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System Boilers

Deciding on a boiler which is both an efficient and practical fit within your home and lifestyle can be a difficult choice to make. Some of the most popular boiler choices are gas and oil boilers, especially system boilers, which are particularly beneficial for those with larger households. Our complete guide to system boilers and what we at Lacey Plumbing & Heating install will help determine what is best for you and your home.

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What is a System Boiler?

A system boiler is a type of boiler commonly found in larger homes which have a greater demand for heating and hot water. System boilers feature a separate cylinder for water storage, so they are able to provide a constant supply of hot water throughout your home; unlike combi boilers which can only heat water on demand.



Depending on the size of your home, you will need to have a particular sized boiler. Adding up the number of radiators in your room will help you understand a suitable system boiler size:

  • Up to 10 radiators should have a boiler of 9 -18 kW
  • 10 -15 radiators should have a boiler of 18 – 26 kW
  • 15 – 20 radiators should have a boiler 26 – 40 kW

System boilers usually don’t need to be as powerful as other boilers because of their separate hot water cylinder for storing and heating domestic water.


How long should a System Boiler Last?

Most boilers, regardless of their type, should last between 10 to 15 years on average.

If you have an older boiler approaching the end of its life, you may want to consider purchasing a new one. The older boilers, especially those installed over 10 years ago, are considerably less efficient than modern ones.


Are System Boilers more reliable?

Despite the fact that system boilers can sometimes be more expensive than other types of boilers, such as combi, they are more reliable.

System boilers can handle high hot water demand with ease, meaning multiple members of the household can use hot water at the same time. This is because a cylinder is installed.

They also use the mains water supply when delivering water to your hot water outlets which provides improved water pressure.


What to consider when comparing System Boilers

When comparing system boilers, there are a few things that you will want to be aware of.

  • Output:

One of the most important factors is output, as you will want to ensure it sufficiently meets your home’s water needs and requirements. System boilers have central heating output ratings which gives a reflection of how many radiators the unit is capable of heating up.

Another factor is efficiency. Boilers work by converting fuel into usable energy, and how much fuel that gets converted is reflected in the efficiency ratings. The higher the efficiency rating, the less energy is wasted when in use.

  • Budget:

System boilers can cost around £500 – £2,000 without the cylinder and before installation. Depending on how much you’re willing to spend, there are many options to suit varied budgets.

  • Reviews:

Finally, you may want to look at reviews from heating engineer specialists. Reviews and opinions from fully-qualified experts will give you valuable insights into the system boilers you may be considering.

Interested in a System Boiler installed by professionals? Speak to us today

How does a System Boiler work?

In a system boiler, the central heating water passes through a coil within the cylinder and transfers from the coil to the water stored within the cylinder, transferring heat into it. The water will stay here until it’s required. As there is a large amount of hot water available, multiple taps and showers can be used at once.


How does a System Boiler heat your home?

Once the hot water is stored in the cylinder, the water can be pumped around your home if required. The water in the system is used over and over, being pumped through the coil in the hot water cylinder and the radiator system.


How does a System Boiler provide hot water?

System boilers provide hot water by heating the cold water from the mains via a heat exchanger. This is either gas or electric, which transfers energy to the water. Once the water is heated, it is pumped and stored in the hot water cylinder until it is required.


What efficiency rating does a System Boiler have?

As all taps and showers in your home receive near instant hot water at the same pressure, a system boiler can help to optimise your home’s water consumption. Many models achieve up to 90% efficiency in optimum conditions.


Can a smart thermostat work with a System Boiler?

Smart thermostats typically work with most boilers, including gas and LPG heating systems, as well as most oil boilers. You can also control your system boiler’s water tank too. However, if your boiler is too old or run down, it may not be possible to connect a smart thermostat to it.

The best way to determine if a smart thermostat can be used with your boiler is by consulting a professional. If you would like a consultation on your boiler in relation to smart thermostats, don’t hesitate to get in touch with us today and one of our expert team members will be able to help you.

Man repairing a boiler
Lacey plumber repairing a boiler

What hot water cylinders do you need?

Knowing which hot water system you need is not the easiest thing to understand, and you may not even be sure about what you’re looking for.

In simple terms, the hot water cylinder you need for your home depends on how you intend to heat your water and how you want the hot water to be distributed.


Direct vs Indirect Cylinders

With direct cylinders, the water is heated directly by the cylinder itself through an immersion heater. Some cylinders have more than one immersion heater installed inside them to allow for a more powerful performance, therefore producing hot water quickly. Direct cylinders come as either vented or unvented models. This impacts how the water is distributed across the property to the hot water outlets and they tend to be used when a traditional central heating system is not available.

Indirect cylinders rely on external appliances to apply the necessary heat to the cylinder. This generates the hot water that is stored, and also used as a backup if the main heat source fails. The water is indirectly heated by other appliances, like a boiler, but it may also include things like solar panels. An internal heat exchanger in the cylinder transfers heat from the external appliance to the water cylinder.

Some indirect cylinders contain immersion heaters that allow the cylinder to add additional heat to the stored hot water. Indirect cylinders are therefore able to generate hot water much more quickly than direct cylinders.


Unvented vs Vented Cylinders

If you need a cylinder which uses the mains water pressure to distribute the hot water around your home for use at your taps and showers, you will need to invest in an unvented cylinder. Unvented cylinders are commonly used as part of a system boiler installation. This enables you to take advantage of the high pressure water supply. Unvented cylinders also take up less space than vented cylinders as no feeder tank is required.

Vented hot water cylinders rely on a feeder tank to provide it with cold water from the mains. This water is then heated and distributed to the hot water outlets as part of a gravity fed system. Vented cylinders are usually used alongside regular boiler installations, whereas unvented cylinders are used with system boiler installations.

What is the difference between a System Boiler and a Combi Boiler?

While a system boiler is designed to work with a hot water cylinder, a combi boiler provides both heating and hot water instantly. Combi boilers are the most popular type of boiler for domestic heating in the UK as it combines both a water heating system and a central heating boiler all in one contained box. System boilers are typically found in larger properties as it can meet the demanding needs of a bigger household.

Converting a System Boiler to a Combi Boiler

Converting to a combi boiler from a system boiler is a worthy investment. Combi boilers take up less space, heat water on demand and require far less maintenance. However, there are circumstances in which a combi boiler may not be the best option for you. If your household uses a lot of water at once, a combi boiler may not be able to keep up with the demand. This means during peak times, the water pressure may be greatly reduced.

The cost of changing to a combi boiler is also something you may want to consider. You can get a combi boiler installed for around £1,800 to £2,000, but this doesn’t include the cost of removing your old system boiler and connecting the water feeder tanks. This may be around £500 altogether which bumps the total installation and removal cost to approximately £3000 – £3500. If you are thinking of changing your system or regular boiler to a combi boiler, contact our specialist team today to see how we can assist with the boiler removal and installation process.


How much does a System Boiler cost?

Depending on your existing boiler type, it can cost between £1,600 – £2,900 for a new replacement system boiler. This price is solely for the boiler, and it does not include the tank. However, system boiler replacement costs are mostly made up of the actual system boiler itself, fittings and labour costs.

For a basic boiler with a low five year warranty, it may cost around £1,600. Mid to high-end boiler replacements cost around £1,900 to £2,900 depending on the make and model.

With new boilers, we can offer a monthly new boiler payment plan including 0% interest-free credit and no deposit fees. Get your instant quote today for some of the most competitive prices on the market.


What is involved in changing to a System Boiler?

Changing to a system boiler is typically only recommended if you are extending your property or adding bathrooms. Depending on what type of boiler you have, there are specific steps that need to be taken:

Combi boilers to system boilers involve installing a new hot water tank. Regular boilers involve moving from an ‘open-vent’ system to a sealed system and changing a copper water cylinder to a pressurised steel one.

Man fixing boiler

What are the pros and cons of a System Boiler?

Just like with any boiler, system boilers come with their pros and cons so it’s best to consider these and do your own research before committing to one.


Pros of a System Boiler

  1. They can handle high hot water demand. The large amount of hot water stored in the tank allows numerous household members to use hot water at the same time without affecting the pressure.
  2. Eco friendly. The insulated water cylinders are an economical way to provide hot water to a number of outlets. Not to mention water storage cylinders and tanks of system boilers can be heated through solar panels after installation.
  3. Improved water pressure. System boilers use the mains water supply when delivering water to your hot water outlets, which provides improved water pressure.
  4. Extra loft space. System boilers only require a small amount of space for water cylinders, unlike regular boilers which require large plastic feeder tanks  to be installed in your attic.

Cons of a System Boiler

  1. Lack of instant hot water. System boilers have a large amount of water to heat up, and this happens relatively slowly. This means if your boiler has been switched off for a significant amount of time, you will be waiting for the water to be heated up from cold, which may take a while.
  2. Take up more space than combi boilers. System boilers require an additional water cylinder that takes up space in your home, unlike a combi boiler which is only the component itself.
  3. Price point. Because of the separate components and size, system boilers tend not to be the cheapest on the market.

Common System Boiler problems

Just like with any boiler, system boilers can have their problems. These can include:

  • Leaking. A boiler leak is usually caused by an issue with an internal component. If the leak comes from your relief pressure valve, it’s likely that the system’s pressure has become too high.
  • Low boiler pressure. You can tell if your boiler’s pressure is too low by looking at the pressure gauge. Low pressure can be caused by system leaks, recently bled radiators or failed components.
  • No heating or hot water. This may be concerning with system boilers as the tank is supposed to store hot water. Your boiler may need repressuring or the thermostat adjusted. If you cannot safely find a solution, it’s best to consult a professional.
  • Boiler breakdown. If the above issues are left unaddressed, the above issues can lead to a complete boiler breakdown. If this is the case, you should contact a gas safe registered engineer as soon as possible.

Hot water only works when System Boiler central heating is on or the immersion heater is turned on?

If hot water only comes out of your taps when the central heating is on, it is likely that the diverter valve isn’t diverting hot water properly. If you think this is the case, we would recommend calling our Gas Safe registered engineers to take a look at it for you. The work carried out on gas appliances can be dangerous and can damage the boiler if it isn’t handled professionally.

Our boiler engineers will be able to run checks to see if a faulty diverter valve has caused the issue, or if there is potentially a different underlying issue. Lacey Plumbing and Heating will be able to offer you a high standard of care and expertise for any boiler repair issues.

Interested in a System Boiler installed by a professional? Get In Touch

Constant running water sound from a System Boiler

If you can hear a running sound or gurgling noise from your boiler, this is fairly common as the water is moving through the system. However, if the noise is constant or becomes loud enough to catch your attention, this could be a sign that something is wrong.

The noise can be caused by trapped air in the system, in which case you will need to bleed your radiators. If you’re unsure on how to bleed a radiator yourself, contact our professional heating engineers for assistance.

Alternatively, if bleeding your radiators doesn’t seem to fix anything, you will have to check the water pressure by looking at the pressure gauge on the boiler. The gauge will likely have a red and a green section, and the needle on the dial should be sitting in the green section. If the dial is in the red section before the green, the water pressure is too low. If it is in the red section after the green, the water pressure is too high.

You will either have to put extra water in if the pressure is low, or take some out if the pressure is too high. Once you’ve fixed the pressure, you can check the dial to ensure it is sitting in the green section.


Which System Boiler is best?

There are a variety of brands on the market with high quality system boilers available, so finding the right one for you is important.

Worcester Bosch System boilers

The Worcester Bosch 4000 system boiler is everything you could need in a boiler, with quiet operations and a high hot water output.

The main features include:

  • Ideal for medium sized homes with multiple bathrooms
  • Easy access with removable side panels
  • Compatible with third party controllers such as Hive, Nest and Tado
  • ErP rating of 94%
  • Available in power outputs of 12kW, 15kW, 18kW, 21kW and 24kW

We are Worcester Accredited Engineers and can offer up to a 12 year manufacturers warranty on Worcester installations.Ideal Boilers


Ideal Logic System Boilers

The Ideal Logic System boiler is one of the company’s most popular system boiler models, and one of the most competitively priced compared to the major brands.

The main features include:

  • Compact design which fits into kitchen cupboards
  • Built-in service indicator that reminds you when it’s time for a service
  • Boiler frost protection
  • Built in schedule programmer

The boiler is available in outputs of 15kW, 18kW, 24kW and 35kW.

The Ideal Logic system boiler is perfect for keeping you on track with your annual service. Simply give us a call when your service indicator reminds you and we’ll be able to arrange a boiler service suited to you.


Viessmann System Boilers

The Viessmann Vitodens 100-W is officially the brand’s best selling system boiler, using hydrogen-ready technology and an upgraded backlit LCD touchscreen display.

The main features include:

  • Class A for energy efficiency (89.5%)
  • Hydrogen-ready for the next generation of fuel
  • Compatible with the Viessmann Vitronic smart thermostat
  • Stainless steel heat exchanger

The boiler is also available in outputs of 11kW, 19kW, 25kW and 32kW.


Alpha System Boilers

The Alpha E-Tec system boiler is one of the most popular system boilers. A budget friendly option, with stainless steel heat exchangers, Alpha rivals the likes of premium brands.

The main features include:

  • Designed to fit neatly into small kitchen spaces like cupboards
  • Easy access front panel
  • Compatible with all leading third party controls such as Hive, Nest and Tado
  • Zinc plated expansion vessel to prevent corrosion
  • ErP rating 93%

The boiler is available in outputs of 18kW and 38kW.

Alternatively, with whatever system boiler you choose, we provide boiler system service plans which will meet all your plumbing and heating needs.


Is a System Boiler right for your home?

System boilers are ideal if you have a large home, with a high demand for hot water. If you don’t want a boiler to take up much room, system boilers work by using the mains water supply. This means it eliminates the need for a cold water tank which typically takes up space in a loft.

If you’re installing a system boiler from scratch, it may be less expensive due to it not needing a tank.

However, if you don’t want it to take up much room at all, it’s important to remember that system boilers still require a water cylinder and are in general bigger than combi boilers. Depending on the size of your house, you may be better off buying a combi boiler.


System Boiler installation with Lacey Plumbing & Heating

For information or advice about system boilers or the type of boiler best suited for your home, please feel free to contact us. Lacey Plumbing & Heating can provide you with a high quality system boiler and offer expert installation from our Gas Safe registered heating engineers.

We are here to answer all of your boiler related queries, no matter what the issue is.  Get in touch with us today to discuss having a combi boiler installed in your home by our friendly, professional team.


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Eton Hill Road, Radcliffe, M26 2ZS

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