What Size Boiler Do I Need?

Boiler size

Finding the answer to ‘what size boiler do I need’ means doing a bit of research into the heating and hot water demands of your home. Boiler sizing relates to the output rating, which is the power the boiler is able to deliver to your heating system. Simply put, a higher output rating will be more suited to homes with more radiators.

While it’s difficult to define a boiler sizing rule of thumb, it is possible to work out approximately what size boiler you need based on some basic information.

Why is boiler size so important?
What type of boiler do I need?
Boiler size calculator
What size combi boiler do I need?
What size system or regular boiler do I need?
Boiler sizing and efficiency tips
Get professional advice on boiler sizing

NOTICE: You can still get boiler quotes during the COVID-19 lockdown. Heating engineers are classed as key workers and are providing quotes as long as no one in your household has Coronavirus symptoms and social distancing guidelines are followed. Many can also offer advice and provide quotes over the phone and via a video call.

Why is boiler size so important?

A new boiler should make your home more comfortable while also helping to lower your energy bills. For a new boiler to do this it has to be the right size. And by boiler size, we don’t mean the physical dimensions of the unit, instead this refers to the power of the unit, otherwise known as the output rating.

Measured in kilowatts (kW), the output rating shows whether a boiler will be able to meet the heating demands of your property. Should you have a boiler installed with an output rating that’s too low, it won’t be able to meet demand. Meanwhile, an output rating that’s too high will waste fuel and lead to your energy bills rising.

For those reasons, getting boiler sizing correct is essential.

What type of boiler do I need?

Before considering what size boiler you need, you should start by finding the right boiler type. In the UK, most boilers run on natural gas. However, this is only possible if your property is connected to the gas network. Properties that aren’t connected to the network will need a boiler that runs on oil or LPG – fuels that can both be stored onsite.

There are 3 different boiler types – combi, system and regular – and there are models available to run on gas, oil or LPG.

Combi boilers

Combi boilers are single cost-effective units perfectly suited to small to medium-sized homes with 1-2 bathrooms. They take heat water directly from the mains and provide central heating and hot water on demand as and when needed. This removes the need for any additional tanks or cylinders which will save space elsewhere around the property.

System boilers

System boilers are similar to combi boilers in the sense that they take water directly from the mains. And while they also directly heat the central heating system, they store domestic hot water in a cylinder. This does mean that additional space is needed for a cylinder, as well as the boiler, but allows system boilers to meet higher demand for hot water.

Regular boilers

Regular boilers are the oldest type of heating system and are commonly found in traditional properties. A regular boiler needs to be installed as part of a heating system that also includes a feed and expansion tank in the loft as well as a hot water cylinder. Cold water is fed to the boiler from the tank in the loft before it is heated and then circulated to the central heating system or hot water cylinder. They’re only recommended for homes that are planning a direct replacement for an existing regular boiler – due to the complexity of the pipework.

A heating engineer will be able to recommend the most suitable boiler type for your home. However, you can get an idea based on the size of your property and the level of demand for hot water.

Find out more about the options available to you when considering a boiler replacement in Types of Boiler Explained: Combi, System & Conventional.

Boiler size calculator

A number of factors have to be considered to get boiler sizing right. And to work it out, you can put together a boiler size calculator using the following information:

  • Type of boiler (system and regular boilers store domestic hot water in a cylinder making them better suited to homes with multiple bathrooms and a high demand for hot water)
  • Total number of radiators
  • Bedrooms
  • Bathrooms
  • Number of people living in the property

As you can see, property size is one of the key factors in determining an appropriate boiler size. We have a selection of guides that will help you to find the most suitable boiler depending on the size of your property:

While you can get a good idea of the ideal boiler size, it’s best to get a professional assessment quote. This would involve a qualified heating engineer visiting your property to assess your heating system firsthand.

What size combi boiler do I need?

Combi boilers are bigger in size than system and regular boilers as they need to produce hot water on demand for domestic use as well as heating. And as they directly provide central heating and domestic hot water, they have two separate output ratings:

  • Central heating (CH) output
  • Domestic hot water (DHW) output

Regular and system boilers only have a central heating output as the domestic hot water is stored in a cylinder.

Combi boiler sizing guide

The central heating outputs of combi boilers can be split into three categories to help you find a suitable model depending on the number of radiators in your property.

Number of Radiators

Recommended Combi Boiler Size (CH output)
Up to 10 24-27 kW
10-15 28-34 kW
15-20 35-42 kW

Best Combi Boilers

The best combi boilers are manufactured by Vaillant, Worcester Bosch, Baxi, Viessmann and Ideal.

The following combi boiler models are highly recommended by professional heating engineers and homeowners. They’re all available in a range of sizes to suit most sizes of home:

  • Vaillant ecoTEC plus Combi
  • Worcester Bosch Greenstar i Combi
  • Baxi 800 Combi
  • Viessmann B2KB Vitodens 200W
  • Ideal Vogue Max Combi

Find out more about the Best Combi Boilers.

What size system or regular boiler do I need?

System and regular boilers do not need to be as powerful as combi boilers. This is because they incorporate a separate hot water cylinder for heating and storing domestic water.

System boiler sizing guide

Using the number of radiators around your property, you can get a good idea of a suitable system boiler output rating.

Number of Radiators

Recommended System Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

Best system boilers

The following system boiler models are highly recommended by professional heating engineers and homeowners available in a range of sizes to suit most sizes of home:

  • Baxi 800 System
  • Ideal Logic Max System
  • Vaillant ecoTEC plus System
  • Viessmann Vitodens 200W System
  • Worcester Bosch Greenstar Si

Get a complete overview of system boilers in What is a System Boiler? Pros, Cons and Costs.

Regular boiler sizing guide

The recommended regular boiler sizing depending on the number of radiators is the same as for system boilers.

Number of Radiators Recommended Regular Boiler Size
Up to 10 9-18 kW
10-15 18-26 kW
15-20 27-40 kW

Best regular boilers

The following regular boiler models are highly recommended by professional heating engineers and homeowners available in a range of sizes to suit most sizes of home:

  • Baxi 400 Heat
  • Ideal Logic Max Heat
  • Vaillant ecoTEC plus Regular
  • Viessmann Vitodens 100W Open Vent
  • Worcester Bosch Greenstar Ri

What size hot water cylinder do I need?

If you’re having a combi boiler installed then you don’t need to worry about hot water cylinder size. This is because they heat and deliver hot water on demand. System and regular boilers, on the other hand, store hot water in a cylinder. So for anyone having a system or regular boiler installed, cylinder size as just as important as boiler size.

While boiler sizing relates to the power of the unit, hot water cylinder size is how much water it’s able to store. Capacity is usually measured in litres and the most suitable size for your home will depend on the size of your property.

Naturally, homes with more occupants and bathrooms are likely to have a much higher demand for hot water than smaller properties.

As well as varying in capacity, hot water cylinders can be vented or unvented and be heated directly or indirectly. Vented cylinders take water directly from the mains so have to be installed alongside a system boiler. Unvented cylinders, on the other hand, receive water from a cold water tank in the loft which means they should be installed with a regular boiler.

Being direct or indirect simply refers to whether the water in the cylinder is heated by the boiler or an immersion heater. Immersion heaters are electrical heaters that sit within the cylinder and can be heated by solar thermal panels to help lower your energy costs. If you want the boiler to heat the water in the cylinder then an indirect cylinder is needed.

The table below is a guide used by professionals. You’ll notice that direct cylinders need to be larger, this is because room is needed for the immersion heater.

Bedrooms Bathrooms Indirect (litres) Direct (litres)
1 1 75/120 120/150
2 1 150 180
3 2 180 210
4 2 210/250 250/300
4+ 3+ 300+ 300+

Converting from a regular to a combi boiler

Combi boilers are a relatively recent development in boiler technology but have quickly become the most popular. And it’s easy to see why. While regular boilers need to be installed alongside tanks and cylinders, combi boilers are single cost-effective units. This means that they don’t take up much space and converting from a regular boiler to a combi could free up more room in your home.

One such component that combis don’t need is a cylinder. Great news if you’re looking to make space but is a concern in the sense that they can’t meet as high demand for hot water. Having said this, unless the hot water demands of your home are considerably high then a combi should do the job.

Benefits of converting from a regular to a combi boiler include:

  • No need for additional components
  • Hot water on demand (no need to wait for the cylinder to fill up)
  • Most combi boilers can be hidden away in a kitchen cupboard

When to swap a regular boiler for a combi

Converting from a regular to combi boiler involves a considerable amount of work that can take up to 3 days.

As you’re likely to be without heating and hot water for a few days, summer is the best time to hire an engineer. Not only will the weather be warmer, reducing the need for heating, heating engineers tend to have a clearer schedule. And you could be able to save money on the installation compared to a winter boiler replacement.

Due to the complexity of the work involved, it’s best to arrange a regular to combi boiler swap during a period of renovation. This is because the pipework needs to be completely rerouted to connect the combi directly to the mains and water outlets.

Would a system boiler be a better option?

For homes with multiple bathrooms, most combi boilers won’t be able to meet the hot water demand. The other downside to combi boilers is a weaker flow rate when water is needed from multiple taps at the same time. As regular and system boilers store water in a cylinder hot water can be delivered to multiple baths, taps, or showers at one time.

One of the big benefits of replacing a regular boiler with a combi is the space being saved. And while system boilers need a cylinder they connect directly to the mains which means the tanks in the loft can be removed. A new cylinder will be needed too. This is because the unvented cylinder, needed for regular boilers, must be replaced by a vented cylinder.

When to replace a regular boiler with a new regular boiler

Of course, if it’s time to replace your regular boiler, you don’t have to decide on a new boiler type. You could simply replace it with a new regular boiler.

While regular boilers are the oldest boiler type, they’ve developed greatly in terms of performance, reliability and efficiency. Replacing a regular boiler with another boiler type would mean rerouting the pipework. This is best done during a period of renovation whereas a like-for-like boiler swap can be done without so much work and in less time.

Our Complete Guide to Boiler Installation will take you through everything involved in replacing a boiler.

Boiler sizing and efficiency tips

You can’t do too much research when it comes to boiler sizing. So we’ve put some extra tips together to help you get boiler sizing right.

Current boiler size

It’s a good idea to know the size of your current boiler as a starting point but don’t assume you need to stick with the same size going forward. Things may have changed since it was installed such as the household’s water demands, the level of insulation or the addition of an extra bathroom. Or, you may have plans to extend or convert part of the property in the future.

If you find your home is regularly running out of hot water it could mean that your current boiler size isn’t up to the task and you need to think bigger. However, if your current boiler is more than 8 years old then it may just be a case of inefficiency rather than a lack of capacity.

Mains flow rate

While regular boilers take their water supply from a cold water storage tank in the loft, combi and system boilers are fed directly from the mains. For this reason, if you are replacing an older system with a combi or system boiler, the size you need will be influenced by the flow rate (pressure) of the water coming from the mains. You need to make sure the boiler you choose is big enough to cope with the amount of water passing through it to be heated.

You can get a rough idea of your flow rate with the help of a bucket with measurements. Run cold water from one of your taps for 1 minute and measure how much you collect in litres. Boiler manufacturers will specify the maximum flow of water in litres per minute so you will be able to see if your new boiler is a good match. The more you collect, the faster the flow from your mains supply and the harder your boiler is going to need to work. The harder it needs to work, the higher the output needs to be.

Alternatively, a professional installer will be able to measure the flow rate and temperature of your mains water for you.

Boiler efficiency tips

A new boiler should increase the efficiency of your home and, as a result, help to lower your energy bills. When the time comes for a new boiler replacement, you should consider these boiler efficiency tips too.


A poorly insulated house will lose heat through the roof, walls, floors or windows and your boiler will have to work harder (i.e. use more energy) to keep your home warm. To keep energy bills as low as possible you may want to think about investing in some more effective insulation or replacement windows.

Radiator size

Not all radiators are created equal as some have higher outputs, measured in British Thermal Units (BTUs), than others and will vary in size. The bigger the surface area of the radiator the quicker it will heat a room, so each radiator should be adequate for the size of room.

Smart controls

Many boilers are now compatible with smart controls which enable you to adjust the temperature of your home from any location via smartphones or tablets. This technology is not only much more convenient but also gives you much more visibility and control of your energy usage. Some systems can even automatically compensate for the weather outside.

Annual boiler servicing

To keep a boiler running at its best for as long as possible it’s really important to get it serviced by a professional once a year. This will catch problems early on before they can develop into faults and potentially a complete boiler breakdown.

Get professional advice on boiler sizing

To get an accurate idea on the appropriate boiler sizing for your home, we highly recommend seeking the advice of a professional heating engineer.

Comparing multiple quotes will help you to find the most suitable boiler size for your home at the most competitive price. And we’ve made finding local heating engineers easier than ever.

All you have to do is complete our simple online form – which should only take a few moments – letting us know a little about the work you need carrying out. We’ll then match your needs with up to 3 companies in your area who are qualified to carry out the work. Each of the companies will be in touch as quickly as possible to provide a free boiler installation quote.