Home  >  Advice Centre   >   System Boiler Replacement Guide

System Boiler Replacement Guide


Are you pondering on the idea of getting a system boiler for your home? In this comprehensive guide, we divulge into system boilers, what they are about, how they work, what benefits and downsides they have to offer and a few figures to chew on when installing the boiler. Basically, everything you need to get your new system boiler going.

Most homes in the UK rely on combi boilers for hot water and heating. For larger homes, your combi boiler might not be sufficient. A system boiler offers a level ground between a combi and regular boiler and could be a better alternative in various circumstances.

What is a System Boiler?

A system boiler differs from a combi boiler because it has an additional storage tank for the hot water. It’s also different from a regular tank because it takes its water directly from the mains.

Like a combi boiler, most of the heating components of the boiler are built into one unit. However, there is an additional hot water cylinder often tucked away in the airing cabinet that stores the hot water and ensures a steady flow.

System boilers also have some added features. In the vented network, the boiler has an expansion vessel and it also has a pressure release valve that allows the boiler to release water when the pressure is too high. This protects the boiler from significant damage and expensive repairs.

How Does a System Boiler Work?

To avoid any confusion, system boilers are also referred to as sealed systems. The functioning of the boiler is more or less the same as with other types of boilers.

The boiler is supplied with water from the mains. The water is heated and pumped into a single storage cylinder. The water is then distributed to the radiators and the showers from the single cylinder.

The boiler might take about an hour to heat the water. It’s advisable to get the boiler running way before you need the hot water. The tank that comes with the boiler is added to prevent such inconveniences and ensure you can get hot water on short notice.

What You Need to Consider Before Getting a System Boiler

Getting a system boiler is not a matter of waking up and heading out. You need to weigh your options carefully to determine if this is the right boiler for your home. Here are some of the factors you need to consider before getting a system boiler:


If you’re short on space, a system boiler is not ideal for you. You need space to install the boiler and the storage tank. The storage tank is significant and poses a risk, so it needs to be in a discreet location. If your house has an airing cabinet, it’s a plus. The tank can be installed there.

The tank has to be insulated

To prevent heat loss and wastage of energy, the hot water tank needs to be insulated. This could adjust the cost of insulation upwards. However, insulating a hot water tank is not expensive, so the difference will not be significant.

Hot water is not instant

With combi boilers, you get hot water on demand. With system boilers, you might have to wait up to an hour before you get the hot water. A quick fix to this is to have the boiler running beforehand.

Advantages of a System Boiler

Easy installation

Most of the individual components of a system boiler are built into the heating unit. It makes the boiler easier, cheaper and neater to install. Having most of the components in one unit allows for a compact and more aesthetically appealing boiler. The only component outside the unit is the cylinder, which goes into the airing cabinet.

Ideal for large homes

Where a combi boiler will not suffice, a system boiler will work great. The addition of the tank allows the combi to provide hot water, which is perfect for large homes with multiple bathrooms that might need more hot water.

High water pressure

One of the downsides of using a regular boiler is reduced water pressure at the taps around the house. This is because, in a regular boiler, the flow of water through the pipes relies on gravity.  A system boiler draws water directly from the mains. As a result, the pressure of the water in the taps around the house remains the same.

Economical and fast response

System boilers have a pump that allows them to speed up the supply of hot water around the house. The faster response also means you waste less energy and the boiler is more economical to run.

You save up on space

You might have to give up a little more room in your airing cabinet for the hot water cylinder. However, you don’t need to worry about giving up even more space in your loft to house the cold-water feed tank. In homes where space is a premium, this can prove to be a vital benefit.

Compatible with solar thermal

With a system boiler, you can make even more savings by hooking up the system to solar. You get to enjoy hot water and a warm home while making substantial savings on your energy bill.

Disadvantages of a System Boiler

As you would expect, there are downsides to having a system boiler, as well. It might be enlightening to know what to expect before you decide to get knee-deep when buying a system boiler.

System boilers take up more space

Compared to combi boilers, system boilers will take up more space around the house because of the hot water storage tank. If you’re strapped for space, it might be a challenge to fit in a system boiler.

High initial cost

System boilers have a higher buying price than combi boilers. The additional costs are because of the tank and the other components like a pump. However, they also tend to be more reliable, so they are worth the premium price tag.

Less efficient

The water storage tank comes with advantages and downsides, as well. One of the glaring disadvantages of having water in a tank is that it loses heat. The tank has to be heated continuously to keep the water ready for use.

How Much Does a System Boiler Cost?

Now that you know all there is to know about system boilers, including the advantages and disadvantages of a system boiler, it’s time you start analysing the numbers.

You have plenty of brands and models to pick from if you decide to go for a system boiler. Each of the different brands and models comes with a different price tag depending on the features and output of the boiler, among other factors.

A system boiler will cost in the range of £500 to £3,500. This does not include the cost of installation. The cost of boiler installation is not as easy to estimate because it depends on a variety of factors. To make the process easier for you, Compare the Boiler Market will help you get up to 3 FREE and no-obligation quotes from installers in your area to make sure you get the most competitive boiler installation rates.

What are the Running Costs of a System Boiler?

With the initial set up cost of the boiler out of the way, you now have to focus on the running costs of the boiler once you start using it.

You should know that system boilers are not built equal. They use different types of fuel (gas, LPG and oil) and they come in different efficiency ratings.

The energy costs are determined by the type of fuel and also the efficiency rating of the boiler. The higher the efficiency, the less fuel the boiler is likely to use.

In addition to the energy bills, you also need to worry about service and maintenance bills. Fortunately, system boilers are reliable and highly durable, so you might not have to worry about repairs for the first couple of years.

However, you will need to have the boiler serviced every year, so you don’t void the warranty. The cost of servicing the boiler will depend on the model and make of the boiler and output. However, this could cost about £80 for a one-off service and about £300 for an annual cover.

Before you commit to anyone boiler engineer, it’s a good idea to shop around for different quotes to make sure you get the best deal.

What’s the Difference Between a System and a Combi Boiler?

Understanding the difference between the different boilers is essential in helping you identify the right type of boiler for your home.

A system boiler is not the same as a combi boiler. On the outside, the units might look similar but, the details are on the inside. A combi boiler heats water on demand and doesn’t come with a storage tank as a system boiler does.

However, you can still use a combi boiler even if you have a large property. But, that calls for a huge combi boiler, which might be expensive to buy and run.

With a system boiler, you can use a smaller boiler that will steadily trickle water into the storage, ensuring you have hot water at all times, even when the boiler has a small output.

Is a System and Heat Only Boiler the Same Thing?

Again, a system boiler is different from a heat-only boiler. A heat-only boiler is also known as a regular, traditional or conventional boiler. The closes similarity that these two boilers have is the hot water storage tank.

However, a heat-only boilers also pack an extra water feed tank that supplies water to the boiler. Regular or heat only boilers are often used in areas with low water pressure.

Which is better: A System or a Combi Boiler

The decision between a system and a regular boiler might be an easy one given the substantial amount of space that a regular boiler takes. Choosing whether to have a combi or system boiler might not be too easy.

However, your decision boils down to the availability of space. Both are great options, but space is the limiting factor.

Storage tanks vary in size from 90 litres to about 300 litres. Even the 90-litre option will have a significant footprint in the house.

If you’re not comfortable giving up the extra space, you can always go for a high capacity combi boiler that will be up to the task.

The bottom line, both options are ideal under the perfect circumstances.

What is the Best System Boiler?

There’s no limit to the variety of options of system boilers in the market. As the second most popular type of boiler behind the combi, each leading brand has invested heavily to provide all users with an option that will work for their homes. Some of the best include:  

Worcester Bosch Greenstar 30CDI

Worcester is one of the leading boiler manufacturers in the UK. The Greenstar CDI is the embodiment of their years of excellence and investment in research. The 30DI is a 30kW system boiler with a hot water cylinder of between 150 -300 litres.

The boiler is perfect from homes with up to 18 radiators and has an energy efficiency rating of 92%. A 5-year warranty safeguards your purchase and the easy to use LED display ensures you have a smooth experience with your new boiler.

Vaillant ecoTEC System Boiler

Another system boiler worth considering is the ecoTEC system boiler by Vaillant. It’s slightly costly, but considering that Vaillant says it’s 85% recyclable, it means you will have a lower carbon footprint and possibly save even more in energy bills.

The ecoTEC has an output rating of 27kW, which is ideal for homeowners that might find the 30kW Worcester Bosch a bit of an overkill, comes with 5 years of warranty comes with an automatic LPG converter which makes it much easier to use the boiler with gas and with LPG.

Alpha InTec 18S System Gas Boiler

Like the other system boilers, this is also a condensing boiler. It’s a little low on the ErP rating and is B-Rated at 88.80. It’s an excellent option for homes that are struggling with space but still need the services of a system boiler because it is wall-mounted and has a decent output of 20kW.

Compared to the former two, this is a budget-friendly option that balances practicality and cost with ease, and to guarantee quality service, the manufacturer throws in a 3-year warranty to sweeten the deal.

System Boiler Size Calculator 

It’s impossible to tell the size of the boiler you will use right off the bat. There are several variables that affect the size of the system boiler that will work for your home.

The size of your home, number of bathrooms, radiators, and hot water taps all play a critical role in dictating the size of the boiler you will need for your home.

With our boiler size calculator, the process of choosing the ideal boiler for your home is much easier. The calculator is easy to use and will take the uncertainty out of picking the right output for your boiler.

With system boilers, it’s vital to remember that the size of the hot water cylinder is worth consideration as well.

Get Free Quotes for a Replacement Today!

The hardest part about getting an old boiler replaced is knowing how much the boiler replacement is going to cost. Nonetheless, the cost of removing the old boiler and installing the new is a significant part of the process and will also take up a substantial chunk of your budget so you can’t afford to go in blind.

Luckily, this is another area where Compare the Boiler Market can help you in. We have an expansive network of boiler engineers in your areas. We can get these installers to give you a call and provide you with a FREE, NO-OBLIGATION quote for your boiler replacement.

The installers will give you a call and discuss details about the project and even ask you a few questions like your area of residence, the type of boiler you have and the type of boiler you want to install, and the state of your central heating among others. Once the installer has detailed information about the task, you will receive a highly accurate quote customised to your boiler replacement.

  • Buy Now Pay Later Schemes!
    • Interest Free Credit
    • Buy Now Pay Later
    • Spread Payments
    Finance Options
  • Free Boiler Quotes

    No-obligation to buy!

    • From the comfort of your home
    • You choose the best company
    • Gas Safe trusted companies
    Get Quote NOW
Need a New Boiler

Get FREE quotes from tradesmen in your area today. What are you looking for?