What is a System Boiler? (Boiler with Water Tank/Cylinder)

The combi boiler is the most popular type of boiler installed in UK homes, providing homes with heating and hot water from a single unit.

They’re what cool people have, basically.

But other cool people, with modern or large houses also have/need system boilers with extra water tanks to quench their thirst for coolness.

• What is a system boiler?

• How do system boilers work?

• System boilers vs combi boilers 

• Who is a system boiler suitable for? 

• How much do system boilers cost?

This guide was so easy to digest, even Kirk off Corrie could understand it.

System Boilers Explained

System boiler vs Combi boiler vs Regular boiler

In addition to the system boiler, which is the star of this show, there are two alternatives types of boiler:

What is a regular boiler? 

Also known as a conventional boiler which is similar to a system boiler but requires an additional cold water storage tank (usually in your loft) which acts as the water supply as opposed to the mains – meaning a lesser water pressure and less room in your attic for dead bodies or prisoners.

What is a combi boiler? 

The UK’s most popular and – in most cases – most efficient boiler, requiring no cylinders or water tanks and providing instant hot water and heating to your radiators. It’s the boiler dream.

The only downside of a combi is that if it does break down, your home will have no heating, and no hot water. You don't have the backup of an immersion heater.

Back to the point, though.

What is a system boiler? 

System boilers are very similar to regular (conventional) boilers in that they deliver hot water directly to the central heating and store hot water within a hot water cylinder.

However, they differ in that their water is supplied directly from the mains water supply, this advantage, means they do not require as much storage space. It also means they deliver water at high pressure, so provide a more consistent flow rate. 

Curious to know which boiler brand performed worst? Find out in our report of the worst boilers. 

System boiler key points: 

• Recommended for properties with multiple bathrooms/hot water outlets. 

• Does not require a cistern tank in the loft. 

• Internal components make installation easier.  

• May not be suitable for areas that suffer from low water pressure. 

• May not be suitable for older central heating systems that cannot tolerate high water pressures. 

• If you are swapping from a combi, you need to consider where you will store the hot water tank. 

How does a system boiler work? 

There are two main components to a system boiler – the boiler itself (the ugly white box)

...and the water cylinder (the other ugly thing that’s probably hiding in your airing cupboard).

How a system boiler heats your home:

Your system boiler uses water direct from your mains water supply. 

When you switch your heating on, the boiler will rapidly heat water from the mains supply and deliver it like Royal Mail straight to your radiators.

How a system boiler provides hot water:

To provide hot water for your taps, bath, and shower – your system boiler heats the water inside your water cylinder (that thing in your airing cupboard again).

This is done relatively slowly, taking up to an hour depending on your boilers power and water cylinder size. So you’ll be having to switch your boiler on well prior to needing any decent amount of hot water.

What is a system boiler suitable for?

System boilers are in fact suitable for any home, but that doesn’t mean they’re the most suitable.

The water storage cylinder might mean that you’ve had seven cups of coffee prior to it being ready, but it also means that once that water is hot – there’s enough to go round for multiple showers or baths at the same time, even if someone decides to do the dishes during - controversial.

This makes them most suitable for larger family homes where hot water is likely to be demanded more than chips for tea.

Note: If you have an older boiler and are suffering from lockouts and lukewarm water, you may want to consider whether you have a faulty heat exchanger. 

Curious to know how condensing boilers differ from combi boilers? Read condensing boilers vs combi boilers. 

Pros of a system boiler

#1 - They can handle high hot water demand with ease

Never judge a water tank by its cover. 

The beast in your airing cupboard might seem like an ugly waste of space but a large amount of stored hot water (that’ll stay hot for hours) will allow numerous household occupants to use hot water at the same time.

#2 - Quick to heat your house and eco friendly

A water pump within system boilers allows for super quick delivery of water to your radiators, and the insulated water storage cylinder is an economical way to provide hot water to a number of outlets and relentlessly showering family members.

#3 - Harvest the power of the sun

Water storage cylinders and tanks of regular and system boilers can be heated through solar panels after installation. Soak it in.

#4 - Improved water pressure over a regular boiler

System boilers use the mains water supply when delivering water to your hot water outlets which provides improved water pressure. This is opposed to a regular boiler that relies on water from a dirty tank in your attic that harbours dead birds and lazily relies upon gravity for its pressure.

#5 - Additional loft space over a regular boiler

Unlike regular boilers that essentially require a steel bathtub to be installed in your attic space, a system boiler only requires a water cylinder, tucked inconspicuously away in your airing cupboard - like a burglar thats heard you come home early.

Cons of a system boiler

#1 - Saint-like patience required – a lack of instant hot water

System boilers heat a large amount of hot water (in the cylinder) relatively slowly which means if your boiler has been switched off for a significant amount of time, you’re waiting for that water to be heated from cold which could well test your patience.

#2 - System boilers use more space than combi boilers

One of the many perks of a combi boiler is that the boiler itself is the only component. System boilers require an additional water cylinder that is going to take up space somewhere in your home, most likely an airing cupboard. That’s space you could be using to store golf clubs, wine or poorly behaved children.

Water cylinders don’t really come in all different shapes but definitely come in different sizes. It’s possible you’ll have a small cylinder and suffer from an inadequate hot water supply or maybe an oversized cylinder and you have to watch an entire series of The Soprano's before it's been heated.

Want to save money and energy on your heating bills? If so, you should consider installing one of the best smart thermostats.  

Recommended heating guides:

• System boilers vs combi boilers

• Best 40kW system boilers

• What is a boiler filling loop?

• How much does a powerflush cost?

• How to turn off a radiator

• How do you know if your boiler is condensing?

System boilers – The best system boilers available

Irrespective of your type of boiler, be it regular, combi or system – the foundations of picking a good one remain the same:

Is it reliable?

Is it well priced?

And does it have a good warranty?

Want an exact price of a new boiler? Use our boiler quote calculator (your fixed price will appear on screen).

#1 - Viessmann system boilers

Viessmann are statistically the UK’s most reliable boilers, Which? Best Buy award winners and through Heatable – all come with a 10-year parts & labour warranty. It’s German magic.

#2 - Ideal system boilers

Ideal boilers are a mainstay in the British heating industry and have been delivering quality boilers for longer than your Dad has been looking for his glasses. They offer outstanding value for money and there’s no finer option for a tight budget.

#3 - Vaillant system boilers

Vaillant has always had boilers in abundance so we’ve just picked the best ones. They’re a sturdy and reliable British company with a great reputation and decent warranties – albeit a touch overpriced in our opinion.

#4 - BAXI system boilers

BAXI are the underdogs of the boiler world, quietly going about their business in the background. They may be guilty of producing too many boilers, but their system boilers are more limited and they’ve done a ruddy good job with impressive 7-year warranties.

#5 - Worcester system boilers

Perhaps the only boiler you think you’ve heard of. An outstanding boiler company, with a great reputation and some fantastic boilers – but beware, they certainly have the price to match it.

How much do system boilers cost?

Typical Heatable costs for Ideal Logic system boilers, based on a straightforward like-for-like system boiler swap.

Typical Heatable costs for Viessmann system boilers, based on a straightforward like-for-like system boiler swap.

Check out our new boiler costs guide for something more comprehensive and if you want to know which boiler to choose, see our best condensing boilers review. 

Swap a system to a combi boiler?

Converting from a pre-existing system boiler to a combi boiler is going to be significantly more expensive due to the extra materials and work required.

Typical Heatable costs for Ideal & Viessmann combi boilers [converted from system boilers]. 

Heatable’s conversion prices include full removal of your old boiler & system cylinder, installation and upgrading of pipework if required and – obviously – your new combi boiler.

The only thing not included in a Heatable order is a stuffed meerkat.

System boiler size calculator 

Sticking with your loyal system boiler? Get the right size...

Getting the wrong size system boiler isn’t quite as bad as forgetting to wear pants in public. People won’t point and laugh. But you will be extremely let down with yourself for such a basic error.

Essentially, for small to medium-sized homes – allow 1.5kW per radiator + 3kW for your cylinder.

For something way more comprehensive, read our boiler size guide.

Don’t like maths? Let Heatable do the whole thing for you – get quotes.

How to get a new boiler 👻

You may need a new boiler.

You may need it fast, and with affordable monthly payments.

You may even need it from a company with top end local engineers, with 10,000 Trust Pilot reviews to back them up. Plus a Which? Trusted Trader award for good measure.

You may need a new boiler quote from us, Heatable 😘