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System Boilers Explained [And How Much They Cost to Install]

This 5-minute guide is going to help you understand how system boilers work, how they differ from combis, and the best system boilers for your new installation.

Need a new system boiler and don’t know where to start?

No problem 🙂 – this guide has everything you need to know.

Want to jump right ahead and get quotes? We’d recommend Heatable. They’re a boiler comparison site, so offer Worcester Bosch system boilers at stupidly low prices. Use this clickable form to get your fixed price on screen today.

Heatable new boilers

How Does A System Boiler Work?

Put simply, a system boiler relies on a hot water storage tank or cylinder. It stores hot water here, rather than heating water on demand (like a combi).

Central heating systems using a system boiler are suited to large properties, with lots of hot water demand (multiple showers and taps being used at once).

The Benefits Of System Boilers

So, what are the major advantages of using a system boiler? And should you switch to a combi?

  1. Large Properties With High Hot Water Demand

Although there are large 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. And, that means if there’s 2-3 showers or taps running at once, you’re going to need a huge boiler to meet that demand.

With system boilers, a property can get away with using a much smaller kw boiler. Hot water is stored in a hot water storage tank. Instead of buying a ridiculously sized boiler, you can fit a cylinder that suits your hot water needs instead.

  1. High Water Pressure Compared To Conventional Boilers

The problem with conventional boilers, is they rely on a storage tank. You’ll usually find this located in your loft. Without additional pumps, the pressure from conventional boiler systems tends to be extremely low.

And this is where system boilers are a major advantage; they get water directly from the mains. The pressure of mains water is going to be much higher than that of a feed tank in your loft. Even better, these systems can be used with pressurised cylinders.

  1. They’re Not Outdated

Some central heating systems are incredibly outdated.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, otherwise the system isn’t going to 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 cylinder, there’s a chance that you can reuse it.

System Boiler Prices

So, how much do system boilers cost? Below, we’ve listed system boiler prices (estimates) from Worcester Bosch, as they’re one of the most popular brands. But, if you need to upgrade your cylinder too, that’s going to be at an additional cost. You can read our guide to cylinder installation costs here.

If you’re looking for cheap, you’ll pay around £600 for a system boiler. That’s a low-end brand, typically, with a low warranty too. 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 with Heatable here.

Worcester Bosch System Boiler Prices

Worcester Bosch system boilers are some of the most popular on the market, so let’s start with these as an example. In terms of prices, these are going to be mid-high range.

If you’re looking for system boiler installation, the best deals I’ve found on Worcester come from Heatable.

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

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. This compares to combi installation costs that typically start at £1,750-£2,000.

The bigger your system, and the more modifications needed (additional pipework, upgrading from a back boiler etc), the higher the cost.

You’ll be able to get competitive prices on screen via Heatable.

What’s the Difference Between A System and A Combi?

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 can constantly trickle hot water into a hot water storage tank or cylinder. Meaning, a smaller system boiler can provide a higher volume of hot water at once (due to it being stored), compared with 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.

Is A System Better Than A Combi Boiler?

So why would you install a combi boiler over a system boiler, if a system boiler can supply more hot water due to its storage facility?

It 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 a small cupboard (and more, on some occasions).

So, the average property in the UK (2-4 bedroom semi for instance), there are plenty of combi boiler solutions that are suitable.

But, if you’re property has 2 or more showers (regardless of how many bedrooms you have), you might want to consider a system boiler instead.

After checking out your heating system and listing hot water requirements, number of radiators and towel rails as well as showers, it will become obvious to them whether a combi or system boiler is the most suitable setup.

What kind of fuel does your boiler use?

Grab your fixed price online by 3PM, and get next-day boiler installation.

Worcester boilers installed from as little as £1,795.

The Best System Boilers in 2020

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

Slight problem…

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, it might be feasible to upgrade to a combi, rather than replacing a system boiler.

So, instead of boring you with a list of the best system boilers, and specs that mean nothing to anyone but a heating engineer, we’ll instead, list the best system boiler brands.

We’ve created a list of the best boiler brands here, and that list stands for 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?

And, this is exactly why we’ve not recommended specific boilers.

Because there are tonnes of variables that affect what size kw boiler you need, and the size cylinder you need.

You can click here to figure out roughly what kw boiler you need. Or, to save you reading, you can jump over to this boiler size calculator (takes about 90 seconds to fill out).

What Type And Size Cylinder Do I Need?

But, you’re going to need to install or replace your existing hot water cylinder too. So, you can click here for a guestimate on what size hot water cylinder you’re going to need.

To further add to confusion, you’ll have a certain type of cylinder (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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