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What Boiler Size Do I Need: Calculator and FAQs

Finding the right boiler size for your home is not an easy feat — there are many variables at play, and specifications sheets are seldom helpful if you’re not a professional installer. So, if you’re wondering “what boiler size do I need?” you’re in the right place: in this guide, we will explain how to identify the right size boiler for your property.

Keep in mind that choosing the wrong boiler size has consequences. If you buy a boiler that’s too small, your heating and hot water won’t get up to the desired temperature. If you buy one that is too big, you will sacrifice energy efficiency and waste money on energy bills.

What size boiler do I need calculator

What Size Boiler Do I Need (Calculator)

A cool and straightforward way to figure out what size boiler you need is to use a boiler installation company’s fixed price quotation tool.

Heatable have a 90-second form that asks you questions about your property to determine which is the most suitable boiler for your central heating system. You don’t have to input any personal information or make any commitment to buy. To use the boiler size calculator, you only have to provide information about a few key things, such as:

  • What type of heating system do you have?
  • What type of property it is?
  • The number of bedrooms in your property?
  • How many baths and showers you have?

Just go and fill out the web form yourself here — you’ll get a range of gas boilers tailored to your property size, central heating demands, and hot water use. And, you’ll probably be pleasantly surprised at some of their great deals.

Worcester Bosch Greenstar 4000


Can I Use Another Boiler Size Calculator

The Heatable boiler size calculator is our favourite online tool and it comes with the added bonus of leading you to some great deals on a new boiler.

But there are other boiler kW calculator tools and charts you can try, including:

  • Glow-Worm – Manufacturer’s boiler size calculator
  • Worcester Bosch – Combi boiler size calculator from this prestigious manufacturer
  • Baxi – Manufacturer boiler size calculator
  • Vaillant – Manufacturer boiler size chart
  • Biasi – Manufacturer boiler size calculator

What Size Combi Boiler Do I Need for My Property? 

There are three major groups of combi boiler sizes: 24-27kW, 28-34kW, and 35-42kW.

The chart below shows you how these boiler size ranges correspond to key variables, like property type, the number of bathrooms, and the quantity of radiators.

What kW Boiler Size Do I Need

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

Consider Your Hot Water Demand

Keep in mind that the chart above is based on a reasonable demand for hot water and combination boilers.

Combi boilers rely on the mains water supply and heat hot water as and when it’s needed, so you don’t have any water stored. Boilers will always give priority to your hot water, meaning if you jump in the shower for 10 minutes, some of the heating output directed at the central heating will likely be pushed towards the shower. If this isn’t a problem (it isn’t for most), a combi boiler will fit your needs perfectly.

However, if your household uses a lot of hot water and frequently runs multiple taps or showers at the same time, a combi boiler may not provide enough water pressure and you might have to explore other boiler types (more on that below).

Also, the chart above assumes you have average-sized radiators. If you have large radiators (such as a 500mm x 2000mm double convector) as part of your central heating, you’ll likely need a higher output boiler than the ones mentioned in the above table.

How Is Boiler Size Measured?

While the chart above gives you a decent ballpark of what the size of the boiler should be, it’s always best to leave it to professionals to zero in on the correct wattage. That’s because each property is unique, and the criteria above are too flexible to give a definitive size every time.

You have 2 options for figuring out the precise boiler size for your property. You can schedule an appointment with a professional Gas Safe engineer to come and survey your property. Or, you can fill out Heatable’s 2-minute survey and see what boiler sizes the calculator recommends for your home. Both options will get you an accurate size, but the latter is 100% free while calling in a licensed installer will cost you money.

Establish What Type of Boiler Your Home Needs

Perhaps the biggest factor in choosing a new boiler is the boiler type you prefer. There are three gas boiler options available for those using mains gas — combi, system or regular boiler.

1. Combi Boilers

Combi Boilers combine heating and hot water in a single unit. They generate hot water on demand, meaning there is no need for a hot water tank.

Combi boilers are ideal for:

  • Flats
  • Apartments
  • 2-4 bedroom house

Combis may not be the optimal solution for large homes with lots of radiators, or busy households with lots of demand for hot water.

2. Regular Boilers

Conventional, aka regular boilers or heat only boilers provide heating directly from the boiler, and work with a hot water cylinder and cold water tank.

Regular boilers are ideal for:

  • Large flats
  • Large Apartments
  • 4+ bedroom house

Conventional boilers are not optimal if you’re short on space at home. The cold water tanks, hot water cylinder, and all the pipework connecting the system take up precious room. So, if you want to free up some loft space, consider converting to a combi or system boiler.

3. System Boilers

System Boilers work with a hot water cylinder, but with an integral pump and diverter valve. Unlike a regular boiler setup, a system boiler will rely on a hot water cylinder that gets its water from the mains; this means that there’s no need for a cold water feeder tank up in the loft.

Ideal for:

  • Homes with robust hot water demand

4. Other Types of Boilers

  • Oil Boilers
  • LPG Boilers

Ideal for:

Oil and LPG-powered boilers tend to only be used in properties where there is no mains gas supply available. This is because the running costs of these boilers will generally be higher.

These days, most people are looking to have a combi boiler installed unless their home is too big for one. So, for the purposes of this guide, we have focused on combi boilers.

What kind of fuel does your boiler use?

Fixed price online with next day installation

Worcester boilers from as little as £1,965.

Boiler Model Recommendations

So far, we have outlined how you can determine the right boiler size for your property. But having done this, which boiler is the best for you?

This is a subjective question and the right boiler for you will depend on a range of things including your budget, the length of warranty you want, and where you decide to buy it from.

But in this section, we will reveal our recommended boiler makes and models suited for different sized properties:

Boiler size for a small 1-2 bedroom property with up to 10 radiators (including apartments and flats)

  • Worcester Bosch 25Si Compact
  • Viessmann Vitodens 050-W
  • Ideal Logic Plus 24

Boiler size for a 3-bedroom property with 10 radiators

  • Worcester Bosch Greenstar 4000 30kW
  • Viessmann Vitodens 100-W
  • Ideal Logic Plus 30

Boiler size for a 4-bedroom property with 14 radiators

  • Viessmann Vitodens 100
  • Worcester Bosch Greenstar 42CDi Classic
  • Ideal Logic Plus 35

Boiler size for properties with 5 or more bedrooms, with more than 16 radiators

It is possible to use a combination boiler for larger properties with this number of radiators, but calculations need to be made beforehand, and we would recommend getting a qualified Gas Safe heating engineer to visit your property to sizer things up and make a recommendation.

Typically, properties of this size will have a system boiler feeding a hot water cylinder that stores water, but this can vary from one property to another.

How Converting a Boiler From Regular to Combi Impacts the Boiler Size

If you’ve got a regular boiler, but want to convert it to a combi and free up that loft space, there are a few thigns to consider. Crucially, know that converting a regular boiler to a combi costs more than a simple replacement for a new boiler of the same type. That’s because the installers will have to dismantle and remove the feeder tanks, hot water cylinder, and all the other infrastructure that the regular boiler needed to operate.

Secondly, you may be worried that a combi won’t meet your central heating and hot water needs.

You may ask yourself:

Should I Oversize My Boiler?

The answer is NO. Opting for an oversized boiler can needlessly increase your energy spending and slash your boiler’s efficiency. An oversized boiler will not be able to dissipate the heat throughout the home’s radiator network, and the return water (water flowing back to the boiler after completing the circuit around the heating system) will be hotter than it should be.

The return water temperature matters because modern condensing boilers only operate in their ‘efficient’ mode when the return water is at about 55°C. At this temperature, a condensing boiler can reclaim latent heat from the escaping flue gases. If the return water is too hot, the flue gases — and the latent heat they carry — cannot contribute to the heating process and are simply wasted.

What’s more, since they can’t dissipate all the heat they produce, oversized boilers operate at higher temperatures and subject their components to unnecessary wear and tear from the excess heat. This can cause the components to fail or reach the end of their useful life much earlier than they’re meant to. 

So, contrary to popular belief, getting an oversized boiler is not any better than getting a boiler that’s too small. 

Our Conclusion About Boiler Size

Getting the right sized boiler for your property is important and in this guide, we have provided all the information you will need.

You can get a good idea of the right sized boiler for your property by using an online calculator. We recommend trying Heatable, as their calculator will also point you in the direction of some great deals as well.

Heatable boiler clickable form

We have also outlined some others and detailed how to choose the right-sized boiler depending on the number of radiators and the size of your property.

You can also take a look at some of the best combi boilers currently available for different-sized properties.

Still got questions relating to which size boiler is right for your home? Any other tips for our readers we haven’t covered in this guide? Please share your thoughts with us using the comment box below.

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  1. damien says:

    have large house 3400 sq foot under floor and about 22 rad how many kw would i need

    1. heatingforce says:

      Hi Damien,

      You’ll need a heating engineer to spec your property based on BTU.

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