What Is A System Boiler? System Boilers Explained

Much like regular boilers, systems boilers heat hot water indirectly through a hot water tank. But instead of using a feed tank, they take their water supply directly from the mains. What is a system boiler is a common question we’re asked. This guide should help you understand everything there is to know about system boilers and how they work.

How do system boilers work?

Mostly installed with unvented cylinders, system boilers work on a sealed system. Meaning water has to be added manually through a filling loop. Although it may seem impractical. This eliminates the need for feed and expansion tanks. And makes the system work much more efficiently. To deal with expansion, system boilers have expansion vessels built-in. Due to system boilers being pressurised they have a PRV (pressure relief valve) which opens if the pressure ever gets too high. This will release water outside or into a drain to stop pipework from developing leaks. Like a regular boiler, system boilers heat the hot water through a coil in the cylinder.

Also, due to the design of the system, they can provide heating and hot water at the same time. Which a combi boiler cannot do. Using a twin channel programmer, the boiler can be instructed to fire up on central heating or hot water, or both. The programmer sends a signal to the zone valves to open, once they have opened a signal is sent to the boiler telling it to fire up. The boiler will fire up and circulate hot water around the central heating system and cylinder until they have reached temperature. Due to them being pressurised, system boilers tend to work a lot quicker. Meaning your home will get hotter faster!

what is a system boiler?

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


Essentially, system boilers are the modern counterparts of regular boilers. They work in a very similar way but by using modern system design, system boilers are much more effective.

Aside from system boilers working on a sealed system. The main difference is that system boilers contain most of their working parts inside, unlike heat-only boilers. There are no external pumps or feed tanks. Eliminating the need for zone valves, some unvented cylinders can even be installed with internal diverters. The loss of the feed tanks and external parts offer many advantages which we will cover in the next section.

What types of homes are system boilers suitable for?

Although they would work brilliantly in any home, system boilers are more suitable for larger properties with multiple bathrooms. Due to the space they take up it would be difficult to find room for them in smaller homes.

System boiler advantages


Better water pressure

When working in conjunction with an unvented cylinder, system boilers can offer considerably better water pressure than their traditional counterparts. System boilers work off mains pressure, so the hot water pressure is the same as the incoming cold water pressure. When the water pressure is good enough this can offer massive amounts of hot water. Enough to feed multiple showers at once. Perfect for large families with multiple bathrooms.

Worcester greenstore unvented cylinder

Works on a sealed system

System boilers work on a sealed system. Meaning that instead of having a feed and expansion tank feeding the system. Water has to be added manually. This offers numerous benefits. Firstly, sealed systems corrode much slower than an open vented system because the water isn’t exposed to open air. Secondly, the chances of debris getting into the system through the tank are eradicated. With feed tanks, sludge can build up and eventually get sucked in. This sludge can cause blockages and clog up components. Thirdly, due to the added pressure airlocks and blockages are much less likely. The added pressure can push airlocks out and keep sediment moving around the system to a point where it can be removed. Without the problems caused by an open vented system, system boilers work much more efficiently.

Flexibility of installation

When installed, heat-only boilers have to be situated below the feed tanks. With system boilers not requiring feed tanks, they can be installed almost anywhere. Giving much more flexibility.

Frees up loft space

Replacing a heat-only boiler with a system boiler frees up a lot of space in the loft, once the tanks have been removed. Leaving you with more space for suitcases, old clothes or even a loft conversion?

More covered under the warranty

Due to most parts being inside the boiler they’re all covered under the manufacturer’s warranty. This can save a lot of money on repairs in the long run. Also, due to most of the parts being inside the boiler it makes maintenance much easier.

Can use solar thermal

When installed in conjunction with an unvented cylinder, system boilers can utilise solar thermal panels to heat the cylinder. Saving a considerable amount of gas needed.

solar thermal

Energy efficient

As with all modern boilers, system boilers are highly efficient and must be at least 88% efficient due to regulations. Also due to unvented cylinders being highly insulated the boiler has to do considerably less work to keep the cylinder up to temperature.

Suitable for large homes

When sized correctly system boilers can heat large houses with multiple bedrooms. Two can be even be installed to work in tandem, to heat extremely large homes where a commercial boiler may have been used.

System boiler disadvantages


Use a lot of space

System boilers systems can take up considerably more space than combi boilers. The unvented cylinder will usually take up and entire cupboard. Some really big ones can be over 6ft tall so have to be installed in garages or plant rooms. This makes them unsuitable for houses short on space.

Take a while to heat up

System boilers heat the water in the cylinder relatively slowly, so once the hot water has run out it can take a while for it to heat up again. This can be made worse if the cylinder is undersized and runs out regularly.

How much do system boilers cost?

The cost of a system boiler installation can vary depending on the type of system you already have. And, what boiler and cylinder you go for. Most manufacturers make their own cylinders, generally, these work out more expensive. But, it can be easier if any faults occur as you will be able to get the boiler manufacturers to check both the boiler and cylinder.

When directly replacing a system boiler, expect to pay between £1,500-£2,500 depending on the boiler make and your location in the country. The further south you get, the more you will pay, unfortunately. If you’re replacing a combi boiler with a system boiler it will work out quite expensive due to the additional work involved. Most installations will cost between £3,000-£4,000.

Replacing a regular boiler with a system boiler may work out a bit cheaper if the cylinder is going in the same place as the previous cylinder. Expect to pay between £2,500-£3,500

Depending on the existing installation system boilers can cost up to £4,000 to install. If you’re directly replacing a system boiler, you may be able to get it done for under £2,500 depending on the brand of boiler installed.

Boiler installations are always expensive, but if you’ve got a high hot water demand, a system boiler and unvented cylinder are well worth it!

What system boilers are best?

Worcester Bosch

The most reputable boiler brand in the UK. Worcester Bosch has developed its fantastic reputation through years of quality products and great service. They offer a wide range of system boilers to suit most homes. Along with fantastic guarantees to go with them.

  1. Worcester Life/Style System – Released in 2019 the Life and Style range is the replacement for the old Classic boiler. Both boilers are highly reliable. And, when fitted by a Worcester Accredited Installer come with a 12-year guarantee. Both boilers have a new modern design, with the Style available in two colours, black or white, and a full-colour digital screen. Available in 30kw or 35kw this boiler is suitable for larger properties with a large heating and hot water demand. The only drawback is its large size, making it difficult to install in tighter spaces like kitchen cupboards.
  2. Worcester Greenstar i System – Worcesters most popular system boiler, this boiler’s compact size makes it perfect for homes tighter on space. Sizes range from 9kw to 30kw, so there’s an option for almost every size of home. Available with a 10-year guarantee when installed by a Worcester Accredited Installer.combi boiler installation cost


A well-known brand based in Hull, Ideal build well-made boilers and back it up with good customer service and warranties. A bit more budget-friendly than Worcester Bosch for those that are watching the pennies.

  1. Ideal Logic+ System – One of the only boilers in the UK to receive the Quiet Mark accreditation. Reasonably priced, the Logic+ offers great value for money. With a 7-year warranty and compact cupboard fit it offers a lot for its reasonable price. Available in 15-30kw sizes.
  2. Ideal Logic Vogue Gen2 System – Ideals premium boiler, the Vogue is a high-quality boiler with 10-year warranty to match (only when installed with an Ideal filter). Built using premium components and with a backlit 3.5″ LCD full-colour display this is Ideal attempt at matching Worcester. They’ve done a good job, but with the price being so close to Worcester prices, we’d go for a Worcester.


Currently the third-largest boiler manufacturer in the UK, Baxi have been going for well over a hundred years. If you’ve had an old regular boiler in the past there’s a large chance it was a Baxi. Baxi is a solid mid-range brand with an extensive product range. Their customer service is very good and they offer good warranties.

  1. Baxi 800 System – With a 10-year warranty and an Adey Micro2 magnetic system filter in the box the 800 is a bit of a bargain. Baxi has a very good reliability reputation, added to its cupboard friendly size this boiler is a great choice. The only drawback is that it’s only available in 18kw & 24kw, making it unsuitable for large homes.
  2. Baxi Megaflo System – Available in sizes ranging from 15kw all the way up to 32kw the Megaflo is suitable for a variety of property sizes. It comes with a 7-year warranty and a good build quality. Its large size makes it impractical for smaller spaces.


Ten years ago Vaillant was one of the best selling boiler manufacturers in the UK, but in recent years their market share has dropped by quite a bit. From personal experience, their customer service can be hit and miss, which may be one of the reasons. They do still make good quality boilers though.

  1. Vaillant ecoTEC Exclusive – Vaillants premium system boiler adapts itself so it only gives you the precise amount of heat you need. Offering a 10-year warranty when installed with a Vaillant Advance Installer. Only available in 27kw so not suitable for smaller homes.
  2. Vaillant ecoFIT Pure System – With five sizes available ranging from 12kw to 30kw there is a multitude of options available. Comes with a 2-year warranty as standard but can be upgraded to a 7 or 10 when fitted by a Vaillant Advance Installer.

vaillant ecofit pure


A big player in the German market, Viessmann has been growing over recent years and offer the typical high-quality German engineering you’d expect. But, in the past, their boilers haven’t been very user friendly so installers don’t really like them. Plus getting parts for them once the warranty has expired can be a bit of a pain in the backside. Another drawback is the flue is higher up, meaning that when directly replacing an existing boiler the new boiler will be lower. Now, this isn’t a problem in an airing cupboard where there’s lots of space. But, in a kitchen with a worktop underneath, it can make installation and maintenance difficult.

  1. Viessmann Vitodens 200-W – Able to modulate its output down to 6% of its maximum, this boiler offers energy-saving features and is up to 98% efficient. Has a colour touch screen offering easy operation of the boiler and can be connected to your phone with the ViCare app. Comes with a 3-year warranty as standard but can be upgraded to a 10-year warranty when installed by a Viessmann trained engineer.
  2. Viessman Vitodens 100-W – Significantly cheaper than that the 200-W the 100 still offers the same level of German engineering but with fewer technological options. It has a backlit LCD touch screen instead of a colour screen and is smaller than the 100-W. Available with a 10-year warranty when installed by a Viessmann Trained Engineer.


Hopefully, by now you know what a system boiler is, how a system boiler works and what are some of the best ones on the market. System boilers can be a fantastic addition when installed in the correct property. Your gas engineer should advise if one is suitable or not. If you need any impartial advice please don’t hesitate to get in touch – info@heatingbritain.com.

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