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What Is a System Boiler? A Detailed Guide

Need a new system boiler and don’t know where to start? You’re not alone; not only are there too many boiler types on the market, but each comes in a variety of models from dozens of brands.

To save you time and trouble, we did some research and came up with this 5-minute guide. Here, you’ll learn how system boilers work, how they differ from combis, and show you the best gas system boilers for your property.

system boiler guide

Want to jump right ahead and get quotes? We’d recommend Heatable. They’re an online boiler installer, and they can offer a Worcester system boiler at stupidly low prices. Use this clickable form to get your fixed price on screen today.

Worcester Bosch Greenstar 4000

How Does a System Boiler Work?

Simply put, a system boiler relies on a hot water storage tank or cylinder to store hot water rather than providing heating and hot water on demand, like a combi.

Central heating systems with system boilers are suited to large family homes with significant hot water demand (i.e. multiple showers and taps being used at once).

The Benefits of System Boilers

Here are the major advantages of using a system boiler:

  1. It’s Great for Large Properties With High Hot Water Demand

With system boilers, a property can get away with using a lower output boiler.

Although there are high-capacity combi boilers on the market, there’s a point where it doesn’t make sense to fit one. Combination boilers heat hot water on demand. So if you have 2-3 showers or taps running at once, you’ll need a huge combi boiler to meet that hot water demand.

System boilers store hot water in a hot water storage tank, which is usually kept in an airing cupboard. Instead of buying a ridiculously sized combi, you can fit a cylinder that provides an adequate supply of hot water instead.

  1. System Boilers Offer High Water Pressure Compared to Conventional Boilers

The problem with conventional boilers is that they require a cold water storage tank in the loft. You’ll usually find this water tank located in your loft. Without additional pumps, the pressure in a regular boiler system relies solely on gravity, and tends to be low.

This is where system boilers offer a major advantage — their water supply comes directly from the mains. The pressure of mains water is much higher than that of a feed tank in your loft. Even better, system boilers can be used with a pressurised hot water storage cylinder.

  1. They’re Modern and Advanced

Some central heating systems are incredibly outdated. Take those running on oil, or even back boilers, for instance. In most scenarios, a simple boiler replacement is going to turn into a full-on central heating upgrade, without which the system wouldn’t be efficient.

That’s not the case with system boilers — if your property has high hot water demand, there’s no need to switch to a combi boiler. And, depending on the condition and size of your hot water cylinder, there’s a chance you can reuse it.

System Boiler Prices

So, how much do gas system boilers cost?

The prices range significantly between brands, models, heating outputs, warranty lengths, and other factors.

If you’re looking at cheaper models, you’ll pay around £600. That’s for a low-end brand with a short warranty. Those prices can rise quickly when you change to superior brands, higher kW outputs, and look for boilers offering a high warranty.

We’d always aim for a warranty of 5-years or higher, and you can get 10-years on a Worcester system boiler with Heatable here.

Worcester System Boiler Prices

Below, we’ve listed estimated system boiler prices from Worcester Bosch (boiler only, inc VAT), as they’re one of the most popular brands. You’ll find their prices in the mid-range. These exclude the cost of the cylinder, though, so that’s going to be at an additional cost if you need to replace yours; you can read more in our guide to cylinder installation costs here.

Boiler Price
Greenstar 12Ri £775
Greenstar 15Ri £810
Greenstar 18Ri £856
Greenstar 24Ri £935
Greenstar 12i £870
Greenstar 15i £920
Greenstar 18i £990
Greenstar 24i £1,030
Greenstar 30CDi Classic System £1,140
Greenstar 35CDi Classic System £1,285

If you’re looking to install a replacement boiler, the best deals we’ve found come from Heatable. You can get your own range of fixed-price boiler quotes (with financing options) by filling in Heatable’s anonymous property questionnaire.

Heatable boiler installations

How Much Does It Cost to Install a System Boiler?

The cost of installing a system boiler starts at around £2,500, including a hot water cylinder. In contrast, combi boiler installation costs typically start at £1,750-£2,000.

The bigger your system, and the more modifications it requires (additional pipework, upgrading from a back boiler, etc), the more you’ll pay.

What’s the Difference Between a System Boiler and a Combi Boiler?

A combi heats hot water on demand and doesn’t require a hot water storage tank. Therefore, properties with 2 or more showers typically need a HUGE combi boiler or a system boiler.

A system boiler, on the other hand, can constantly trickle hot water into a hot water storage tank or cylinder. This way, a smaller system boiler can provide a higher volume of hot water at once compared to a combination boiler.

Are System and Heat Only Boilers the Same Thing?


A heat-only boiler (also known as a conventional or regular) is similar, but they also require a cold water storage tank. It’s this cold water storage tank that feeds the hot water cylinder.

Combi Boiler vs System Boiler

So why would you ever install a combi boiler over if system boilers can supply more hot water?

In the system vs combi boiler match-up, it all comes down to one thing — space.

Hot water cylinders range from 90L (incredibly small) to 300L (extremely large), and beyond. These are not small units, and typically, they’ll take up at least cupboard.

This is why the average, 2-4 bedroom UK property can save space by opting for a combi instead of a system boiler.

Now, if you’ve got 2 or more showers (regardless of how many bedrooms you have), you might want to consider a system boiler instead of a combi.

After checking out your central heating system and listing hot water requirements, the number of radiators and towel rails, as well as showers, a Gas Safe engineer will know which boiler suits your property best. You can also use Heatable’s online calculator to see whether your home needs a combi or a system boiler.

What kind of fuel does your boiler use?

Fixed price online with next day installation

Worcester boilers from as little as £1,965.

The Best System Boilers in 2024

You’re going to see a tonne of websites recommending specific system boilers.

But there’s a slight problem with this…

It’s useless recommending a boiler without seeing the property. One 3-bedroom semi isn’t the same as the next (extensions, radiators, number of bathrooms), and in some cases, a combi upgrade may be more feasible than a system boiler swap.

So, instead of boring you with a list of the best system boilers, and specs that mean nothing to anyone but a Gas Safe heating engineer, we’ll go over the best system boiler brands instead. (We’ve created a full, detailed list of the best boiler brands here; it applies to system boilers, too).

The Best System Boiler Brands

  1. Worcester Bosch
  2. Viessmann
  3. Ideal
  4. Baxi
  5. Vaillant
  6. Potterton
  7. Glow-worm

What Size System Boiler Do I Need?

This is exactly why we’ve not recommended specific boilers — there are tonnes of variables that affect the required boiler output and cylinder size.

You can jump over to this boiler size calculator to get a precise answer — it takes about 90 seconds to fill out.

What Type and Size Cylinder Do I Need?

If you’re getting a new system boiler, you’ll need to install or replace your existing hot water cylinder too. You can read this post to get a guestimate for your hot water cylinder size.

To further add to the confusion, you’ll have a certain type of cylinder — either vented or unvented. And, you HAVE to make sure you buy the right one. You can click here to read our guide on the difference between vented and unvented heating cylinders.

What’s Next?

Thanks for reading our 5-minute guide to system boilers. Ready to get one installed? Get a fixed price on screen here in less than 90 seconds.

Still got questions? Drop a comment below and we’ll reply as soon as we can.



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