as with all things, there are drawbacks too. Here are the cons of a system boiler:

System boilers require more space than a combi boiler for the hot water cylinder

Whilst system boilers take up less space than heat-only boilers, they do still require a hot water cylinder.

System boilers need more space in your home than a combi boiler; which houses all of its component parts in one compact unit.

System boilers don’t have instantaneous hot water

Combi boilers produce hot water instantly, on demand. They will never run out of it (although they struggle to supply multiple outlets at once). This is different from system boilers which heat water and store it in a cylinder, for use later.

Since system boilers use stored water; it is possible to run out of hot water (just until the cylinder refills). But you can minimise this problem with the correct sized cylinder and properly set up controls.