System Boiler

System boilers heat domestic hot water and central heating separately by making use of a hot water cylinder. System boilers are a great choice for larger households that have high hot water demands. It’s possible to find system boilers suited to every budget and property.

System boilers work by heating water for central heating directly. This hot central heating water circulates through a home’s radiators and through a coil in the hot water cylinder. The hot water cylinder is a storage tank for hot domestic water, and the coil inside it makes the water hot for taps and showers. A major benefit of system boilers is that they are compatible with twin coil hot water cylinders, which permit you to add a secondary heat source, such as solar power.

System boilers run out of hot water if you use up all the water stored in the hot water cylinder. It takes 20 – 30 minutes for the cylinder’s coil to bring your domestic hot water back up to temperature if this happens to you.

System boilers use unvented cylinders that work in a pressurised heating system, which means they do not use cold water storage tanks to maintain water pressure. Cold water tanks are a hallmark of regular boilers, which use vented cylinders. System boilers differ from combi boilers in that system boilers use hot water cylinders while combi boilers do not store pre-heated water. Instead, combi boilers heat water on demand.

System boilers are available in a range of sizes from as low as 9kW to as high as 33kW. The range of system boilers available makes it easy for you to find the perfect boiler for your property. Smaller 9 – 18kW system boilers support up to 10 radiators while larger system boilers with outputs over 26kW support 15 – 20 radiators to heat the largest properties.

System boilers take up more space than combi boilers because they require a hot water cylinder to operate. System boilers typically need one square metre of space for the boiler itself and 2 square metres for the hot water cylinder.

A new system boiler costs £2,000 – £3,800 fitted, which breaks down into £1,000 – £2,800 for the boiler itself and £800 – £1,200 for the installation costs. The best system boilers are manufactured by Worcester, Viessmann, Ideal, Vaillant, and Baxi. The best individual system boilers are the Worcester 8000, Ideal Logic+, The Viessmann VItodens 100-W, and the Vaillant ecoTEC Plus.

How does a system boiler work?

A system boiler works by heating water for the radiators and indirectly heating domestic hot water inside a cylinder. System boilers burn fuel and transfer the heat to the central heating water via a heat exchanger. Heated central heating water then circulates through all the radiators in your home, heating the rooms. Hot water in a separate circuit passes through a coil in a cylinder storing domestic hot water. The heating and domestic hot water systems stay entirely separate so you never drink the water from your central heating.

How long does a system boiler take to heat water?

A system boiler takes 20-30 minutes to heat the water in a cylinder. The hot water cylinder gets refilled from the cold water mains. Thus, your taps and showers run cold once you’ve used up all the heated water in the cylinder. You have to wait for that cold water to heat before you can use it. The following two factors determine how long it takes for your system boiler to heat water.

Tank size: A larger hot water tank takes longer to heat fully. A small 115 – 150 litre tank heats up considerably faster than a larger 230 – 300 litre tank. That said, a larger tank stores a higher volume of hot water, which lasts longer.
Boiler output: The higher your system boiler output, the more energy it creates to heat up the water in your central heating system. This increased capacity boosts the speed at which your domestic hot water is reheated.

How long does water stay hot in a tank?

Water stays hot for 1-2 days in a tank. A typical hot water tank loses between 1kWh and 2.5 kWh per day. How long the water stays hot in a tank depends on its insulation. The better insulated your cylinder is, the longer the water retains heat.

How does a hot water tank work?

A hot water tank works by storing pre-heated water for use in your shower or hot water taps. The hot water cylinder that works with a system boiler does not heat the water itself. Instead, a coil passes through the hot water cylinder carrying heated water from the boiler.

The most simple hot water cylinder is a single-coil hot water cylinder. Single-coil cylinders have one coil through which hot water circulates, and rely on a convection current to spread the heat from the coil throughout the tank. A convection current moves hot water around the cylinder as the less-dense heated water naturally rises to the top of the boiler and cooler, denser water falls to the level of the coil. The result is that the warmest water in the boiler is located at the top of the cylinder, where the exterior pipe carries it to your shower or hot water tap.

An alternative type of hot water cylinder is a twin-coil hot water cylinder. Twin-coil hot water cylinders contain the primary coil (which carries hot water from the system boiler) and a secondary coil. This secondary coil typically runs through the bottom section of the hot water cylinder and adds additional heating options for your domestic hot water, as it allows connections to a secondary heat source. Solar energy is often used as a secondary heat source in twin-coil cylinders.

System boilers use unvented cylinders, while regular boilers use vented cylinders. A vented cylinder is connected to a cold water tank stored in the loft. The higher location of the cold water tank uses gravity to create natural pressure in the water circulation system. An unvented cylinder simply does not have this second cold tank and therefore system boilers rely on the pressure from the cold water mains to create water pressure.

The disadvantage of a vented system is that it tends to have less consistent water pressure. An unvented system boiler’s advantage is that it does not need an external expansion vessel. Vented systems are closed systems so they need an external expansion vessel to accommodate the excess water that is created as the stored water expands as it heats.

What is the difference between a heat-only and a system boiler?

The two differences between a heat-only and a system boiler are listed below.

Heat-only boilers have cold water tanks: Heat-only boilers use two cold water tanks in addition to a hot water cylinder. One of these tanks stores cold water for use in taps and showers, whilst the other contains water for the heating system. System boilers only require a hot water cylinder to operate.
Heat-only boilers have an external expansion tank: System boilers have an internal expansion vessel whereas heat-only boilers use a feed and expansion tank to release excess water pressure.

What is the difference between a combi boiler and a system boiler?

The following are the two differences between a combi boiler and a system boiler.

Combi boilers do not have cylinders: System boilers use cylinders to store hot water for domestic use, while combi boilers do not.
Combi boilers use mains water pressure: Combi boilers heat water from the mains but do not have pumps to generate water pressure. Using mains water pressure means that combi boilers do not need expansion vessels.

System boiler vs combi boiler running costs

A system boiler has higher running costs than a combi boiler for the following two reasons.

Lower energy efficiency: Combi boilers are generally more energy efficient than system boilers and so cost less money to operate than system boilers.
Hot water storage: System boilers require a hot water storage tank, in which the water must be kept sufficiently hot. This process consumes more energy than heating water on demand.

How do I know if I have a system boiler?

Take the following four steps to know if you have a system boiler.

Check for a cylinder: System boilers use a hot water cylinder to store domestic hot water, which is generally located in an airing cupboard. Having a hot water cylinder means you either have a system boiler or a regular/heat-only boiler.
Check your loft: System boilers do not use an external cold water tank, unlike regular/heat-only boilers. You will not find a cold water tank in your loft if you have a system boiler.

Check for pipes coming out of your boiler: System boilers typically have three copper pipes leading from the base of the boiler. You may have a combi boiler if you have four or more such pipes and a regular/heat-only boiler if you have fewer.

Check for an external pump: System boilers do not use an external pump whereas regular/heat-only boilers do.

What sizes do system boilers come in?

System boilers come in sizes ranging between 9kW and 33kW, which makes them suitable for all types of properties, from small apartments to larger houses. The following are the three most common system boiler size ranges.

9-18 kW system boilers: System boilers with 9 – 18 kW outputs are suited to smaller properties with up to 10 radiators, 1-2 bedrooms, and 1 bathroom.
18-26 kW system boilers: System boilers with 18 – 26 kW outputs are able to heat medium-sized properties with 10 – 15 radiators, 2 – 3 bedrooms, and 2 bathrooms.
26-40 kW system boilers: System boilers with 26 – 40 kW outputs are suitable for larger properties with 15 – 20 radiators, 4+ bedrooms, and 3+ bathrooms.

What system boiler size do I need?

Multiply the number of radiators in your home by 1.5 and add 3 to figure out what system boiler size you (roughly) need. However, calculating an accurate boiler size requires a professional tool that factors in various property parameters. Heatable’s online tool helps you find the perfect system boiler for your property, taking into consideration your property’s size, number of radiators, number of showers, and number of baths. Using Heatable’s tool means you don’t need to call a gas engineer to your home to find the right boiler size for your property.

How much room does a system boiler need?

A system boiler needs up to three square metres of room. This space includes one square metre for the boiler itself and up to 2 square metres for the hot water cylinder. System boilers are typically 700 – 800 mm in height, 200 – 400 mm in width, and 400 – 500 mm in depth. Their attached hot water cylinders are generally 1.5 – 2 metres in height and 500 – 600 mm in diameter.

How efficient are system boilers?

System boilers are highly efficient, with modern boilers rated “A” for energy efficiency with typical seasonal space heating efficiency ratings of 90 – 98%. Switching to a more efficient “A-rated” system boiler like the ones on this list compared to an inefficient “G-rated” boiler saves you £840 if you live in a detached home, according to the Energy Saving Trust. Making a similar switch in an apartment could save you as much as £270 a month.

However, system boilers are generally less efficient than combi boilers because they have to keep a hot water tank heated.

How much is a new system boiler?

A new system boiler is £2,000 – £3,800, including installation costs and the boiler itself. That price breaks down into £1,000 – £2,800 for the boiler itself and £800 – £1,200 for installation costs. A system boiler also needs a hot water cylinder in order to work, so you will need to purchase a hot water cylinder if you do not already have one. Hot water cylinders typically cost £400 – 1,600 to purchase and install.

How much does it cost to install a system boiler?

It costs £800 – £1,200 to install a system boiler, excluding the £1,000 – £2,800 cost of the boiler itself. The following five factors influence how much you pay to install a new system boiler.

Location: Location plays a big role in how much you pay to get a new system boiler installed. More expensive locations, like London, typically have higher boiler installation costs.
Installer: Your choice of installer heavily influences the cost of installing a new system boiler. Some installers offer more efficient services by cutting out operating costs, while other national brands charge more because they’re well-known.
Scope of work: Expect to pay more for a new system boiler installation if you are changing the location of your boiler or switching from a different kind of boiler.
Warranty: System boiler installations are more expensive if your boiler’s manufacturer’s warranty requires your installer to follow specific steps when installing your new system boiler.
Extras: Adding extras, such as third-party boiler controls, increases the price of your boiler installation.

It is best to go through a professional installer to get a quote suited to your particular property and situation. Heatable is a great option if you want to get a fixed quote on a new system boiler without having to call an installer to your home. Their simple online calculator gives you an accurate quote for a new system boiler in just 90 seconds.

How much does changing from a conventional boiler to a system boiler cost?

Changing from a conventional boiler to a system boiler costs £800 – £1,200 beyond the normal costs of installing a boiler. This cost covers the removal of the conventional boiler system cold boiler water tank and changes to the piping necessary to complete the transformation to a new system boiler.

What is the cheapest system boiler in the UK?

The cheapest system boiler in the UK is the Baxi Megaflo. The Baxi Megaflo retails for £700 – £900 for the boiler alone with installation costs typically running you an additional £800 – £1,200 you should expect to pay for installation. The Baxi System is a good boiler for a small apartment with 15kW of energy output and a 91% energy efficiency rating delivering low maintenance costs.

Where to find the best new system boiler deals in the UK?

Heatable’s easy online quote calculator lets you find the best new system boiler deals in the UK. Heatable deliver the best prices by running on very low overhead. The installer does not operate any storefronts and maintains excellent relationships with boiler distributors across the UK, both of which enable them to deliver you the best value.

Heatable is a reputable installer with decades of experience installing all types of boiler systems. The installer is so confident of the quality of their installations that they offer a 10-year warranty on any boiler they install even if the installer’s standard warranty is much shorter.

Which is the best system boiler in 2026?

What the best system boiler in 2026 is depends on your individual preferences. However, you cannot go wrong by choosing a new system boiler from one of the five brands below.

Worcester Bosch
Viessmann
Ideal
Vaillant
Baxi

Below is a thorough guide to the best system boiler models from the five aforementioned brands.

Worcester system boilers

Worcester system boilers are some of the best on the market, but they come at a premium price. The two advantages of Worcester Bosch system boilers are their 10-year guarantee and their market-leading features. The disadvantage of the Worcester system boilers is their price, which is generally higher than alternative boilers on the market because of Worcester’s brand name and the quality of their product.

Worcester Greenstar 4000 system

The Greenstar 4000 is a premium boiler with advanced capabilities. The 4000 has fewer bells and whistles than the Greenstar 8000 but is still one of the top of the range system boilers on the market for the moderate sum of £3,100 – £3,600.

There are four advantages to the Greenstar 4000 system. First, The Greenstar 4000 has wireless connectivity via Worcester EasyControl, empowering you to adjust your boiler settings no matter where you are. Second, the Greenstar 4000 is compatible with Greenskies solar panels. Third, the Greenstar 4000 comes with a 10-year guarantee which is competitive with other manufacturers of similar quality. Finally, the Greenstar 4000 has an intuitive LCD display that makes it easy to do simple troubleshooting yourself.

There are two disadvantages to the Greenstar 4000 system. First, it has fewer of the advanced features you get with the Greenstar 8000. Second, it is expensive at £3,100 – £3,600.

Worcester Greenstar 8000 system

The Greenstar 8000 system is a top-of-the-line boiler with advanced features that deliver in terms of durability and energy efficiency. The 8000 is not cheap at £3,200 – £3,800, but you get one of the best boilers on the market for that price.

There are three advantages to the Greenstar 8000 system. First, it comes with a 12-year guarantee as standard. This guarantee is not only longer than other leading brands – Viessmann offer a 10-year warranty as standard – it is also more rigorous as Worcester is obligated to repair any damage. Second, The Greenstar 8000 system has wireless EasyControl, a wifi-enabled smart thermostat that connects to your smartphone and allows you to adjust your boiler settings from anywhere. Finally, The Greenstar 8000 has a host of energy-conscious features like intelligent filling that help deliver a 94% efficiency rating. That rating means that for every £1 you spend on heating your home, the 8000 system uses 94 pence for heating and wastes just 9 pence.

There is one disadvantage to the Greenstar 8000 system – the price. The Greenstar 8000 system costs £3,200 – £3,800, which is more expensive than alternative system boilers like the Vitodens 100-W with a price tag of £3,000 – £3,600.

Viessmann system boilers

Viessmann make premium system boilers that rival and sometimes exceed Worcester for quality and features. The two advantages of Viessmann system boilers are their advanced stainless steel parts and impressive energy efficiency and durability. The disadvantage of Viessmann system boilers is their short warranties.

Viessmann Vitodens 100-W

The Vitodens 100-W is a reliable boiler that offers some of the most advanced components for delivering high energy efficiency and durability. The major downside of the 100-W is a short warranty that fails to compete with those offered by other premium boiler manufacturers.

The Vitodens 100-W has three advantages. First, the 100-W comes with a stainless-steel Inox-Radial heat exchanger and MatriX-Plus burner that improve the boiler’s durability due to stainless steel’s resistance to damage. Second, the MatriX-Plus burner and Lambda Pro combustion system modulate the Vitodens 100-W’s use of fuel to achieve the best possible energy efficiency. Finally, the Vitodens 100-W comes with wifi integration so you can control your boiler wherever you are.

There is one disadvantage to the Vitodens 100-W – a short warranty. The Vitodens 100-W comes with only a 2-year standard warranty which extends to 5 years when you register your boiler with Viessman. This warranty is far shorter than warranties for comparable boilers like the 10 or 12-year guarantees Worcester offer on their system boilers.

Viessmann Vitodens 200-W

The Vitodens 200-W is a reasonably priced boiler that offers features and functionality to satisfy most families. The 200-W offers a high level of durability and good energy efficiency and includes sizes suitable to most homes.

The Vitodens 200-W has four advantages. First, the 200-W is one of the few boilers that can run on a hydrogen mix, an efficient and clean fuel type that is becoming increasingly popular with manufacturers and regulators. Second, the 200-W’s stainless steel parts help extend the lifetime of its boiler. Third, the 200-W uses some of the most advanced parts of any system boiler to deliver a high level of energy efficiency and durability. Finally, the 200-W is priced well for its features and quality at just £2,300 – £2,850.

The Vitodens 200-W has one disadvantage – the warranty. The 200-W comes with a 5-year standard warranty that is considerably shorter than the 10 or 12-year guarantees offered by competitors like Worcester. It is possible to extend the warranty to 10 or 12 years for an additional fee.

Ideal system boilers

Ideal make a range of system boilers that cover different budgets. Good warranties back up all Ideal system boilers and more premium models like the Vogue Max have the longest warranty on the market for a system boiler. Ideal make their products a pleasure to use with easy-to-use displays.

Ideal Logic+

The Logic+ is a great all-around boiler with an affordable price, easy-to-use design, and a decent warranty. The unit is unfortunately held back by being a little less energy efficient than the other boilers in this guide.

There are three advantages to the Ideal Logic+. First, the Logic+ has a 7-year warranty on the boiler and a 10-year warranty on its heat exchanger. These warranties are competitive with other premium boilers in terms of length and speak to the durability of the unit. Second, the Logic+ comes with a fully back-lit LCD display so you can easily change the settings on your Logic+. Finally, the Logic+ is available for a reasonable price of £1,500 – £2,100.

There is one disadvantage to the Ideal Logic+ system – its energy efficiency. The Logic+ system has a seasonal space heating efficiency rating of just 89.5%. This level of efficiency is still impressive compared to inefficient boilers but falls significantly behind other boilers on this list.

Ideal Vogue Max

The Ideal Vogue Max is a premium boiler with impressive features and a price to match. The Vogue Max boasts an LCD display, weather compensation, and wireless control.

The Ideal Vogue Max has four advantages. First, the Vogue Max has a 3.5-inch full-colour backlit LCD display, which makes adjusting your system boiler’s settings easy and intuitive. Second, the Vogue Max comes with an impressive 12-year warranty as standard, a warranty length matched by only Worcester. Third, the Logic Vogue Max system boiler is compatible with Ideal Controls, which allow you to set your boiler no matter where you are. Finally, the Vogue Max offers weather compensation, meaning that it measures external weather patterns and automatically adjusts your heating without your intervention.

The Ideal Vogue Max has one disadvantage – its price. The Ideal Vogue Max is expensive at £2,000 – £2,600 for full installation. This price tag comes with advanced features that may make it worth it for the right consumer.

Vaillant system boilers

Vaillant manufacture a small range of high-quality system boilers. Vaillant system boilers do not have all of the premium features of a Worcester or Viessmann boiler but are reasonably priced and offer a longer guarantee than many other brands.

Vaillant ecoTEC Plus system

The Vaillant ecoTEC Plus system boiler is a durable and high-performing boiler with outputs suited to any sized property. The ecoTEC Plus is priced reasonably given its features at £1,700 – £2,400 for full installation. The only major drawback for the ecoTEC is its poor energy efficiency when compared to other premium boilers.

There are four advantages to the Vaillant ecoTEC Plus. First, the ecoTEC Plus has wifi connectivity and allows you to adjust your boiler settings remotely via the vSMART app. Second, the ecoTEC Plus comes with a 10-year guarantee when you book a Vaillant-accredited installer. Third, the ecoTEC Plus uses a stainless steel heat exchanger which extends the life of your boiler’s most difficult-to-replace component. Finally, the ecoTEC Plus has output sizes ranging from 12kW to 37kW so there is a model suited to every property.

There is one disadvantage to the ecoTEC Plus – its energy efficiency. The Vaillant ecoTEC has a lower energy efficiency rating than other boilers on this list with a seasonal space heating efficiency percentage of just 89.3%. The 89.3% rating means that for every £1 you spend on heating your home, the ecoTEC Plus wastes 10.7 pence. But, the ecoTEC is still rated “A” for energy efficiency so is much more efficient than boilers with lower efficiency grades.

Baxi system boilers

Baxi build reliable and practical boilers for an affordable price. Baxi system boilers generally lack the advanced features of some of the more premium boiler brands on this list like Worcester or Viessmann. However, Baxi boilers suit the budgets of most families and do a good job of meeting their needs.

Baxi 600 system

The Baxi 600 is a reasonably priced boiler that offers decent energy efficiency and solid durability. The 600 may lack the advanced features of more premium boilers but meets the needs of most families and does so with quality at an affordable price.

The Baxi 600 system boiler has four advantages. First, the 600 comes with a 7-year parts and labour warranty, which is decent compared to its competition. Second, the Baxi 600 is compatible with the Megaflo Eco cylinder, a market-leading hot water cylinder that helps it achieve a moderate 93% seasonal space energy efficiency rating. This 93% rating means that for every £1 you spend on heating your home, the Baxi 600 uses 93 pence for heating and wastes just 7 pence. Third, the Baxi 600 can run on a 20% hydrogen blend, putting you at the forefront of clean boiler fuel. Finally, the Baxi 600 is affordable at £1,800 – £2,100 for full installation.

There are two disadvantages to the Baxi 600 system boiler. First, the 600 does not have the LCD display of other boilers in this guide, which means it’s harder to customise your boiler settings or troubleshoot issues without the assistance of a gas technician. Second, the Baxi 600 lacks some of the advanced features of more premium brands on this list, like intelligent filling or weather compensation.

Baxi Megaflo system

The Baxi Megaflo is an affordable boiler that has the big advantage of being compatible with the Megaflo cylinder, the market leader in hot water cylinders. However, the Baxi Megaflo is less energy efficient than other boilers on this list and lacks the features of more advanced boilers.

The Baxi Megaflo system boiler has four advantages. First, it comes with a 7-year parts and labour warranty. 7 years is a decent warranty length for a system boiler in this price bracket. Second, the Baxi Megalfo is built from top-quality brass components. Third, the Baxi Megaflo system boiler is compatible with the Megaflo cylinder, one of the most advanced hot water cylinders on the market. The Megaflo has energy-saving features, higher capacity than most other cylinders, and a whopping 25-year warranty. Finally, the Megaflo is affordable at just £1,600 – £2,100 for full installation.

The Baxi Megaflo comes with three disadvantages. First, the Megaflo has an analogue interface which is significantly less useful than the touchscreen displays of more advanced system boilers. Second, the Megaflo has fewer energy-saving features than other, more premium boilers, resulting in a seasonal space heating efficiency rating of 91%. That 91% rating means that for every £1 you spend on central heating, the Megaflo wastes 9 pence. Third, the Megaflo lacks advanced features like wifi-connectivity or weather compensation that you could expect from more expensive boilers.

What are the advantages and disadvantages of a system boiler?

The advantages of a system boiler are that it comes with a large supply of hot water, is solar compatible, and takes up less space than a standard boiler. The disadvantages of a system boiler are that it is less efficient and larger than a combi boiler. Below is a more comprehensive look at the pros and cons of system boilers.

What are the system boiler pros?

Below are the three pros to system boilers.

Large hot water supply: System boilers can produce a larger supply of hot water than combi boiler systems because they store pre-heated domestic water in a hot water cylinder. This extra capacity means they are better able to meet the higher hot water demands of multiple people using appliances at once.
Solar energy compatible: System boilers are able to integrate with dual-coil cylinders, which are compatible with photovoltaic (solar) panels. Dual-coil cylinders have secondary coils that can be connected to a second heat source, specifically a solar energy system.
Need less space than a heat-only boiler: System boilers have a smaller profile than regular/heat-only boilers because they do not need a separate cold water tank. This smaller scale may be an advantage if you live in a smaller home.

What are the system boiler cons?

The following are the four system boiler cons.

Need more space than a combi boiler: System boilers take up more space than combi boilers because they store water in a hot water cylinder, while combi boilers only heat water on demand. This extra storage unit means that system boilers may not be suitable for small apartments.
Water supply limited to cylinder volume: System boilers can run out of water if your household exhausts the domestic hot water capacity of your cylinder. Running out of hot water is not a problem if you have a combi boiler as a combi boiler provides hot water on demand.
Less efficient than a combi boiler: System boilers are less efficient than combi boilers because their hot water cylinder loses some heat and has to thus be regularly heated via the central heating coil even if you haven’t used any hot water.
Relies on mains pressure: System boilers are unable to generate their water pressure, a feature of the cold water tank maintained by regular boilers. This inability to generate water pressure means your system boiler must rely on the water pressure from the mains, a weakness shared by combi boilers.

Is a system boiler right for me?

The following four considerations should help you decide if a system boiler is right for you.

Budget: System boilers are generally more expensive than combi boilers. You may not wish to buy a system boiler if you’re operating on a very tight budget. System boilers are also a little less energy efficient than combi boilers so a combi boiler may be better if you need to cut down on your gas bill.
Storage space: System boilers take up more space than combi boilers because they operate a hot water cylinder and less space than a regular boiler because they do not need a separate cold water tank. This size profile makes system boilers a great choice if you have moderate to large amounts of storage space but a poor choice if you live in a small apartment.
Central heating needs: Larger properties are likely to come with more radiators that need a higher-capacity boiler. System boilers are often a good fit for larger properties with more radiators although you need to select a model with the right capacity for your property.
Hot water needs: System boilers are a particularly good choice if your household uses a lot of domestic hot water. System boilers store dedicated domestic water in a cylinder ready for use and so can handle multiple occupants using appliances or showers at the same time, compared to the poor performance of combi boilers under these conditions.

FAQs about system boilers

We answer the most frequently asked questions about system boilers below.

Can a system boiler run out of hot water?

Yes, a system boiler can run out of hot water. The limitation on how much water a system boiler can supply at any one time is the size of its hot water cylinder. Your system runs out of hot water once the hot water in your cylinder is exhausted and replaced with cool water. You then need to wait 20-30 minutes to heat a hot water cylinder full of cool water.

Does a system boiler need to be on for hot water?

A system boiler does not need to be on for hot water to be available at the taps and showers. Your hot water cylinder stores hot water and that water will stay hot for at least 1 day without any additional heating. However, a system boiler does need to be on to keep your central heating system working and to heat the cylinder once it has lost heat.

How does a gas boiler work?

A gas boiler works by burning gas in a sealed combustion chamber to heat water. A gas valve releases the gas into the combustion chamber. A connected pipe carries central heating water through the combustion chamber and then carries the heated water to the radiators in your home.

How does a central heating system work?

A central heating system works by circulating heated water through the home’s radiators to heat each room. The hot water heats the metal casing of your home’s radiators as it passes through them. The heated radiators then emanate heat into each room. Your central heating system is a hot water circuit that passes through both your home’s radiators and also through a boiler that keeps the hot water at an appropriate temperature. This hot water also passes through a coil in your boiler to heat domestic hot water if you have a system boiler.

Does central heating use water?

Yes, central heating uses water. Most UK central heating systems circulate hot water through a home’s radiators. This water also routes through your home’s boiler, which ensures it stays at the appropriate temperature to heat your home. Forced air or electric baseboard heating systems are more popular in the US and elsewhere in the world and do not use water.

What is a gravity-fed system?

A gravity-fed system is a heating and plumbing setup that uses cold water tanks to maintain pressure in the central heating and domestic hot water systems. The cold water tank feeds into your hot water cylinder to replace used heated water. The weight of the water from the cold water tank creates pressure in your taps and showers. Gravity-fed systems function with heat-only boilers.

Why do I have a boiler and a hot water tank?

You have a boiler and a hot boiler water tank because you have a system or regular boiler, which is used to heat your home and provide hot water for showers. The hot water tank holds pre-heated water for use in your domestic water system while the boiler heats water for your central heating, which is used to indirectly heat the water in your hot water tank via a coil that passes central heating water through your tank.

How to turn off the hot water supply from a cylinder?

You turn off the hot water supply from a cylinder by switching the boiler off using your system boiler’s control panel and also shutting off the water supply to the hot water cylinder using the supply valve. Turning the mains valve off will not turn off a system boiler as the system boiler has its own stored hot water that it circulates through the domestic water system.

Does a condensing boiler need a water tank?

A condensing boiler does not need a water tank. Some combi boilers are condensing boilers and they do not use a water tank. There are also condensing system boilers and they do have a water tank. Condensing boilers pass the hot water over the excess vapours produced by burning fuel for a second time, allowing them to make use of the heat remaining in that vapour that might otherwise escape to the atmosphere.

Does a combi boiler need a water tank?

No, a combi boiler does not need a water tank. A combi boiler heats water directly from the mains so there is no need for water storage facilities.

Can you use a Megaflow system with a combi boiler?

No, you cannot use a Megaflow system with a combi boiler. Combi boilers do not need and cannot use a hot water tank like the Megaflow. Combi boilers heat water on demand rather than storing pre-heated water ready for use in a cylinder and so you cannot compare combi boilers and the megaflo system cylinder. A megaflo is a stainless-steel hot water storage cylinder. Megaflo cylinders are unvented cylinders, meaning they refill themselves from the cold mains rather than via a cold water tank.

What type of boiler do I have?

Follow the four steps below to figure out what type of boiler you have.

Check your loft: Regular/heat-only boilers use an elevated cold water tank to maintain pressure in your hot water system. You likely have a regular boiler if you find a cold water tank in your loft. Otherwise, you likely have a system or combi boiler.

Check for a cylinder: Both regular/heat-only and system boilers use a hot water cylinder to store domestic hot water. You have one of these two types of boilers if you find a hot water cylinder in your storage closet.

Check for pipes coming out of your boiler: Combi boilers typically have four or five copper pipes coming out of the bottom of the boiler unit whereas system boilers typically have three copper pipes and regular boilers have one pipe on the bottom and one on the top of the boiler unit.

Check for an external pump: Regular/heat-only boilers use an external pump to move water through your central heating system while system and combi boilers do not. You likely have a regular boiler if you find an external pump.

How long does a boiler take to heat water?

A boiler takes 20 – 30 minutes to heat water. System boilers or regular boilers rely on a hot water cylinder to store pre-heated domestic water for taps and showers. Both boiler types need 20 – 30 minutes to reheat the cylinder if you fully use all the heated water. Combi boilers heat water on demand and so never run out of hot water, although using several appliances at once strains their hot water capacity.