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The Best Small Combi Boiler Models [2023 Update]

Small combi boilers

Are you looking for a small combi boiler? In the heating world, “small” can refer to boiler power output and physical size, both of which we’ll cover in this article.

We’ll also walk you through small boiler sizing, brands, models, and installation costs.

Compact vs Small Combi Boiler: What’s the Difference?

They might sound like the same thing, but in the case of boilers, a small boiler and a compact boiler mean two different things.

So, you’ll need to determine whether you need a compact boiler, a small boiler, or one that’s both small and compact before reading the rest of the guide.

Also, keep in mind that this post deals specifically with combi boilers, which supply hot water on demand, without any water tanks or cylinders.

When we refer to “small” gas boilers, we’re talking about the heat output.

Essentially, they’re designed to power small properties, such as a 1-bedroom flat through to a small 2-bedroom terrace house.

If your property fits this criteria, and your household demand for heating and hot water is modest, a small boiler may be right for you.

“Compact” refers to physical dimensions and usually implies that the boiler unit can fit in a small space, such as a kitchen cupboard.

So, if you prefer to stick your boiler in a kitchen cupboard, out of sight and out of mind, a compact boiler is right for you.

Note that buying a compact boiler can add anywhere between 5% and 20% to the purchase price. If you’re looking compact combi boilers that are also suitable for smaller properties — i.e. a compact small boiler — you can expect to add around £50-200 on top of the total cost.

What Size Is a Small Boiler?

Getting the boiler output right is important, especially if you don’t want to overpay on your heating bills.

Typically, a small gas boiler unit that’s ideal for flats, apartments, and small houses will have a power output of 25kW or less.

That said, if a household’s demand for hot water is relatively low, a 25kW boiler can easily power a 3-bedroom property. So, if you don’t run multiple taps or showers simultaneously and have few radiators, you can even get away with a boiler that’s smaller than 25kW.

Simply put, each boiler is sized based on flow rate and CHW (central heating output) in kW. If you have more than the average number of radiators for your property size, or regularly run taps/showers at the same time, you’ll want DHW (domestic hot water) and CHW on the high end.

We could go through heating calculations and so on, but it’s much easier to use this multiple choice form from Heatable to work out which gas combi boiler you need. Click the options relevant to your property, and you’ll get suggested boilers on screen, along with a total price including installation by a Gas Safe heating engineer.

Alternatively, you can ask a Gas Safe engineer to survey your property and suggest an optimal boiler output.

The Best Small Boiler Manufacturers

There are literally dozens of boiler brands available in the UK. That just adds to the confusion, as a suitable small boiler could cost £400 from one manufacturer (boiler cost only), and £800 or more from another.

Typically, the difference in price boils down to these factors:

  • Brand reputation
  • Customer service
  • Online reviews
  • Supply & demand
  • Warranty

Of the above, brand and warranty are the two most important things to consider when.


We always say that there’s a reason behind any warranty term you get from a small boiler manufacturer.

Short warranties mean that the manufacturer expects their product to fail soon; long warranties show that the manufacturer has confidence in their boilers.

Consequently, if you invest in a boiler with a short warranty to save £100 on an installation cost of £2,000+, it could come back to bite you.

We’d always aim for the longest warranty possible; think 5 years as minimum. You’ll get a nice, long warranty as standard from the likes of the Ideal Logic Plus (7 years) and the Worcester Bosch range (up to 10 years), when you get your small combi boiler from Heatable.

Choosing a Small Boiler Brand

Oone local installer might prefer a certain brand, with another installer leaning elsewhere. However, the majority will agree that these are the top brands for gas combi boilers:

  • Worcester Bosch
  • Ideal Boilers
  • Baxi
  • Potterton

We’ve written a full guide covering the best boiler makes being supplied in the UK, and that’s worth a quick read.

The Best Small Combi Boiler Models

As we’ve already mentioned, some installers will prefer one brand over another. Typically, that’s because in their experience, they are the most reliable, and offer excellent value for money.

If that’s the case, it’s likely they install them in volume, and get a volume-based discount from their supplier (which they can then pass onto you, getting you a better deal).

Here are two small combi boilers worth mentioning…

Worcester Bosch Greenstar 4000 25kW 

During my time working at a plumbing merchant, this was one of the most popular small combis we sold ever since it launched in 2021. It’s got a high efficiency rating, is easy to use, and incredibly reliable, with a heat exchanger that’s guaranteed for 10 years.

And, it’s MORE than adequate for smaller properties. In fact, you’d be able to power an average 3-bedroom house with a small boiler like this without overspending on your gas bills.

Alternatively, you can check out Worcester’s earlier small combi boiler model, the Greenstar 25i; this boiler range is also quite popular, and has won the Which? Best Buy award on several occasions. We’ve written a full review of this boiler here and another one for the larger, 30i version.

You can get fixed price boiler quotes on screen, including a 10-year warranty, finance options, and other juicy extras by clicking here — that’s Heatable, and boiler prices from these guys are extremely competitive.

Worcester Bosch Greenstar 4000

Ideal Logic Plus 24kW

Ideal Logic small combi boiler

The Ideal Logic Plus 24kW is a comparable small boiler model to the Worcester Bosch 4000 25kW, offering similar efficiency levels and overall performance.

When using a local installer, the Worcester may be £100-200 more expensive. However, as we mentioned above, Heatable provide small boilers from Worcester Bosch at extremely competitive prices. Typically, Ideal and Worcester boilers cost about the same when you get them from Heatable.

If you’re using a local installer and looking to save a little, it’s worth a look. The warranty is strong at 7 years (so already above what we recommend), and Ideal Boilers are quickly becoming one of the most popular boiler brands in the UK.

One thing worth noting, too — the Logic Plus is a compact combi boiler. So, that means it’s going to fit into a kitchen cupboard, whereas the Worcester 4000 won’t. If you want a Worcester boiler that’s compact, you’ll need to go for the Worcester Bosch CDi Compact.

Want to read more about Ideal Boilers? We’ve reviewed the Ideal Logic Plus 24kW here.

How Much Does Installation Cost for Small Boilers?

Depending on size, brand and the complexity of the install, we’d expect the average small boiler installation cost to be in the £2,000-2,300 range. The price can increase with a range of factors; for instance, if you have to move your boiler to a new location.

If you want to jump straight to it and check out on-screen prices for recommended boilers, you can use this clickable, anonymous form from Heatable. It takes around 90 seconds to fill in, and you don’t need to supply any personal details, like a phone number or email address.

You’ll get a range of fixed price boiler quotes for models that are best-suited for your property.

Heatable Condensing Boiler Quotes

FAQs About Small Combination Boilers

Before we wrap up, we’ll do our best to answer some common questions about small combis.

What is a compact combi boiler?

The term “compact” is used to refer to boilers that are physically small and able to fit into a standard kitchen cupboard. Typically, a compact boiler will measure around 690mm (H) x 390mm (W) x 280mm (D). Anything larger than this wouldn’t leave sufficient clearance between the boiler and the surrounding cabinetry.

What size boiler do I need for a 2-bedroom flat?

You’d need a boiler with an output of 24kW or higher for a 2-bedroom flat. Note that the size of the property isn’t the only determining factor. You should also consider your hot water demand — if you often have multiple taps and showers running at the same time, consider a higher-output combi or a system boiler.

How much space do you need around a combi boiler?

It really depends on the model, so the best thing you can do is find the clearances in the boiler’s manual.

That said, a compact boiler may only need a front clearance of ~20mm (with the kitchen cupboard door closed), whilst side clearances can usually be reduced to a mere 5 milimetres. To accommodate a flue elbow, the boiler may need a top clearance of up to 200mm, and a front clearance of ~500mm is usually required for servicing.

How many radiators can a 30kW boiler run?

A 30kW combination boiler can usually run up to 15 radiators.

However, there’s no one-size-fits-all formula here, since there are other factors that may affect this number. For instance, if your home is poorly insulated or your household has great demand for hot water, your 30kW combi boiler may struggle to heat this many radiators sufficiently.

Is it better to undersize or oversize a boiler?

It’s always best to get the boiler size right instead of oversizing or undersizing the appliance. However, an oversized boiler — albeit wasteful and more expensive upfront — won’t leave your family shivering on chilly winter days. So, if we’re talking stricly about comfort, it’s best to end up with a radiator that’s too large than with one that’s too small.

Again, ideally, you’d get a boiler that’s perfectly sized to meet your heating and hot water needs. To zero in on the optimal boiler size, you can either use an online calculator like this one from Heatable, or consult with a local Gas Safe boiler engineer.

Can a combi boiler run 2 showers?

Theoretically, a combi could run 2 showers simultaneously, but you’d be left with unsatisfyingly low water pressure. That’s because a combination boiler setup relies on mains water pressure and lacks stored volumes of hot water. If your household often runs 2 showers at the same time, you should consider getting a system or regular boiler instead.

Can you get an oil combi boiler?

Yes, you can get a combination boiler that runs on oil. Worcester’s Greenstar Heatslave is an excellent option, which handily fits under a kitchen countertop and works with renewable energy sources, like solar panels.

As an alternative, Worcester’s Danesmoor oil boiler range also offers system and regular oil boilers.

What’s Next?

Thanks for reading our guide to the best small combi boilers. Whether you’re researching combi boilers to serve a heating system in a smaller home, or models that fit in kitchen cupboards, we hope our guide has given you some helpful information.

Got questions, or need some advice relating to an upcoming boiler purchase? Drop us a line below and we’ll get back to you as soon as possible.



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