How To Size A Boiler For Your Household Needs

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Many factors affect how to calculate boiler size for your home as we shall explain in this article. After you have read through to the end you may know more than most salespeople do, and reach a decision that suits your interests best. Then you can find a gas-safe engineer to install it safely for you.

There’s no one-size-fits-all in our industry. Every home is different and the people living there are unique. Therefore choosing the right boiler size for a house is a lifestyle decision.

The larger the boiler, the greater the energy consumption and your utility bills too. You could spend more than you need with an oversized one. You, therefore, need a balance between having enough hot water, and heating too much. Like we said, sizing a boiler is a lifestyle decision you are going to make.

I Am Not Technical How Do I Choose the Right Size UK Combi Boiler?

No worries, let’s try a boiler sizing rule of thumb first. If you are replacing a boiler in your house ask the family whether the old boiler gave enough hot water or not. Check its size on the maker’s plate.

You already have one important clue that tells you how to size a boiler! If the house was not warm enough in winter or the showers ran cold you need a bigger one. However, if you had more hot water than you needed that’s another question.

If you think the old boiler might be too large, compare your gas bill with neighbours who have similar size families. If they are paying less ask about their boiler sizing, which will give you information on how to calculate boiler size for your home.

The Boiler Size for a House Matters but Kilowatts May Be More Important

The kilowatt rating matters more than the physical boiler size when it comes to the utility bill. We can compare these units of energy to a bonfire on Guy Fawkes’ night. The more logs (kilowatts) the brighter and hotter it burns. But the more wood this needs.

You need a certain amount of kilowatts for a medium-size boiler, but you don’t want to pay for too many of them. These are how kilowatts stack up against boiler size.

  • A reasonable size for a small home 24kW – 27kW
  • The above plus a ‘cushion of comfort’ 28kW – 34kW
  • A larger home  (extended family) 35kW – 42kW,

This may help you home in on the right size UK combi boiler but this is not the end of the journey yet.

Let’s Apply This Knowledge to Combi Boilers

The most popular heating systems nowadays are combi boilers. That’s because they have artificial intelligence that gives bathrooms and kitchens priority over radiators. This makes sense because radiators stay warm for a while after we turn them off.

It does however mean you need to hold back on washing up, or taking a bath while the radiators warm gradually after a service. That’s unless you are willing to pay for more kilowatts for the best of both worlds. There are, however, tricks of the trade to let you know how to size a boiler. It’s a question of balancing supply and demand with the right size UK combi boiler.

Larger radiators require more energy to heat up properly and remain at the set temperature. If the water is not piping hot in the kitchen for washing up, then the radiators may be stealing the heat meaning the boiler is not the problem. 

The opposite situation may apply if you have a larger family all needing to shower in the morning or after work when they come home. A combi boiler may not have the energy to keep the radiators going while they do. If you find the temperature in the house dips, then you may need a conventional boiler with a storage tank.

Turning to Water Flow Theory Next

Your local council aims to supply water under consistent pressure. However, it limits the flow to each home according to the diameter of the delivery pipes. A combi boiler must therefore be sufficiently powerful to be able to heat all the water it receives. A qualified gas safe engineer can measure the flow rate to your current boiler and will know how to calculate boiler size best suited for your needs.

When you know your flow rate you can also shop around for a combi boiler you can afford that can handle the flow. If it is too small, it is simply unable to heat the water properly. It’s like trying to boil a large pot of water on the smallest burner in the kitchen with the heat turned down low.

Reaching the Decision Which Combi Boiler to Buy

If you have a large house with many rooms it follows you have more radiators (and probably bathrooms) than a two-bedroom terrace. The hot water will also have to travel a longer distance resulting in a small loss of heat too. 

A combi boiler specialist is able to calculate the minimum combi boiler size you need for decent showers and warm rooms. However, there are boiler sizing rule of thumb calculations to help you get started with a budget:


Bedrooms Radiators Boiler Range
Large House 4 + 15 to 20 35 – 42 Kilowatt
House / Flat 2 – 3 10 to 15 28 – 34 Kilowatt
Smaller Unit 1 – 2 Max 10 24 – 27 Kilowatt

Table 1: General Guideline for Choosing Combi Boiler Size

The above is only a general guideline for choosing the boiler size for a house because of all the other factors we have been writing about. It’s always best to have an expert visit your home because they will know for sure how to calculate boiler size for your home.

Always consult with a registered gas safe technician when sizing a boiler. They will also be able to comment on the safety of your appliances and estimate the cost of installing your new combi boiler, assuming this is the right solution for you.

However, you are at least a little closer to discovering the right size UK combi boiler for your family. A boiler really is the heart of a home as it faithfully keeps the rooms warm all the time.