Heatable uses cookies to give users the best possible experience.

View Policy

Black Friday Boilers
We've knocked £100 off, everything.

COVID-19 Notice
We're still installing boilers, safely. Read more.

Best Small Boilers: Smallest Combi Boilers of 2021

If you need to fit a boiler in a tight spot, it'll need to have compact dimensions. Here's our guide to picking the best small boiler for your home.

Are you looking for a small boiler?

Not just any small boiler, one that's compact, but'll deliver all of the hot water and heat your hearts desire?

You're in the right place.

The quick answer? The smallest boiler in terms of dimensions is the Worcester Bosch CDi Compact, designed to fit into kitchen cupboards. Check the latest price here

Compact vs. Small Combi Boilers 

Small Boilers vs Compact Boilers

You may assume that compact and small are used interchangeably, but in the context of boilers, they mean slightly different things. 

...How are they defined? 

Small Boilers 

A small boiler relates to the boilers heat or power output (measured in Kilowatts) i.e. it is considered to have a “small” heat output or one that is adequate for properties with smaller hot water demands. 

Check our reviews of some of the smallest boilers on the market (ranging from 25 kW - 30 kW): 

Compact Boilers 

A compact boiler relates to the dimensions of the boiler unit itself. 

The good news is that many 'small' boilers are also compact too, which makes perfect sense considering small boilers are usually installed in smaller properties with limited space. 

Compact boilers are ideal for people wanting a boiler that will fit in a kitchen cupboard or comparable small space.

When it comes to boilers, size really does matter


Read our in-depth Worcester Bosch Greenstar CDi Compact Review.

What is Considered a Small Boiler? 

What is a Small Boiler?

For those that live in a small property such as a flat or small house, a boiler size with a power output between 24kW – 28kW is considered to be a small boiler and adequate. 

Sometimes these small boilers will be sufficient for a 3 bedroom or bungalow too, it largely depends on the number of radiators you have and your hot water usage. 

Since they provide both hot water and heating, combi boilers have two separate output ratings: 

  • Central Heating (CH) Output
  • Domestic Hot Water (DHW) Output

Typically, boiler brands will offer their boiler’s in a range of different sizes (the Worcester Bosch 8000 for example comes in 30kW, right up-to 50kW!)

30kW-50kW Boiler SizesTo determine how powerful it needs to be, the main factors when it comes to combi boilers are the number of hot water outlets your home has (such as bathrooms, showers etc).

For example, if you live in a small bungalow with one bath/shower, then a small combi boiler is probably going to be recommended.

However, if you live in a home with many rooms, with multiple bathrooms, then a large-sized boiler will be required to meet the heating demands.  

So, what size boiler do you need? 

You can use your current central heating setup to give you an idea of what size boiler you require.

This is based on the number of bathrooms in your property (or you can find out using our free tool): 

Bathrooms/Showers Ideal kW  
1 24-28kW
2 28kW -32kW
3 32kW-35kW
4+ 35kW+

Note: Combi boilers are not reccommended for homes where there is a very high demand for hot water. If you have multiple bathrooms, consider a hybrid/storage combi boiler, such as the Viessmann 111.

What makes the Best Small Boilers? 

What Makes The Best Small Boiler?

The majority of boiler manufacturers offer their boilers in a range of different sizes, but how do you know which one to choose? 

Ultimately, the same factors apply as they would for any other type of boiler - you want your boiler to be reliable, offer maximum value or money and to last the test of time. 

Here is a good summary of the factors we always consider when recommending new boilers (regardless of their size): 

#1 - Which? Best Buys

The consumer association was established back in 1963 and their Which? Best Buy Awards are highly respected by both consumers and industry insiders.

Which? Best Buy

Their reviews and ratings are based on feedback from installers, consumers, and a thorough testing process. 

One of the leading boiler brands Worcester Bosch (and also ranked number 1 in our best boiler brands list) has maintained the Which? Best Buy Award for 10 years in a row. 

More specifically the brand was awarded top marks for their build quality and availability of parts and spares.

#2 - The Warranty Period 

The warranty period is an essential part of shopping for a new boiler since you want to be covered for parts and repairs in the unfortunate event that something goes wrong. 

Ideally, you want the warranty period to be as long as possible.

10 Year Small Boiler Guarantee

The majority of boiler brands offer a warranty from 5 to 10 years, with Worcester Bosch offering a market-leading 10-year guarantee (when you buy through Heatable). 

That’s correct, they offer a 10-year guarantee and not just a warranty. 

...What’s the difference? 

A guarantee is an agreement from the brand that they will attend to your property and repair or offer a boiler replacement within a reasonable timeframe. 


Most other brands just offer a warranty as standard, this is an agreement to investigate the problem, but is not an unconditional agreement to fix or repair it for free.

If you live in an average-sized home, see our guide on choosing the best boiler for a 3 bedroom house

#3 - Efficiency Ratings 

The efficiency rating isn’t just important for those that care about the environment, but a high efficiency also means you will use less energy and therefore save money on your heating bills. 

Energy Saving Trust

The highest efficiency gas combi boilers are now available with an A-rated efficiency rating of 94%, meaning that for every £1 you spend, only 6p is wasted. 

Want to have your boiler moved to a different location? Check out our "cost of moving boiler" guide.

#4 - Availability of Parts 

What if your boiler breaks down due to a faulty part? Such as a faulty heat exchanger or perhaps even a faulty diverter valve

Sometimes a boiler can only be repaired and brought back to a functioning state by replacing a faulty part. Therefore, you want new boiler parts to be available and delivered to your engineer fast. 

That is why it is essential to secure yourself a boiler with a well-established network and parts that are easily accessible in the UK. 

The Best Small Combi Boiler

Best Small Combi Boilers

With the above factors in mind, here are our picks of the best small boilers currently on offer, providing reliability, value for money and substantial warranty periods. 

Worcester Bosch Greenstar 25i

Worcester Bosch is one of the most well established and renowned boiler brands in the UK, well respected by installers and homeowners alike (that’s why we ranked them as #1 in our list of the top boiler manufacturers). 

Greenstar i Best Small BoilerThe Worcester Bosch Greenstar 25i is from the 'Greenstar i' range and is Worcester's cheapest and most popular combi boiler. See our boiler size calculator for more details on kW's.

The Greenstar i is often considered to be Worcester Boschs entry-level combi boiler range, offering an A-rating for efficiency, an extremely user-friendly digital interface and awarded the Which? Best buy award.  

Looking for a small boiler that’s also compact? So much so that it can fit in most kitchen cupboards? Check out the Worcester Bosch CDi Compact

Summary of the Greenstar i: 

  • High efficiency A-rated boiler
  • Available in 25kW or 30kW 
  • Which? Best Buy Award winner for 10 consecutive years 
  • 10-year guarantee (not just short warranty)


Order your Greenstar i boiler today 

At Heatable, we fit Greenstar i's from as little as £1,745, all over the UK.

What's more, as we've already covered, our boilers come with warranties which are up to 10 years long. That's loooooong.

What's even more, we're able to offer low monthly payments with boiler finance, and even interest free options.

Get a fixed quote on a new boiler installation in 60 seconds, here.

More advice

New boiler deals

Buy your new boiler, better

Get a fixed price on a Which? Best Buy in minutes

Heatable is registered in England and Wales No. 08804726 (VAT No. GB205805920 ). Trading address is 1 Station Road, Great Sankey, Warrington, Cheshire, WA5 1RH. Heatable acts as a credit broker (not as a lender) and offers credit products through a panel of lenders. Heatable is authorised and regulated by the Financial Conduct Authority (reference no. 805259).

[1] Price based on a like for like combi swap in a 2 bed, 1 bathroom terraced home with a standard flue

[*] Installation dates are subject to installer availability

[ยน] only available on combi boiler installations

Financial Support: If you are in receipt of certain benefits, you may be entitled to claim a free boiler grant.