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24kW Combi Boiler Guide – How Much And Which One’s Best?

The 24kW combi boiler is the workhorse of modern British flats. But with the myriad models out there, picking one is neither easy, nor fun.

To make your research easier, we’ve done our homework and put together this boiler guide to 24 kilowatt combi boilers. Here, you’ll learn which boiler manufacturers make the best models, how to get them installed cheaper, and which features to look for when choosing your new boiler.

24 kW combi boiler guide
If you’re not sure if this size boiler is right for you, you can quickly fill out this boiler size calculator and the most suitable boilers will be listed towards the top.

NOTE: The central heating output is seldom obvious from a boiler model’s name. For instance, many combi boiler models labelled “30kW” actually have a central heating output of 24kW — which is what you’re really after.

Do You Need a 24kW Combination Boiler?

A 24kW combination boiler will supply sufficient heat and hot water flow rate for a smaller property. To get a general idea of how to pick boiler sizes, you can have a look at the table below:

Boiler SizeRadiatorsBathroomsProperty Type
24kW-27kW 10 or less1Smaller properties. Typically, apartments and 2-bedroom terrace houses.
28kW-34kW 10-15 radiators2Medium-sized properties. 2-4 bedrooms.
35kW-42kW 15-20 radiators2Large properties

Bear in mind, however, that the numbers above are estimates, based on typical heating and hot water needs. To make sure that you’re choosing the right boiler size for your property, it’s always best to consult with a heating engineer.

Alternatively, you can use this online calculator from Heatable. There’s no need to supply personal details or credentials — no email address, password, sign in info — just answer a few questions about your property and you’ll get a range of fixed price boiler quotes on your screen.

Again, note that boiler designations don’t always reveal the real central heating system output. Instead, they point to the maximum output for domestic hot water. So, if you see a boiler that’s advertised as 30kW, be sure to look at the specifications — chances are, it’s ~30kW for hot water and ~24kW for heat.

Top Boiler Manufacturers with 24kW Combis 

There are literally dozens of boiler makes that supply 24kW combis in the UK, some better than others.

We’ve already created a guide to the best combi boiler brands here. However, in summary, you’d want to opt for the following brands:

  • Worcester Bosch
  • Ideal
  • Viessmann
  • Baxi
  • Vaillant

These boiler manufacturers are known for excellent customer service, long warranties, and building boilers that last.

Choosing the Right 24kW Combination Boiler for Your Home 

Each brand mentioned above will offer a range of 24kW boilers, so you still have plenty of choice. You’ll want to narrow this down further by considering the following factors:

  1. Physical Dimensions of the Boiler

Firstly, you should decide where you will install the boiler. If you’d like to fit the appliance into a kitchen cupboard, you’ll need a “compact” boiler. Examples of compact boilers include the Baxi 600, Ideal Logic Plus, and the Worcester Bosch CDi Compact. Note that compact boilers typically cost a bit more.

  1. Boiler Price

Narrowing down your budget will help filter your shortlist further. Typically, you’d expect all the top brands to be comparable on price. However, you will find that certain installers fit more of one brand than another, so can offer that specific brand at a slight discount.

For example, Heatable is an online installer that fits boilers at attractive rates because they have a low overhead and good relations with leading boiler manufacturers. Generally, you’ll find that Heatable can fit Worcester Bosch and Viessmann boilers for less than your local trades.

That said, it always pays to get as many quotes as possible, so be sure to ring up your local boiler installers — at least 2 or 3 — and see what they’ll quote you. You can use your fixed-price boiler quote from Heatable as a benchmark for comparison.

  1. Warranty

Although all of the brands mentioned above are known for their reliability, not all of their boiler models come with a sufficiently long warranty.

And generally, a boiler warranty speaks volumes about the product quality.

Short warranties mean that the manufacturer doesn’t expect their boiler to last long, and vice versa.

Naturally, a long warranty term also guarantees that you won’t be footing the bill for expensive repairs during the warranty period.

Long story short: Some boiler models will come with a warranty as low as 2-years — avoid these.

We’d personally recommend heading for 5-years as a minimum; you’ll still have plenty of combi boiler options there.

As an example, Heatable mentioned above install combi boilers with a 10-year warranty, which is huge!

The Best 24kW Combi — Worcester Bosch Greenstar 4000

There’s a tonne of choice, and the best 24kW combination boiler is certainly up for debate. But as an overall package, including warranty, price, product quality, and extras, we’d pick the Worcester Bosch Greenstar 4000 30kW. Again, the “30kW” here refers to the boiler’s hot water output; the central heating output is 24kW.

Overall, we put the Worcester 25i up there as one of, if not the best combi with a 24kW heating output.

It has an exceptional warranty at 10 years, you can fit the boiler for under £2,000 and has plenty of useful features such as frost protection, not to mention it’s compatible with a wide range of smart controls.

Worcester Bosch Greenstar 4000

Below, we’ll walk you through its key features and explain why this model is out top pick.

Built-in Frost Protection

Winter is the busiest time of the year for any Gas Safe engineer, simply due to the number of boiler breakdowns.

One of the most common breakdowns relates to a condensate pipe blockage, which will cause you boiler to lock out and lead to expensive emergency repairs.

Worcester aim to solve this problem with their CondenseSure. The feature helps to avoid condensate ice blockages by protecting your boiler down to -15°C for up to 48 hours.

In simple terms, the CondenseSure on the Greenstar 4000 causes condensate to leave the pipe at intervals, meaning they have less of a chance of freezing over.

Advanced, Straightforward Controls

Worcester Bosch Greenstar 4000 has endless options when it comes to the types of controls you can use with the boiler. Obviously, you could go with a cheap (£40 or so) analogue thermostat from the likes of Honeywell.

However, most people nowadays opt for a smart thermostat. The two best options for Greenstar 4000 are the Worcester Wave or the Nest Learning Thermostat.

Currently, you can get a smart thermostat (~£250 value) fitted as part of an installation deal from Heatable.

Boiler Warranty

Warranty is a big deal maker — or a deal breaker, depending on how long it the term is. The Worcester Greenstar 4000 is leaps and bounds above most of the competition, coming with an impressive 10-year parts and labour warranty, and a separate, 10-year warranty on the heat exchanger.

However, the standard warranty is lower, and you’ll need to use an accredited installer and fit certain Worcester-branded parts. This means you won’t be able to get this 10-year warranty with all local installers — so be sure to check with your installation company before you purchase. Heatable, for instance, offer the 10-year warranty term on all their Worcester installs.


Typically, Worcester Bosch boilers tip towards the higher-end price bracket compared with other brands, although trades do like to fit them thanks to their easy installation and relatively low lift weight.

Generally, you can expect a local installer to fit the Greenstar 4000 for about £2,500, whilst a national company will probably charge far more than that.

However, thanks to its low overhead, you can typically get a better deal from Heatable.

24kW Combi Alternatives to Worcester Greenstar 4000

Not sold on the Greenstar 4000? No problem — there are plenty of other excellent combi boiler models with a 24kW heating output.

Viessmann Vitodens 100-W 30kW

For instance, look at the Viessmann Vitodens 100-W. It’s slightly pricier than the Worcester, but you get space heating efficiency of up to 98%, and tired-and-tested German reliability. If you get the boiler from Heatable, you also receive a 10-year warranty on parts and labour.

Much like its Worcester counterpart, this 30kW natural gas boiler actually has a 1:10 modulation range, with heating output ranging between 2.9 and 23kW, whilst the domestic hot water output is 31.7kW with a flow rate of 15.6L/ minute. That’s more than enough for a small-to-medium property.

You can ask a local installer for a price, or get an online quote from Heatable here.


Ideal Logic Plus 24kW

24kw combination boiler from Ideal BoilersWant to get a premium boiler without a premium price tag?

Look no farther than UK’s very own Ideal Logic Plus 24kW. It’s highly efficient — endorsed by the Energy Saving Trust, in fact — and compact, meaning it’ll easily squeeze into a kitchen cupboard.

At the time of writing (March 2022), you could get an Ideal Logic Plus 24 installed for around £1,900 — you can check prices here.

Want to explore more options? You can compare multiple 24kW boiler prices from a range of brands here.

You can also check out our review of Worcester’s older, 25i model here.

What’s Next?

Thanks for reading our guide to 24kW combi boilers; we hope you now have all the info you need to make an informed purchase.

Is there anything we’ve missed, or do you have any comments? If so, please leave us a comment below.

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