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Best Combi Boiler For 3-Bed House (Buying Guide)

If you’re looking for a new combi boiler for a 3-bedroom house, this quick guide has everything you need to know to make that all-important purchase decision.

We’ll cover what kW output you need, explain what compact combi boilers are, and even give you an idea of how much the installation of your new boiler will likely cost.

best combi for a 3-bed house

What kW Combi Boiler You’ll Need For a 3-Bedroom House

Sizing a combination boiler isn’t particularly easy. One 3-bedroom house is going to have different hot water and heating demand than the next. Your property might have tonnes of 1.6m radiators and two showers, whereas another might have much smaller 1.2m radiators, and just one shower.

And that’s why most Gas Safe engineers will prefer to do a site visit to spec a property before determining which boiler is most suitable.

As a minimum, you’ll want 24kW output for hot water. You’ll get that with a boiler like the Worcester Bosch Greenstar 4000 25kW, which also gets 25kW of central heating output.

For properties that have a higher demand for domestic hot water (large 3-bed homes, or households that consume lots of hot water), an output boost is crucial, so you may want to consider the Greenstar 4000 30kW. Note that this model still has a 24kW output for central heating.

If you need a boost on both central heating AND hot water output, you’ll need a boiler model like the 29CDi Classic Combi; that gets 30kW on central heating and 30.8kW on hot water.

For a 3-bedroom home with a high water demand, it may be wise to consider a system boiler instead of a combi. System boilers work with hot water storage cylinders, and can typically supply more hot water than combis.

As you can see, this isn’t a one-size-fits-all approach.

To get ensure you get the best boiler for your home, you can use this boiler sizing calculator. It’s going to consider both hot water and heating demand (based on questions you answer about your property), and give you suitable boilers and prices on screen automatically.

It’s most likely the Worcester Greenstar 4000 25kW is a suitable gas boiler if you live in a small 3-bedroom property, with low-medium hot water and heating demand. You’ll go for the Worcester 4000 30kW if you need additional hot water demand. And, for those needing a boost on hot water and heating, the 29CDi Classic combi boiler would be a good choice.

You can use Heatable’s sizing calculator to see their combi boiler recommendations for 3-bedroom houses:

Heatable boiler installations

Installation Area

Note that the Greenstar 4000 25kW isn’t be suitable if you need to fit the boiler into a small installation area, such as a kitchen cupboard. In fact, none of the boilers mentioned above will be. They’re better suited to more open installation areas such as airing cupboards, garages, or being wall-hung.

So, if you want to save space in your 3-bedroom house, you’ll need a “Compact” boiler.

For example, you’ll notice if you scroll through the boiler options after filling out your details the 25Si Compact Combi boiler and the 30Si Compact Combi boiler.

These are the compact versions of the 25i and the 30i. Essentially, they’ll fit in a kitchen cupboard, and the 25i/30i boilers, and the newer 4000 25/30kW won’t.

What Are the Best Combi Boilers for a 3-Bed House?

We’re mentioning Worcester Bosch a lot here, and that’s because most homeowners either know the brand, or already have a Worcester boiler installed. They’ve also won the Which? Best Buy award multiple years running.

Worcester are a quality brand, but there are others out there worth a mention. So, whilst specific boiler recommendations are hard to make without knowing the exact details of your property, we’ve already created a list of the best boiler makes here, and that’s worth a quick read.

There, we list the likes of Viessmann, Vaillant, Baxi and Potterton, and explain exactly why we’ve picked each model.


Assuming you’ve picked an online installer or local firm that’s using a quality boiler brand, you should then focus on the boiler warranty. If you type the name of the boiler make and model into Google, plus “warranty”, you’ll see what’s being offered.

This is where you need to be REALLY careful. Some combi boiler models will only come with a 2-year warranty. And, in our opinion, that’s just not enough.

5 years would be our minimum recommendation for a boiler warranty.

However, bear in mind: there are manufacturers offering 7, or even 10-year terms. For instance, the boiler calculator we mentioned on Heatable will highlight Worcester boilers with a 10-year warranty.

Admittedly, this comes at an extra cost compared to a cheaper boiler with a 2-year warranty (£200-300 on an average installation), but those extra years of peace of mind are certainly worth it.

Installation Costs for Combi Boilers in a 3-Bedroom House

We’d expect a minimum installation cost for a combi boiler with a reasonable warranty to be in the region of £2,000-2,200. That’s if you choose a non-compact boiler in the 24kW range. You can get your own quotes for a range of suitable combi boilers by filling in this form from Heatable.

Worcester Bosch Greenstar 4000

Will A New Boiler Save You Money?

New boilers are naturally more efficient than older ones. So, if your boiler hasn’t completely given up just yet, you might be on the fence as to whether to repair or replace it.

The Worcester Greenstar 25kW, being A-rated, runs at around 94% efficiency, 99.7% when running at 30% capacity or 87.7% when it’s maxed out.

Compare that with an older G-rated boiler, which could be less than 70% efficient (that’s bad, by the way).

What that boils down to is a potential savings of around £840 per year, depending on the size of your property, and the efficiency rating of your current boiler.

new boiler energy savings


In many scenarios, the savings on your energy bills could cover monthly finance, if that’s the payment route you choose to take.

FAQs About Combi Boilers for 3-Bedroom Homes

Before we wrap up, we will do our best to address some common queries about combination boilers for 3-bed properties.

How many kW boiler do I need for a 3-bed house?

As a bare minimum, you’ll need a boiler with a 24kW capacity for domestic hot water and central heating. That said, if your home is on the larger side, or you’ve got above-average demand for hot water, you should consider a 30kW combi boiler instead.

How do I work out what size boiler I need?

To get an accurate boiler size, you can either ask a Gas Safe heating engineer to come and spec your property, or use this online calculator from Heatable. If you opt for the latter, all you have to do is supply some information about your property and heating system, and you’ll get a range of recommended boiler sizes, along with fixed-price boiler quotes.

Whilst the 2 options above will give you the most correct sizes, you can use the table below as a rough sizing guide, too:

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

Does a higher kW boiler cost more to run?

Not necessarily — the size of your property, the quantity of radiators, and the hot water consumption are all variables that need to be taken into account. Simply put, a boiler that’s sized correctly will be the most cost efficient.

For example, if you’ve got a large, 4-bedroom home with 15+ radiators, a 24kW boiler will have to stay on longer to keep up with the demand for heating and hot water. Meanwhile, a 30kW will be able to produce sufficient heat faster and stop burning fuel sooner.

Note, however, that oversizing your boiler isn’t necessarily cost-effective. Firstly, you pay more to fit a larger boiler. Secondly, if the boiler’s capacity is far too high for your heating and hot water needs, it will consume more fuel than required to produce enough heat.

To make sure you get a boiler that’s sized correctly, either call a Gas Safe heating engineer or use this online calculator from Heatable.

How long should a Worcester boiler last?

You can expect a Worcester boiler to last 15+ years — as long as you maintain your appliance properly.

Worcester Bosch are one of the most reliable boiler manufacturers in the UK, and their boilers are made from quality parts. However, even the best boiler brands are susceptible to harm caused by limescale and sludge build-up.

So, to ensure that your Worcester lasts as long as possible, you should:

  • Fit your boiler with magnetic and limescale filters
  • Get the boiler serviced annually

Doing these simple things can lower the risk of defects developing in your boiler’s expensive parts, and ultimately, prolong the boiler’s lifespan.

Can you run 2 showers off a combi boiler?

In theory, yes, you could use a combi boiler to run 2 showers at the same time; however, your flow rate will likely be low. Unless your mains pressure is high enough (15-17 litres per minute) and your combi can produce 15+ litres per minute, it’s unlikely that you’ll have a satisfying experience with 2 showers running at once.

If your household often has 2 showers on simultaneously, it may be best to consider a system boiler instead of a combi.

What’s Next?

Thanks for reading our guide to combi boilers for 3-bedroom homes. To get more information about getting a new boiler, you can jump over to our boiler replacement cost guide. Or, click directly from our home page to get boiler quotes for your property.

Still not sure which combi boiler to get for your home? Drop us a line, and we’ll do our best to help you out.



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