What Size Boiler Do I Need?

Residential Boiler Sizing Calculator App

What Size Boiler Do I Need? Residential Boiler Sizing Calculator App

Choosing a new boiler is a big decision. Our boiler size calculator app is designed to help you find the right boiler best suited for your home.

More often than not when we move into our home we are already supplied with a working boiler, but in some scenarios, we need to get a new boiler and it’s hard to know what size boiler is best for the home you live in.

There are multiple elements you must consider when looking for a new boiler. You must also have expectations in place to ensure you are not left disappointed.

Whilst you will need an expert’s advice when it comes to the installation, it can be integral for your wallet and your peace of mind that you do your own research first.

But, before you do any further reading, here’s a handy boiler size calculator app to get an idea of the boiler size you’d need.

Boiler Size Calculator App

What type of boiler do you need?

Heat Only

What is the age of the property?

Pre 1900
1920s - 1930s
1950s - 1980s
2000 Onwards

How many bedrooms do you have?


How many bathrooms do you have?


Recommended Models


Fill in your details to receive a free quote for your chosen system.

Does boiler size matter?

Some of us find ourselves sitting at home asking questions like “how many radiators can a 24kW boiler run?” or maybe “what is the best combi boiler for 4 bedroom house” whilst many others don’t think about size at all!

A common misconception is “bigger is better’” whilst others look to cut corners and costs by getting what is the most affordable not what is the most suitable for their home.

A boiler that isn’t big enough will struggle to heat your home and produce enough hot water, meanwhile, a boiler that is oversized will produce excess heat.

Recommended reading: No Hot Water from Boiler: Our Ultimate Troubleshooting Guide

This is the heat that you can’t use which results in a waste of energy – causing unnecessary damage to our environment and increasing your bills greatly.

It may seem like common sense to some of us, but finding the perfect boiler sizing fit for your home is the first big decision you need to make when buying a boiler.

What are you heating?

The key thing to consider is how big is the house and how many radiators will it need to run?

For instance, ask yourself what size boiler do I need for a 3 bedroom house or what size boiler do I need to run 16 radiators?

But remember, it isn’t just about radiators and bedrooms, it’s about bathrooms too.

Your boiler will provide the hot water for your showers, your baths and your taps.

If you know you have a lot of bathrooms that may be in use all at once, you may need to consider a bigger boiler.

Another potential influencer is how many people in the house.

Are all the bedrooms occupied, does the house get busy, are the bathrooms in constant use all morning before school/work?

Recommended reading: Best Conservatory Heating Solutions

Why are you buying a new boiler?

The last thing to think about is why are you getting a new boiler in the first place?

  • Did your existing boiler breakdown?
  • Was there a chance it was working too hard?
  • Or are you updating your boiler as part of a renovation, commercial fit-out or a new build project?

Therefore are you looking for something more efficient?

Asking these questions helps set your expectations in order to find exactly what you’re looking for.

Once you’ve considered all of these things and you know what you expect your boiler to heat, you can look at our boiler sizing guide below.

What size combi-boiler do I need?

There are many complex combi boiler size calculators online that ask for in-depth information and will then provide a free boiler installation quote once they’ve got your contact info – you’ll also find 9 of the most well-known manufacturer’s boiler calculators at the end of the article.

But that’s not what we’re here to do. We’re just here to provide the facts. So what is a combi or combination boiler?

A combi boiler is a type of boiler which provides heated water instantly, it doesn’t store hot water in a tank, it just takes water straight from the mains and heats it up.

Combi boilers on average are available in a range of sizes from 24 to 27 kW, 28 to 34 kW and 35 to 42 kW. If you consider these are in Small, Medium and Large sizes it is easy to relate to the size home they fit best.

It is relatively simple to think in terms of kW. The higher the demand for hot water there is in a home, the higher the kW of the boiler will need to be in order to increase the flow rate of hot water.

The 24 to 27kW combi boiler will supply flats and small homes that have no more than 10 radiators and one bathroom.

Meanwhile, the next size up will provide a 3-4 bed medium size house with roughly 15 radiators and perhaps two bathrooms or a family bathroom, plus an en-suite.

Finally, the largest size combi boiler would be best for a large house with 2-3 bathroom/ensuites and 20 radiators.

It is important to remember in any home with multiple bathrooms if two showers are running together a combi boiler may not cope as well and the pressure can drop. If there are high chances of multiple occupants in multiple bathrooms at the same time, a system boiler may be a better option.

Recommended reading: Types of Boiler: Condensing, Conventional, System, Combi, Biomass

What size of system boiler do I need?

Unlike the combi boiler, a system boiler provides central heating whilst ALSO heating water for a storage tank.

Due to the tank’s ability to hold more hot water than a combi boiler can produce instantaneously, these boilers are much more suited for homes with a high water demand. As water can be drawn from a tank, multiple taps can run without affecting the pressure and everybody can enjoy their showers!

However, unsurprisingly, these tanks and their boilers can take up more space so naturally, you will want a big space to store the tank in that is also well insulated.

A poorly insulated storage area for your tank will cost you money and waste energy, as heat is lost before it even reaches its destination. The more water your home uses, the larger the tank requirement will be.

Updating old systems

If you had a conventional or system boiler in an older house and you want to update it to a combi boiler, the size combi boiler you need will also be affected by the speed at which the water comes in from the mains.

How do you work out the speed, or as industry professionals would call it, the flow rate? It’s quite simple. Get a bucket with measurements on the inside and measure how much water you can collect from a cold water tap in 60 seconds.

The more water you’ve collected, the faster the flow rate is. This means more water will be passing through your boiler and therefore it will need to work harder to heat all the water as it flows through. The colder the water that comes of the mains is, the harder the boiler will need to work to increase the temperature.

Most boiler manufacturers will let you know the maximum flow of water in litres per minute, so you will be able to work out if the boiler is suited to your mains flow.

However, if you request the help of an expert they can check the temperature and the flow rate to ensure you get the perfect boiler.

Recommended reading: What Should Boiler Pressure Be When Heating Is On

Residential Boiler Sizing Calculator

As we mentioned earlier, there are several combi boiler size calculators on the web that can offer you an idea – as does this article.

However, you should never commit to buying something like a boiler without receiving multiple quotes from a number of professionals in the area.

To ensure you have a good engineer quote for your boiler, make sure they are willing to visit your home, run thorough checks on your mains water flow rate and water temperature as well as make time to ask you questions about what you want.

Having read this article thoroughly, you’ll be able to answer all the questions that the installer will have for you and you’ll be ready to look forward to having the best boiler for your space soon.