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System Boilers: What they are, how they work and what they cost

Here, we deep dive in to the mystical world of system boilers. Their inner workings, pro's, con's, and a brief guide to the best system boilers on the market (along with potential costs).

What the hell is a system boiler?

How do system boilers work?

How much do system boilers cost?

We’re literally going to answer all three of those questions.

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

First up… Boilers come in 3’s.

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

Regular boilers

Regular BoilerAlso 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.

Combi boilers

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.

How a system boiler works

How Do System Boilers 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 cylinders size. So you’ll be having to switch your boiler on well prior to needing any decent amount of hot water.

Unfortunately, water that’s heated slowly doesn’t share the bonuses of a slow cooked lamb leg – it feels, cleans and tastes exactly the same. Your skin will not fall off the bone.

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What is a system boiler suitable for?

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 shows 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.

The pros of a system boiler

System Boiler Pros

#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 the large amount of stored hot water (that’ll stay hot for hours) will allow numerous household occupants use of 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.

All of this extra space means you can store even more trash and Christmas decorations in your loft. A real bonus for your dust collection.

The cons of a system boiler

System Boiler Cons

#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.

It’s not how you use it. Tank size matters

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 its been heated.

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?

#1 - Viessmann system boilers

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.

Viessmann Model Warranty  Price
Vitodens 100 19kW 10 Years
Vitodens 100 26kW 10 Years
Vitodens 100 30kW 10 Years
Vitodens 100 35kW 10 Years

#2 - Ideal system boilers

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.

Ideal Model Warranty  Price
Ideal Logic 15kW 5 Years
Ideal Logic 18kW 5 Years
Ideal Logic 24kW 5 Years
Ideal Logic 30kW 5 Years

#3 - Vaillant system boilers

Vaillant System Boilers

Vaillant have 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 option.

Vaillant Model kW Outputs Warranty  Price
ecoTEC Plus 12 - 37kW 5 Years
ecoFIT Pure 12 - 30kW 2 Years
ecoTEC Exclusive 27kW 5 Years

#4 - BAXI system boilers

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.

BAXI Model kW Outputs Warranty  Price
Megaflow 15 - 32kW 7 Years
600 15 - 24kW 7 Years

#5 - Worcester system boilers

Worcester Bosch 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.

Worcester Model kW Outputs Warranty  Price
Greenstar i 9 - 24kW 5 Years

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How much do system boilers cost?

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

Boiler Warranty Price  Price
Ideal logic 15kW 5 Year (Parts & Labour) £1,895
Ideal logic 18kW 5 Year (Parts & Labour) £1,995
Ideal logic 24kW 5 Year (Parts & Labour) £2,195
Ideal logic 30kW 5 Year (Parts & Labour) £2,345

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

Boiler Warranty Price  Price
Viessmann  100 19kW 10 Year (Parts & Labour) £1,895
Viessmann  100 26kW 10 Year (Parts & Labour) £1,995
Viessmann 100 30kW 10 Year (Parts & Labour) £2,195
Viessmann  100 35kW 10 Year (Parts & Labour) £2,345

Check out our new boiler costs guide for something more comprehensive.

How much to ditch your system boiler and convert to a combi boiler?

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

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

Combi Boiler Warranty Price  Price
Ideal Logic Combi 24kW 5 Year (Parts & Labour) £2,790
Ideal Logic Combi 30kW 5 Year (Parts & Labour) £2,990
Viessmann  050 29kW 10 Year (Parts & Labour) £2,939
Viessmann 100 30kW 10 Year (Parts & Labour) £3,370

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.

What size boiler for... Boiler Size Calculation Minimum kW Size
6 Radiators 6x 1.5kW + 3kW 12kW
8 Radiators 8x 1.5kW + 3kW 15kW
12 Radiators 12x 1.5kW + 3kW 21kW
15 Radiators 15x 1.5kW + 3kW 26kW
20 Radiators 20x 1.5kW + 3kW 33kW

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 the best price on a new system boiler

How? Without boasting, buy from us.

We're an online, nationwide boiler installation business with 500+ 5 star reviews from a shed load of happy customers.

Fixed Online Boiler PricesBecause we're online, our overheads are low which means we pass those savings on to our customers, ensuring they're always getting the best deal on a new boiler. Plus, because of the volumes that we install, we're able to access exclusive warranty lengths from our partner manufacturers. For example, we can offer 10 years of protection on the Viessmann 050-W range, whereas your traditional installer can only offer you 7. Boom.

To get the best deal on a new combi boiler, answer some quick online questions and we'll give you an instant fixed price.

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Heatable is registered in England and Wales No. 08804726 (VAT No. GB205805920 ). Trading address is 1 Station Road, Great Sankey, Warrington, Cheshire, WA5 1RH. Heatable acts as a credit broker (not as a lender) and offers credit products through a panel of lenders. Heatable is authorised and regulated by the Financial Conduct Authority (reference no. 805259).

[1] Price based on a like for like combi swap in a 2 bed, 1 bathroom terraced home with a standard flue

[*] Installation dates are subject to installer availability

[¹] only available on combi boiler installations