Your guide to system boilers.

Learn how system boilers work and how they’re different from combi and regular boilers.

System boiler

What are system boilers?

A system boiler is made up of two parts. The normal boiler you’re used to seeing in your kitchen or air cupboard, and a separate cylinder for storing hot water. This means that system boilers are capable of providing a constant supply of hot water throughout your home, and are commonly found in larger homes which have a greater demand for heating and hot water.

System boilers explained.

Check out our quick video guide and learn if it could be the right boiler type for your home.

How do system boilers work?

System boilers provide hot water and central heating slightly differently depending on whether they’re a gas-fired system boiler or an electric-powered system boiler.

For gas-fired system boilers which are connected to the mains gas, firstly a flame heats up water in an enclosed hot water system. A coil in the water cylinder carries the central heating water at high temperatures through the cylinder, heating its contents and getting the hot water to where it needs to be.

Electric-powered system boilers work in the same heat but use an electric heat exchanger to warm up the circulated water rather than a gas flame.

This means that the water that comes out from the taps is from the cylinder, rather than the boiler, as it’s constantly being pumped through the coil in the hot water cylinder and the radiator system.

System boiler

What’s the difference between a system boiler and a combi boiler?

Whilst system boilers have a water storage cylinder which can store hot water for when it’s needed later, combi boilers are a stand-alone heating system and only heat water on demand. This means that combi boilers take up much less space than system boilers, ideal in smaller homes.

Because they don’t store hot water, new combi boilers are suitable for homes with one to three bedrooms and one bathroom as they’ll struggle to meet the needs of a larger household.

System boilers are able to manage the requirements of a large home as they can manage the hot water requirements of multiple radiators, taps and even underfloor heating.

Learn more about combi boilers.

What’s the difference between a system boiler and a regular boiler?

Regular boilers, or heat-only boilers, are similar to system boilers in that they also store hot water for later use. They are often found in large houses, such as four bedroom homes, and come with a header tank, or feed and expansion tank, in the loft.

This header tank in the loft keeps the regular boiler artificially under pressure and also uses an external expansion vessel to maintain the water pressure. System boilers have a pressurised heating circuit and use an internal expansion vessel.

This means that regular boilers often take up more space than a system boiler and can require more complex piping between the water storage tank and boiler. Because of this, system boilers are more commonly installed in modern properties with multiple bathrooms.

Learn more about regular boilers.

What are the advantages of having a system boiler installed?

Hot water from multiple taps at the same time.

Ideal for larger homes which have a high hot water demand, for example if you have multiple bathrooms.

Good water pressure.

The hot water comes directly from the mains instead of a water tank, which means you’ll get good water pressure.

No tank in the loft.

Unlike a regular boiler, you won’t need a tank installed in your loft, perfect if you’re considering that loft conversion.

Works with solar energy.

Because system boilers have a separate hot water cylinder, they can be compatible with solar panels or solar thermal.

How much do new system boilers cost?

That depends on a number of factors including:

  • The size of heating boiler needed to warm your home.

  • If you need to move your boiler system.

  • If you’re switching from a different boiler type, such as a combi boiler, or keeping like-for-like.

  • Any installation costs.

To help you get started, E.ON can give you a quote for having a new boiler installed and recommend the best Worcester Bosch system boiler for your home.

All our boiler installation costs are included in the price for your new boiler.

Are system boilers expensive to run?

System boilers are designed for larger homes and properties, where they’ll be more cost effective than a combi boiler or regular boiler.

The boiler installation costs might be a bit higher, however the aim will be to save money over time as the heating system will be designed to make the use of the water cylinder and ensure you can use multiple hot water points at the same time.

However if you’re in a smaller home then it may be more cost effective to have a combi boiler or regular boiler.

Can I get a new boiler on finance?

Yes, you can pay with interest-free finance rather than paying the full price of your new boiler upfront. You can choose to spread the cost of your Worcester Bosch system boiler flexibly over monthly payments, at no extra cost, with interest-free credit (0% APR) for up to 2 years.

We also have other affordable options to help you finance your new boiler in a way that suits you. We’re a credit broker and not a lender.1

1 E.ON Energy Installation Services Limited acts as a credit broker not a lender and work exclusively with Creation Consumer Finance Limited who acts as the lender. Credit subject to status, terms and conditions apply and can be found at, E.ON reserves the right to amend its finance offering at any time and may withdraw at short notice. E.ON Next Energy Limited is an Introducer Appointed Representative of E.ON Energy Installation Services Limited who are authorised and regulated by the Financial Conduct Authority in relation to credit broking services under number 750410. Registered in England and Wales under company number 09965944 with its registered office at Westwood Way, Westwood Business Park, Coventry, CV4 8LG.

Get warm and cosy and save up to £475 on your annual gas bill.2

Plus if you’re an E.ON Next customer save £150 when you buy a boiler through E.ON. T&Cs apply.

Get a new boiler

2 An average household could save up to £475 on their annual gas bill. Savings information for boilers in the UK provided by the Energy Saving Trust, based on a detached house with a boiler rating of G.

Why choose a Worcester Bosch boiler?

They’re fast and economical to run.

Worcester Bosch boilers are A-rated for efficiency, guaranteed to be 94% efficient, helping you to reduce your energy bills.

Smart connected thermostats.

Control your central heating and hot water with the wireless Bosch EasyControl, plus get easy-to-understand graphs on gas consumption and how efficient your heating programmes are.

Awarded Best Buy by Which?

Our new Worcester Bosch boilers build on the award-winning Greenstar range, offering smarter and more powerful condensing boilers with advanced technology to maintain water pressure.

Find out more about our Worcester Bosch Greenstar boilers.

How to get a Worcester Bosch Greenstar boiler installed in four easy steps.

1. See how much a new boiler costs.

Get a no-obligation quote and see which new boiler is right for your central heating needs.

2. Book your boiler installation.

Your boiler installation costs are included in the price and we’ll install your boiler when it works for you.

3. Get your boiler installed.

Our heating engineers will remove your older boiler, flush your heating system, check the pipework and show you how to use your new boiler and thermostat.

4. Post boiler installation support.

Once your boiler is up and running we’re here to help with any boiler repairs or boiler replacement parts.

You can get more information on boiler installation and boiler maintenance.

What Worcester Bosch Greenstar system boilers do you offer?

We have three Greenstar system boilers currently available. We’ll help you choose the right one for your central heating system:

What are the disadvantages of a system boiler?

Like any type of boiler, there are several points to bear in mind before you pick a new boiler system:

  • More space is required for the cylinder.

  • Busier times (when you have lots of visitors for example) may mean you have to wait for the cylinder to reheat.

  • May need to add insulation to stop heat and energy loss.