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What size boiler do I need?

When you’re searching for a new boiler, size matters. Investing in the right size boiler will keep you and your family comfortable, without wasting energy and money. The tricky thing is, there’s no such thing as a 'one size fits all' boiler. Your boiler needs to be the correct size for your particular household. If you choose a boiler that’s too big, you’ll be wasting energy and paying over the odds on your bills. But if it's too small, it might not be powerful enough to heat your home, or give you enough hot water. So, you're facing the Goldilocks conundrum of finding a boiler that’s ‘just right’. If you want to gain a better understanding of how different types of boilers are sized, this blog’s for you!

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When you’re searching for a new boiler, size matters. Investing in the right size boiler will keep you and your family comfortable, without wasting energy and money.

The tricky thing is, there’s no such thing as a ‘one size fits all’ boiler. Your boiler needs to be the correct size for your particular household.

If you choose a boiler that’s too big, you’ll be wasting energy and paying over the odds on your bills. But if it’s too small; it might not be powerful enough to heat your home; or give you enough hot water.

So, you’re facing the Goldilocks conundrum of finding a boiler that’s ‘just right’.

This can be a daunting task; because there’s lots of conflicting advice out there; and unlike Goldilocks, you can’t try them all out.

But fear not, because The Heating People pride ourselves on finding the perfect boiler to fit your needs and budget.

If you want to gain a better understanding of how different types of boilers are sized, this blog’s for you!

You might also like our related blogs: Your options with a boiler replacement and 6 top tips for buying a new boiler.

What is the best practice for choosing a size of boiler?

This might come as a surprise, given the number of boiler finding ‘calculators’ you’ll see online. But counting radiators is NOT the answer! This is completely inaccurate.

The correct way to size a boiler is through a whole house heat loss calculation. This is the industry standard for sizing boilers under Part L of the Energy Efficiency Regulations.

These regulations provide guidance on the conservation of fuel and power under The Building Regulations Act 2010.

This Energy Efficiency Best Practice requires that heat loss calculations be conducted before a boiler is installed.

Rest assured, that The Heating People conduct heat loss calculations as standard.

Should I just get the same size boiler as I have currently?

No. Whilst it seems logical to use the size of your current boiler as a guide; it should be just that; a starting point in beginning your choice.

A boiler is a significant investment, and it’s important you get it right. You’ll be expecting your boiler to work hard for you around the clock for at least the next ten years.

Simply assuming that you need the same again, could be a costly mistake. Lots of circumstances have probably changed since your old boiler was installed. And it might have been incorrectly sized years ago, when it was first installed.

What is the problem with oversizing a boiler?

Worcester Bosch Greenstar 8000 next to a ruler and a question mark.When it comes to boiler sizing, bigger isn’t necessarily better. Despite what manufacturer’s boasting their 50kW combi boilers seem to have you believe! Whilst it can be tempting to oversize the boiler, ‘just in case’, this is a mistake.

If you choose a larger boiler that has a much higher capacity than you need; you’ll be wasting money not only on the initial purchase; but also on escalating energy bills.

Too much power isn’t a good thing when it comes to central heating. It’s inefficient; costly; and can generate more power than your central heating system can handle; which will take its toll on your boiler.

Oversized boilers heat up too quickly; run for a short time; and then will turn off; before the cycle starts again.

This constant stop-starting, or ‘boiler cycling’ uses more fuel, costing you money on bills; and potentially on repairs and replacements.

What is the problem with undersizing a boiler?

Too small isn’t a good thing either. If you try to save money by choosing a smaller boiler than you and your family actually need; you’ll find that you’re feeling the chill; and running out of hot water, because your undersized boiler can’t keep up with your demands.



How is boiler size measured?

A boiler next to a boy dressed as a superhero.Boiler size doesn’t refer to the physical dimensions of the appliance itself; but rather how much energy is output by the boiler in terms of heat. This is measured in kilowatts (kW).

A kilowatt is equal to 1,000 watts. This unit helps us to measure the power your boiler can provide to your heating system.

For example, a 25kW boiler uses 25,000 watts (25 kW) of power per hour; to heat the water for your central heating.

Generally, the more heat and hot water you require for your home, the higher the kW boiler you will need. But the type of boiler you have will affect this.

If you have a heat-only or a system boiler; the boiler size will be based on the heat requirements of your property.

For combi boilers, the size of the boiler you need looks ‘bigger’. This is because it produces instantaneous hot water on demand, rather than storing it.

Does a Higher kW Boiler Cost More to Run?

As you would expect, the larger your heating and hot water demands; the larger the size or kW boiler you will need, and the more it will cost to run.

This is one of the reasons why you’ll want to avoid choosing an oversized boiler for your home.

The 3 types of boiler available

A man looking at different doors.There are three main types of boilers (heat-only or conventional, combi and system).

Within these types of boilers, different sizes are available to cater for different household demands – and the type of boiler helps to determine the correct sizing.

●     Heat-only or conventional boilers

Heat-only boilers provide heating and hot-water, with the use of an additional cold-water tank and hot-water cylinder.

These boilers need a large feed tank which is usually stored in an attic or loft. This tank receives cold water from the mains and feeds it down to your boiler.

Once the fuel has been ignited; the heat exchanger warms water; which is then transported to the hot-water cylinder by a pump.

This stored hot-water is then sent to your taps and radiators when you need it.

These boilers work well in large households with multiple bathrooms. They can supply large amounts of hot water to multiple outlets at the same time.

●     Combi boilers

‘Combi’ is short for ‘combination’ boiler because it provides both your heating and hot water from one all-in-one (combined) unit.

Combi boilers don’t need any external tanks or cylinders. They heat water directly from the mains when you turn on a hot tap.

They typically have two heat exchangers, one for your heating (radiators) and the other for your hot water supply.

The primary heat exchanger focuses on your radiators and carries hot water around your home. The secondary exchanger is for heating the water that comes out of your taps.

●     System boilers

System boilers feature aspects of both a conventional boiler and a combi.

Like a heat-only boiler; a system boiler needs a hot water storage cylinder to heat and store hot water for your taps.

But it doesn’t need a tank in the attic. Like a Combi; it takes its water supply directly from the mains.

This cold water is then heated via a heat exchanger, which transfers energy from the gas jets to the water. Once heated, the water is pumped into a large hot water cylinder, where it is stored until required.

Since there is a large store of hot water always available; a steady supply can be maintained even if multiple taps or showers are in use at the same time.

Why are the different types of boilers sized differently?

Heat-only and system boilers operate differently from combi boilers. So the way in which we choose the correct size of each, must reflect this.

Heat-only and system boilers only have a central heating output because they store the domestic hot water in a cylinder.

Therefore, these types of boilers are sized using a heat-loss calculation, which establishes the heat requirements of your home.

Combi boilers work differently, because they directly provide central heating and domestic hot water on demand; so they have two separate output ratings: Central heating (CH) output and Domestic hot water (DHW) output.

Whilst heat-loss calculations are still relevant for sizing a combi boiler; the main factor in determining their size, is hot water demand.

Why do combi boilers have higher kW than system and conventional boilers?

Question marks by a piece of paper.For combi boilers, the size of the boiler you need in kW looks ‘bigger’ than for heat-only and system boilers. This is because of the way in which combis work.

It doesn’t mean that combi boilers are less efficient than their counterparts.

Combi boilers need more power because they heat up the hot water on demand; rather than storing hot water in a cylinder for later use.

A combi boiler will only be using this maximum power when it’s producing hot water for your hot taps; baths; and showers.

Is a larger size boiler always better for bigger homes? 

Although larger homes will generally have a higher demand for heating and hot water; it doesn’t necessarily mean that a bigger boiler is required.

In the past, the physical size of your property and the number of radiators you had; was used to determine boiler size. And you’ll probably find a lot of installers still using this crude method. But this is inaccurate, and doesn’t follow the industry standard under Part L of the Energy Efficiency Regulations.

To illustrate the inaccuracy of this approach; imagine that there are two neighbouring houses of the same physical size; and each having exactly the same number of radiators as the other.

One house is very old with single glazed windows, and no insulation. The other house is a modern new-build with double glazing and lots of insulation.

The first draughty old house will lose its heat very quickly; and will need much more power from a boiler to keep it warm.

Conversely, the second house will lose its heat much more slowly; and will therefore need less power from a boiler to keep it warm.

And it wouldn’t matter if we installed more radiators either. The boilers would still need to output the same amount of power to match the heat loss.

Heat loss not home size!

So, it isn’t the size of your home; or the number of radiators it has; that matters when it comes to choosing the size of your boiler. 

To provide enough heat to your rooms; the heating system output must be greater than the heat loss.

Therefore, a boiler shouldn’t be installed unless a heat-loss calculation has been conducted.

And this is why heat loss calculations are the industry standard for sizing boilers; under Part L of the Energy Efficiency Regulations.

The Heating People always conduct a heat loss calculation before we recommend the size of boiler you require.

Sizing a boiler: Heat loss calculations

A calculator next to a thermal image.So here’s the science… Heat loss calculations work out a building’s heat transfer coefficient. They look at the thermal transmittance or ‘U-values’ of all of the external fabric of the building; along with its volume and average ventilation rate.

This provides the total space heating energy flow rate in watts; divided by the temperature difference between the inside and outside air.

Or, in layman’s terms; a heat loss calculation determines how quickly heat can escape out of the fabric of your property.

Different properties are built using different methods, materials, and levels of insulation; all of which affect the rate at which heat transfers through them.

As in our previous example; a draughty old house with no insulation and single glazing; will have a high U-value, indicating poor thermal performance.

Whereas the new-build with double glazing and plenty of insulation, will have a low U-value, indicating good thermal performance. Even altitude and exposure will impact this.

A heat loss calculation works out how much heat your home will lose on the coldest day of the year; which allows us to advise you on the optimum size of boiler for your needs.

How much heat can be lost from a building?

Thermal image of a houseWhether we like it or not, all buildings lose heat. The Climate Change Committee says: “UK housing stock is one of the oldest and worst insulated in Europe; with 38% of homes built before 1946 and only 15% since the 1990s.”

As such, according to the Energy Savings Trust:

  • 25% of your heat can be lost through the roof;
  • 35% through outside walls;
  • 25% through doors and windows; and
  • 15% can be lost through ground floors.

Insulating and draught-proofing your home is one of the best ways of reducing your home’s heat loss. And then sizing your boiler correctly will ensure that you always have adequate heat.

How do The Heating People perform heat loss calculations?

A calculator next to a thermal image.We always perform a whole house heat loss calculation to determine the correct size boiler for your home.

Our whole house heat loss calculation is included as standard as part of your quote for a boiler replacement.

It is conducted as part of your survey, and only takes about ten minutes on site. We use heat engineer software to size gas or oil boilers.

For more detailed calculations for renewable energy systems; or if your house is borderline between two boiler sizes; we can complete a room by room cross calculation for a more accurate result.

What is a whole house heat loss calculation?

With a whole house heat loss calculation, we will:

  • measure the outside of your building;
  • record what the fabric of your building is made from;
  • count the number of skylights you have;
  • look at how many stories your home has;
  • measure the height of the stories of your home;
  • record the location of your home and the altitude.

Our standard heat loss calculation is based on an indoor temperature of 21°C.

The outdoor temperature varies depending on the altitude and location of your home. In Merseyside and most of Cheshire the outdoor temperature for our calculations would be -2.2°C.

What is a room-by-room heat loss calculation?

The room-by-room method of heat loss calculations is much more detailed and is a separate chargeable service. It takes up to two hours on site, and is then completed away from site on a desktop computer.

With room-by-room calculations, we start with the same basic information as for the whole house calculations. But we then measure each room, window, door and skylight individually.

This allows us to consider small variations in building fabric, for example, where some walls have been insulated, but others haven’t.

Sizing a combi boiler

Lots of different boilers.Although heat loss is still a relevant factor in determining the correct size of combi boiler; the most significant factor when sizing a combi is its hot water performance, rather than space heating.

This is because unlike heat-only and system boilers; combis don’t store hot water, so they need to have enough power to meet your hot water demands.


How do I work out what my hot water demand is?

When thinking about how much hot water you need from your central heating system; a good starting point is looking at how many bathrooms you have.

Generally, homes with multiple bathrooms have a higher hot water demand, and therefore a larger boiler size is needed.

The number of bathrooms you have will not only affect the boiler size; but also the recommended type of boiler for your home.

If you have two or more bathrooms; a heat-only or system boiler is probably a better choice than a combi boiler.

This is because combi boilers don’t store hot water. And a combi’s hot water flow rate is reduced if two showers are running simultaneously.

Since heat-only and system boilers store hot water in a cylinder; they can supply multiple outlets at the same time; without the hot water flow rate being affected.

How do you use your hot water?

Now this doesn’t mean that you can’t have lots of hot showers with a combi boiler, quite the opposite!

A combi will never run out of hot water because it heats water instantly on demand; unlike heat-only and system boilers, that might run out of hot water for a short time while the cylinder refills.

So, this is where it’s important to consider how you use the hot water in your home.

If showers are taken consecutively in the morning, a combi will be fine. But if you want multiple showers on at the same time, a heat-only or system boiler will often be the best choice.

The Heating People will be able to advise you on what size will be just right for your home after completing a survey and a heat-loss calculation. But here is a rough size guide:

  • 24 – 30 kW Combi

Suitable to provide heating and hot water to a home with one bathroom. If you have an en suite with an electric shower, this size of boiler will still be suitable.

  • 30-36 kW Combi

Suitable for homes with two bathrooms. 

  • For larger homes with more than two bathrooms, a storage combination boiler or stored hot water should be considered. Although larger Combi boilers are available, they’re often not the best solution.

We hope that you’ve found this blog useful. For tailored advice on finding the correct size boiler for your needs, contact us today to book your free survey.

Useful Links:

A study for the Climate Change Committee to inform the 6th carbon budget

Report by the Climate Change Committee on heat in buildings

Domestic heating sizing method by the Energy Saving Trust

Government factsheet on Boiler Plus

Heat in Buildings – Government Consultation Paper on Boiler Plus