What Size Boiler 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.

Choosing the wrong boiler size has consequences, too. 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 end up waste money on running costs. That’s why in this guide, we will explain how to identify the right size boiler for your property and even help you zero in on the correct boiler size by answering a few quick questions.

Read on to find out more.

What size boiler do I need

What Size Boiler Do I Need (Calculator)

A cool way to figure out what size boiler you need is to use a 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 heating system. You don’t have to input any personal information or make any commitment to buy. You just provide information about a few key things such as:

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

Worcester Bosch Greenstar i from Heatable

They then list a range of boilers with the cheapest and most suitable first.

Just go and fill out the web form yourself here. You will be pleasantly surprised at some of their great deals.

Viessmann Vitodens 050-W from Heatable

Can I Use Another Boiler Size Calculators

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 size calculators you can try too including:

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

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

Combi boilers heat hot water as and when it’s needed, so no water is stored Boilers will always give priority to your hot water, meaning if you jump in the shower for 10 minutes, some of the output directed at the central heating will likely be pushed towards the shower. If this isn’t a problem (which it isn’t for most), a combi boiler will fit your needs perfectly.

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 onesmentioned in the above table.

How Are Boiler Sizes Measured?

While the chart above gives you a decent ballpark of what your boiler size 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 installer 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. B

oth options will get you an accurate size, but the latter is 100% free while calling in a licensed installer will cost you money.

Types of Boiler For Your Home

Perhaps the biggest factor in choosing a new boiler is the type of boiler you choose to have. There are three main options available for those using mains gas:

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 cylinder.

Ideal for:

  • Flats
  • Apartments
  • Smaller houses

2. Regular Boilers

Regular boilers provide heating directly from the boiler and work with a hot water cylinder.

Ideal for:

  • Large flats
  • Large Apartments
  • Medium/Large houses

3. System Boilers

System Boilers are heat-only boilers that work with a hot water cylinder but with an integral pump and diverter valve.

Ideal for:

System boilers tend to only be found in new build homes.

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?

Grab your fixed price online by 3PM, and get next-day boiler installation.

Worcester boilers installed from as little as £1,795.

Boiler Recommendations

So far, we have outlined how you can determine the right sized boiler 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)

Boiler size for a 3-bedroom property with 10 radiators

  • Worcester Bosch Greenstar 25i or 30i
  • Viessmann Vitodens 050
  • 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 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

Should I Oversize My Boiler?

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.

Avoid Expensive Breakdown Costs
Boiler Replacement Promise: If it's under 7 years old and we can’t repair it, we’ll replace it.
Try YourRepair. From £9/mo
All plans include: annual boiler service, all parts and labour, unlimited support and 24hr helpline.

Leave a comment

Your email address will not be published. Required fields are marked *


Fix things and tutorials

Get a Boiler on Finance

The best pay monthly packages available

Read guide
Heatable Boiler Replacement

Want a new boiler from HEATABLE?

Read guide
WarmZilla Boiler Replacement

Want a new boiler from WARMZILLA?

Read guide
Boiler Installation Costs

Confused about new boilers and their costs?

Read guide