Best System Boilers 2023 Re January 20, 2023

Best System Boilers 2023

engineer servicing a boiler

What are the top system boilers in 2023?

In this blog, we’ll look at the top 5 best system boilers on the market, along with their reliability, efficiency and overall performance to make sure you get the most bang for your buck.

We will also look at factors that may affect the price of your new boiler, including warranty, installation costs, and kW output. 

By the end of this blog, you will be completely clued up with what the best system boilers are on the market. If you’re looking for more information on the best cheap combi boilers, we’ve already covered it. We also have a blog on the best boilers to buy in 2021, which includes combi, system and regular boilers.

Additionally, we also have a Boiler Comparison tool that allows you to compare up to three boiler models or brands at one time, to help you find the perfect new boiler for your home. 

What is a system boiler?

A system boiler can usually be found in homes that have a high requirement for hot water and can often be found in the airing cupboard of your home. 

A system boiler, in contrast, features a separate cylinder for storing hot water. Although the major heating and hot water system components are built into the boiler, the hot water is stored in the cylinder. This means that a constant supply of hot water can be provided to a large number of taps and showers at the same time. This makes it the ideal choice for a home with multiple bathrooms.

You may have overheard your Gas Safe Engineers talk about either a vented or unvented, or a direct or indirect hot water cylinder, if you’re not sure what the difference is, we explain all of this in a separate blog. Head on over when, once you’ve finished reading this blog, of course 😉.

How do system boilers work?

A system boiler is supplied with cold water directly from the mains. This water is then heated by a heat exchanger within the boiler which transfers heat to the water. An electric pump then pushes the hot water through to your radiators and taps. This heat is then used to heat water that returns from your central heating system.

Best system boilers on the market

This next section of the blog aims to discuss the top five best system boilers on the market ready for 2023. We will be taking a look at the overall brand scores from the Which? Best Boiler Brand Survey in 2021, which rates each boiler on reliability, efficiency, and overall performance. We also get to see what engineers and customers think about these boiler brands. 

This section will then discuss why we have ranked these boilers in this order, to help make buying a new or replacement system boiler a little easier for you.

Worcester Bosch Greenstar 4000 System Boiler


The Worcester Bosch Greenstar 4000 System boiler is a part of Worcester Bosch’s latest edition to the family and a great system boiler at that! 

The Greenstar 4000 system boiler is available in five kW output models, including the 12kW15kW18kW, 21kW and 24kW. 

Not only does the Greenstar 4000 system boilers offer reliable performance, but Worcester Bosch is also known for outstanding build quality. Here’s what customers and heating engineers awarded Worcester Bosch during the Which? Best Boiler brand 2021 survey results.

Worcester Bosch was awarded a huge 5 stars for their customer scores and reliability, which supports our claim that Worcester Bosch is one of the best boiler brands on the market. Not only do their boilers off a high-efficiency rating of 94%, but all their boilers also come with a ten-year standard warranty (not that you’ll need it). 

Worcester Bosch is a boiler brand that has become synonymous with reliability and performance, a brand regularly recognised by Which? Awards. In the 2021 Boiler Brands report carried out by Which?, Worcester Bosch scored brilliantly once again, being awarded Which? Best Buys across its entire Greenstar gas and oil boiler ranges. Worcester Bosch performed exceptionally for reliability and customer satisfaction.

Worcester has built an enviable reputation for customer aftercare, which is reflected in their 4.6/5 average boiler review score on Trustpilot. Customers are obviously happy with the product and the customer care they receive.

Many may be concerned when looking at their 4 stars from engineers. When looking further into the scores from engineers, they admitted to finding it difficult to fix a common fault, however, many would argue that this may be down to the robust build and design of Worcester Bosch boilers, a quality that many of us would find reassuring.

Here’s what customers have to say about their Worcester Bosch Greenstar system boilers:

“We are perfectly happy with the boiler and its performance.”

“New Worcester Bosch boiler fitted to replace my ageing boiler. Very happy with the boiler after 1 year use. Very reliable, heats water quickly and does exactly what you would want from a quality product.” 

“We had a Worcester boiler prior to this one and it lasted almost 20 years! It was a no brainer when it came to choosing a new boiler”.

As you can see, from only a few online testimonials, that Worcester Bosch customers are more than satisfied with their Worcester Bosch boilers.  

Boiler ModelWorcester Bosch Greenstar 4000 12kWWorcester Bosch Greenstar 4000 15kWWorcester Bosch Greenstar 4000 18kWWorcester Bosch Greenstar 4000 21kWWorcester Bosch Greenstar 24kW 
Warranty length 10 years10 years10 years10 years 10 years 
kW Output (CH)12kW15kW18kW21kW24kW
Efficiency   94%94%94%94%94%
Price* (inc VAT)£948.98£1,008.98£1,045.98£1,077.98£1,123.98

As you can see, all of the Worcester Bosch Greenstar 4000 systems boilers are consistent in performance across the board. Regardless of the kW output, the efficiency remains the same. 

It is important to note when purchasing a new or replacement system boiler, that you will need to accommodate for an additional hot water storage tank, typically located in an airing cupboard. Similar to regular boilers, system boilers are not ideal for homes with limited space. However, if space is not an issue in your home, then a system boiler could be a perfect choice, especially if you have a high demand for instant hot water. 

This is due to the fact that system boilers are able to store hot water, ready to be used when needed. Unlike a combi boiler, with a system boiler, the hot water that is heating in the boiler unit is transferred into a hot water storage tank for later use. 

For an instant quote on a Worcester Bosch Greenstar 4000 system boiler, click here and complete our 90-second survey. 

Viessmann Vitodens 100-W System Boiler

vitodens 100-w

The Viessmann 100-W includes four kW boiler output models, the 11kW19kW26kW, and 32kW, so there is an option to accommodate all house sizes. 

Viessmann Which? Best Boiler Brand Survey Results:

Viessmann boilers scored an overall brand score of 81% and a massive 5 out of 5 stars for their reliability and customer score, revealing that Viessmann boilers are loved by their customers.

During the Which? Best Boiler Brand Survey, Viessmann scored a 4 out of 5 stars for their overall engineer recommendation score, which takes into consideration the availability of spares and parts and ease of fixing a fault. 

However, we still believe that the Viessmann 100-W system boilers are an all-round great boiler. 

Viessmann has a score of 4.5 stars out of 5 on Trustpilot, with 90% of their reviews being rated above average. Here are what some of Viessmann’s customers have to say about their boilers: 

“My heating seems to be much hotter and quicker to heat up than my old boiler also it is very quiet”.

Boiler is very good. Quiet in operation and retains pressure well. No issues with water or heating temperature. Significantly cheaper than comparable boilers from other manufacturers.” 

Boiler ModelVitodens 100-W 11kWVitodens 100-W 19kWVitodens 100-W 25kWVitodens 100-W 32kW
Warranty length Up to 12 yearsUp to 12 yearsUp to 12 yearsUp to 12 years
kW Output (CH)11kW19kW25kW32kW
Efficiency   94%94%94%94%
Price* (inc VAT)£1,124.98£1,162.98£1,200.98£1,258.79

The first reason being efficiency. The Vitodens 100-W system boilers have an A-rated efficiency rating of 94% across the board. It is important to consider the efficiency of your new boiler to ensure it is suitable for your home. Around 70% of what you spend on energy bills annually can be attributed to the efficiency of your boiler, so it’s important to invest in the most efficient unit within your budget. 

The Vitodens 100-W system boilers are slightly more expensive than the other boilers in this blog, such as the Ideal Logic System boilers and the Baxi Megaflo System boilers, however, when looking at the Which? results, they are deemed as very reliable boilers, with 4 out of 5 stars in the reliability category. They also come with up to 12 years warranty (when purchasing with WarmZilla). 

Remember, cheaper isn’t always better. Purchasing the cheapest option may cause you to ultimately spend more as you may have to fork out for frequent repairs, meanwhile, with a warranty of up to 12 years with a Viessman Vitodens 100-W boiler when purchased with WarmZilla, this worry can be taken off your shoulders. Thinking about it? Go on, go get yourself a new boiler quote! 😉

Vaillant EcoTec Plus 600's System Boiler

Vaillant Ecotec Plus (Heat Only) Boiler

The Vaillant EcoTec Plus System boiler range is one of Vaillant’s best-selling boilers, with outputs of 12kW15kW, 18kW, 24kW, 30kW and 37kW available for small to medium households with up to 20 radiators and multiple bathrooms. In addition to this, Vaillant has also created the 48kW and 64kW EcoTec Plus System boilers for larger homes and commercial applications. 

Vaillant Which? Best Boiler brand 2021 survey results:

We can see from the survey results that Vaillant are consistent in all categories with nothing below 4 stars, conveying that they are a great boiler brand with efficient and reliable boilers which their customers love. 

Similarly to Worcester Bosch, Vaillant also has 4.6 stars out of 5 on Trustpilot, with 90% of their reviews being graded above average. Customers have also shown a lot of love towards their Vaillant boilers. Here are only a few of Vaillant’s Trustpilot reviews:

“Pleased with the speed of response along with the efficiency of planning and installation. Boiler performance is very good and quiet, very pleased with our choice.”

“We had a Valiant boiler and the Google nest thermostat system installed a few months ago, we are bringing a tired old bungalow up to date, and as such not all the radiators are fitted yet, so the boiler has not reached its full potential yet, however, we have seen lower energy bills already, so I would say this is a very efficient boiler and we are very pleased with our purchase.”

We can see from the comments below that Vaillant’s customers are very pleased with their efficient Vaillant boiler and have already started to benefit from upgrading their older boiler to a new, more efficient boiler. 

According to the Energy Saving Trust (2021), condensing boilers are 25% more efficient than non-condensing boiler units. If you have an old non-condensing boiler, upgrading to a newer condensing unit could potentially save you between £45 and £300 on your annual bills alone. Upgrading to a more efficient boiler could save you even more money. We know, madness right?! 

In financial terms, if we say for example that your old boiler has an efficiency rating of 89%, you are currently wasting 11p for every £1 you spend on your energy bills. It may not seem like a lot now, but if you were to have your heating on a little more than usual this winter, and your energy bills rise to around £100 a month, that’s £11 you’ve thrown in the bin, just because you’re boiler is not performing as efficiently as a new, more efficient boiler. 

If this has made you stop and think, which it should have, we have a blog discussing the most energy-efficient boilers on the market at the moment, to help you find the best new boiler for your home. 

Boiler ModelVaillant EcoTec Plus 612 Vaillant EcoTec Plus 615 Vaillant EcoTec Plus 618 Vaillant EcoTec Plus 624Vaillant EcoTec Plus 630 Vaillant EcoTec Plus 637
Warranty length 5 years5 years5 years5 years 5 years 5 years 
kW Output (CH)12kW15kW18kW24kW30kW37kW
Efficiency   92%93%94%94%94%94%

Price* (inc VAT)


Each boiler from the Vaillant EcoTec Plus System boiler range has an efficiency rating above 90%, making them A-Rated boilers, meaning that depending on your current boilers efficiency rating, these system boilers can help you to reduce your energy bills. 

The EcoTec Plus System boiler range comes with a standard 5-year warranty, however, when you purchase your Vaillant EcoTec Plus system boiler through WarmZilla, you will receive an 8- year warranty on your new boiler

In addition to the Vaillant EcoTec Plus system’s boiler’s being highly reliable and loved, they also come with a range of great features such as built-in frost protection for extra peace of mind during the colder months, an LED digital display screen, easy to use controls, a gas valve that automatically optimises heat and hot water and more! To see all the benefits for yourself, why not check out our Boiler Comparison tool which allows you to compare your current heating system to this bad boy and see if you need to get yourself a new boiler!

Baxi Megaflo System Boiler


The Baxi Megaflo range is available in five kW output models, the Baxi Megaflo 15 (15kW boiler), the Baxi Megaflo 18 (18kW), the Baxi Megaflo 24 (24kW), the Baxi Megaflo 28 (28kW), and the Baxi Megaflo 32 (32kW boiler), so there is an option for everyone. 

In the Which? Best Boiler Brand Survey in 2021, Baxi boilers were rewarded an overall brand score of 76%. 

Baxi Which? Best Boiler Brand Survey Results:

As you can see from the survey results, the Baxi Megaflo System boilers are seen as an overall great boiler brand. Baxi has been rewarded 5 out of 5 stars for reliability, which is on par with other great system boiler models from brands such as the Viessman 100-W system boilers and the Worcester Bosch Greenstar 4000’s. 

Looking at the ratings from the survey, Baxi boilers are loved by their customers as well as by engineers. Overall, Baxi boilers are a great competitor for the best cheap system boilers on the market. 

Baxi has over 36k reviews on Trustpilot and over 86% of them are scored above average, showing that customer loves their Baxi boilers. Similarly to Ideal’s reviews, Baxi’s feedback focuses more on the aftercare and service from the brand itself, with some not too happy with their customer experience. With this being said, customers have also left very happy reviews:

“From the off my experience with Baxi was great. They kept me well informed about my appointment and arrived in good time.” 

 Baxi Megaflo System 15Baxi Megaflo System 18 Baxi Megaflo System 24Baxi Megaflo System 28Baxi Megaflo System 32
Warranty length 7 years 7 years7 years7 years7 years 
kW Output (CH)15kW18kW24kW28kW32kW
Efficiency   91%91%92%92%92%
Price* (inc VAT)£792.84£817.78£903.54£936.89£961.87

Although the efficiency ratings for the Baxi Megaflo System boilers are slightly lower than the Ideal Logic range, they still offer a high-efficiency rating of 91% and 92% (A-rated).

The Baxi Megaflo System boilers are very similar in price in comparison to the Ideal Logic System boilers, with a price range between £639.75 – £1,130 for the boiler alone. Unfortunately, WarmZilla does not sell Baxi boilers, however, if you like the look of a Baxi boiler, you can use our Boiler Comparison tool which allows you to compare this system boiler to others that we do sell, to help you find the perfect new boiler for your home. 

Ideal Logic System Boiler

Ideal Logic with 8 year guarantee

The Ideal Logic System boiler is available in four kW output models, the Ideal Logic S15 (15kW boiler), the Ideal Logic S18 (18kW), the Ideal Logic S24 (24.2kW), and the Ideal Logic S30 (30.3kW boiler)

Not sure what kW means when it comes to boilers? It’s basically how powerful the boiler is, the bigger the home, the more powerful the boiler needs to be to heat it. If you want to learn more about what kW output means in your boiler, jump over and have a little read. 

Having four kW boilers ranging from 15-30.3kW means that the Ideal Logic System range has an option for small, medium, and large homes.

In the Which? Best Boiler Brand Survey in 2021, 7,500 customers were asked to rate their boiler on efficiency, reliability, and build quality. Ideal boilers were rewarded an overall brand score of 74%. 

Ideal Which? Best Boiler Brand Survey Results:

As you can see from the survey results, Ideal boilers were awarded 5 stars for reliability and 4 out of 5 stars for customer score, showing that customers are happy with the performance of their Ideal boiler.

During the survey, Which? Trusted Traders were also asked how they felt about Ideal boilers, asking them how easy they are to service, what the technical support is like, what they make of the build quality, how easy it is to get hold of parts and spares, and ultimately whether they would recommend it.

Although Ideal was given 4 stars for their overall engineer score. Build quality, the availability of spares and parts, and ease of fixing a common fault are all taken into consideration as a part of this score. 

On Trustpilot, Ideal has bagged themselves an astonishing 4.8 stars out of 5 and over 93% of their 12k+ reviews are above average, clearly showing that although they are seen as more of an affordable boiler manufacturer, customers are more than happy with their Ideal boilers. 

One of Ideal’s customer have said:

“Extremely pleased with the work carried out by this company. The boiler is of a very high quality and I am extremely pleased with the installation.”

Reflecting on Ideal’s Trustpilot reviews, they seem to be more about the boiler installation from the engineer and aftercare rather than reviews regarding the boilers themselves. 

Fancy an Ideal Logic system boiler for yourself? Get a quote now! 

Boiler ModelIdeal Logic System S15Ideal Logic System S18Ideal Logic System S24Ideal Logic System S30
Warranty length 2 years2 years2 years2 years
kW Output (CH)15kW18kW24.2kW39.2kW
Efficiency   93%93%94%93%
Price* (inc VAT)£809.06£843.53£879.08£913.50

Ideal System boilers are one of the cheaper system boiler ranges on the market with a high-efficiency rating of 93% and 94%, making it ideal for those looking for a reliable boiler on a budget. 

The Ideal Logic System boilers come with a standard 2-year warranty from the manufacturer, which is significantly lower than other boiler brand warranties. However, when purchasing an Ideal boiler with WarmZilla, you will receive 8 years warranty!

Although system boilers need an additional hot after tank, typically installed in the airing cupboard of your home, Ideal boiler units are designed to be compact and lightweight, perfect for installation in a standard kitchen cupboard. 

The prices shown in the comparison tables are taken from an online central heating supplier and do not include installation or labour costs. This is probably a great time to mention that all WarmZilla boiler prices include Gas-Safe installation as standard. It also includes a system filter, a system cleanse, guarantee, smart thermostat, and any other bits and bobs you need to get your boiler up and running. 

What can affect the price of your new boiler?

There are many factors to consider when purchasing a new or replacement system boiler. Installation is a major factor to consider. 

As mentioned above, when purchasing a new or replacement boiler, it is important to consider all aspects, including whether or not installation is included in the price. Not all prices include boiler installation, make sure your quote does as you’ll be looking at £500 – £1,500 to get a boiler installed.

Now you know that all of this is included when purchasing with WarmZilla, you may as well save yourself time and find a new boiler the WarmZilla way – where your price is fixed, everything is included and our online survey will work out exactly what boiler you need based on your answers to some simple questions.

This takes us to the next factor that can also affect the price of your new boiler, kW output.

What kW output system boiler do I need for my home?

The price of your boiler can also be affected by the kW output required for your home. It is important to get several quote from local heating engineers to ensure that you are buying the correct kW output boiler for your home. 

Or, you could just take the WarmZilla online survey to get a boiler quote in minutes, which does all that for you. Nice and Easy. 

We have attached a system kW size guide below to give you a little more guidance on what size boiler you would need for your home

System Boiler kW Size Guide

No of bedrooms and bathroomsPoorly insulated propertyModerately insulated propertyWell insulated property
1 bed8kW7kW5kW
2 bed8kW7kW6kW
3 bed10kW9kW6kW
4 bed12kW10kW7kW
5 bed15kW12kW9kW
6+ bed18kW15kW10kW

System boiler advantages:

  • Ideal for homes with 2 or more bathrooms
  • Relatively easy to install
  • Doesn’t require a cold water tank
  • Compatible with solar thermal energy

System boiler disadvantages:

  • Large storage tank required, not suitable for flats or smaller homes
  • Loss of heat from the hot water storage tank, no matter how well insulated
  • Hot water is limited to the capacity of the storage tank, once the hot water from the tank is used up you will have to wait for it to heat the water again. A combi boiler heats water straight from the mains so it will always be hot.

What's included with a new boiler from WarmZilla?

Your WarmZilla boiler package will include:

  • Gas Safe Installation – £450 (minimum)
  • System Cleanse – £30
  • System Filter – £120
  • Wireless controls – £80
  • Flue – £80
  • Plume Kit – £100 (if required)
  • Moving Boiler – £150 – £500 (if required)
  • Gas pipe upgrade – £300 – £500 (if required/included in your fixed price quote)

With WarmZilla, you also get the convenience of purchasing your boiler and installation all in one go. Online. In a few minutes. You can also choose a convenient boiler install date that suits you, within 48 hours of your purchase date. 

To get a boiler replacement quote tailored to your home without having to part with your personal details, try WarmZilla. It only takes a few minutes and you’ll get a fixed price boiler quote with boiler installation included.

Overall, all of the boiler brands discussed below offer reliable and efficient system boilers, which is why we recommend them at WarmZilla. 

WarmZilla do not currently sell Baxi boilers, however, we think they are a great boiler option, other than that, WarmZilla stock Vaillant, Viessmann, Ideal, and Worcester Bosch boilers, so there are many to choose from. 

*Please note that these prices were taken from an online central heating supplier at the time of publication. Prices may vary depending on your home’s demands. These prices include VAT but not installation. 

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