Knowledge Builders

how big a boiler do i need uk

by Dr. Rusty Davis Published 1 year ago Updated 4 months ago

Like system boilers, conventional/regular boilers have the same recommended central heating output and so do not need to be as powerful as combi boilers.
What size regular boiler do you need for your home?
Number of RadiatorsRecommended Regular Boiler Size
Up to 109-18 kW
10-1518-26 kW
15-2026-40 kW
Apr 12, 2021

Full Answer

What size boiler do I need for my house?

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.

What does boiler size mean?

For the record – boiler size relates to the output rating, not the actual physical size of the boiler. In short, output rating is the power your boiler can provide to your heating system.

What happens if you have an under sized boiler?

An under-sized boiler has difficulty in meeting your home’s central heating needs, resulting in a not powerful enough heating system for your radiators, hot water for your baths/ showers or daily needs. Opting for the larger size boiler for your house than needed will cost more money than is necessary.

Do I need a larger boiler for my hot water usage?

This guide is simply a recommendation for normal hot water usage, and you will need to choose a larger boiler if you use an excessive amount of hot water on an average day, or have more people living in a smaller home. This should provide you with some indication on which boiler to go for.


How do I work out what size boiler I need?

A good way of determining the size of boiler you need is to count your radiators around the home....As a ball-park estimate, you'll need the following power outputs for the number of radiators for a combi boiler:Up to 10 radiators: 24–27 kW.10 to 15 radiators: 28–34 kW.15 to 20 radiators: 32–40 kW.

How many kW boiler do I need for a 4 bed house?

As your home has four bedrooms, you'll need a boiler with an output between 27kW-40kW and a cylinder with a 250-300 litre capacity. If you have 2-3 bathrooms, you'll likely just need a 27kW system boiler, whereas a home with 3+ bathrooms will likely need a 40kW boiler.

Do I need a 25kw or 30kW boiler?

As a rough guide most apartments and smaller houses, with up to 10 radiators will require a 24-25kw boiler. A 28-30kw would be installed for a medium to large 3-4 bed house with up to 15 radiators, and a 33-35kw and a 40kw would be for a large house with anything up to 20 radiators.

How many radiators can a 30kW boiler run?

15 radiatorsHow Many Radiators Can a 30kW Boiler Run? For combi boilers, sizes from 28 to 32kW can keep between 10 and 15 radiators nice and toasty. On the other hand, 30kW heat only and system boilers can heat 15 to 20 radiators, making them the ideal choice for running larger properties.

How big a house will a 30kw boiler heat?

For an average three or four-bedroom house that typically has around ten radiators a small or medium boiler (for example 24-30 kW) should be sufficient. A larger house with substantially more radiators or an en-suite bathroom will generally need a larger one, typically 30-35 kW.

What happens if a boiler is oversized?

Oversized boilers 'cycle' on and off excessively, i.e. they heat up too quickly, run for short blasts and then turn off because they are too hot. Each fire up is something close to full capacity which dumps a huge amount of heat into the system and overheats the radiators.

Is it better to get a higher kW boiler?

A kilowatt is a unit that defines how much energy your boiler outputs in the form of heat. As a general rule, the more heat and hot water you require for your home, the higher the kW boiler you will need.

How many kilowatts do I need for a 3 bedroom house boiler?

Sizing a system or regular (heat-only) boiler The average 3 bedroom property will require between 4-10 kW a day depending on the age of the property and level of insulation.

Is 20kW boiler enough?

If you have a flat, you'll want a machine with a 20kW to 24kW output – or, if your flat is particularly small, a 15 to 18kW boiler will do. If you have a two-bedroom house, look at boilers with outputs between 24kW and 30kW. For a three-bedroom house, we recommend considering 30kW to 35kW boilers.

What happens if my boiler is too small?

If your new boiler is too small, it might not be powerful enough to properly heat your home, or give you enough hot water. On the other hand, getting a bigger boiler than you need could lead to wasted energy which'll take its toll on your bills and on the environment.

What size combi boiler do I need for a 4 bedroom house?

As a rough estimate most flats, apartments and smaller houses, with up to 10, average size radiators and 1 bathroom* will require a 24 to 27kw Combi Boiler. For a medium to large 3-4 bedroom house with up to 15 average size radiators, 1 bathroom and an ensuite* bathroom, a 28-34kw Combi Boiler would be installed.

How much gas does a 24kw boiler use per hour?

A 24 kW boiler will use 24 kWh of energy per hour. According to Choose, the cost is approximately 3.8 pence per kWh, which means it would cost around 91 pence to run a 24 kW boiler for one hour.

How big of a boiler do I need to heat my house?

Boiler Capacity An easy rule-of-thumb for BTU requirements is to figure that you need about 50 BTU per square foot of interior space in a cold climate; 35 BTU per square foot in a moderate climate; and 20 BTU per square foot in a hot climate.

Does a higher kW boiler cost more to run?

Does a higher kW boiler cost more to run? No, it doesn't. Bear with us here. It's true that a 36kW boiler will cost more to run than a 24kW model – after all, you pay for each kW you use.

How many kW combi boiler do I need?

The average three or four-bedroom house usually has about ten radiators. For this reason, it would require a 24-30 kW combi boiler. A larger house that has more radiators and more bathrooms would need a larger 30-35 kW boiler.

What size electric boiler do I need for my house?

As a basic rule, you should add 1.5 kW for every radiator in your home. For example, if you have six radiators, you should get a 9 kW electric boiler. This rule may vary depending on how high your ceilings are, whether you have single or double glazing and how effective your insulation is.

What is boiler size?

Boiler sizes quoted on various models are not actually dimensions or capacity. The sizing measured in kilowatts is actually referring to the power output that a boiler provides. Typically, the more heat and hot water needed for the property and household requirements, the higher the kilowatt output, or boiler size will need to be.

What type of boiler do you need?

When needing a new boiler one of the first tasks in the process is to establish which type of boiler would be most suitable. Commonly both the property requirements and household usage will determine which type of boiler should be installed.

Why do you need a larger boiler output size?

A property with lots of radiators and towel rails will require a higher boiler output size to heat the property effectively.

How much power do you need to add to a hot water tank?

However, if you have a boiler with a hot water storage cylinder/tank you will need to add an additional 3kW to power the tank.

What is the factor that determines which time of boiler would be more suitable?

Water Pressure – As discussed, the pressure of water is a factor that determines which time of boiler would be more suitable. Should water pressure fluctuate, the boiler efficiency can be impacted.

What determines the size of a boiler?

The number of bedrooms – The size of a property to be heated will determine the size of a boiler needed. The more bedrooms that a property has would illustrate a larger size property which would require a higher output boiler.

When needing a new boiler, what is the first task?

When needing a new boiler one of the first tasks in the process is to establish which type of boiler would be most suitable. Commonly both the property requirements and household usage will determine which type of boiler should be installed.

How much efficiencies does a condensing boiler have?

Actual efficiencies are closer to 80% .

How to make a boiler more efficient?

Achieving even better efficiencies with compatible thermostats. Sizing the boiler correctly is the first step to an efficient heating system. The next step is to pair the boiler with an advanced heating control that can adjust the output to suit the outside temperature.

How fast does a combi boiler heat water?

Hot water flow rates vary from 9.8 LPM to 25 LPM! For houses with multiple bathrooms, the hot water flow rate will be halved if two showers are running simultaneously so it’s important to look at the flow rates. If you have multiple bathrooms you may wish to consider a boiler with a higher flow rate. NB for multiple bathroom properties it is really much better to use a system boiler and hot water tank.

Why is a combi boiler bigger?

For combi boilers, the size is ‘bigger’ because it includes hot water production.

Why do boilers cycle on and off?

Oversized boilers will ‘cycle’ on and off, as the boiler heats up too quickly, runs for short blasts and then turns off. Like the clutch of a car stuck in town centre traffic, the stop-start cycles will use more fuel and put the components under greater stress than when cruising along a motorway.

How much heat does a hot water cylinder need?

Sizing for your hot water cylinder. One caveat is the output you need for your hot water cylinder, which is often above 8kW. Therefore consider your hot water requirements. Small cylinders commonly need 12kW of heat whilst larger cylinders can take 25kW and upwards.

Do you need a boiler for multiple bathrooms?

If you have multiple bathrooms in use that the same time then you will need a larger boiler on a conventional setup . The best advice is to pick a boiler than can go down really low on the heating side to get the best efficiencies. See our Guide to the Best Boilers for the most efficient boilers for every situation.

Why is choosing a suitably sized boiler so important?

In the world of boilers, size equals power and output. So it’s very important to have the right amount of heat output to heat and provide hot water to your home:

How much hot water does a combi boiler use?

The combi boiler size really dictates the hot water output as most combis boilers have a limit of 24kW for heating. Therefore if you purchase a 35kW combi, only 24kW will be used to heat the home whereas when you open the hot tap you get the full 35kW power output. So how many bathrooms and/or showers do you have? This is a simple calculation that gives you the most accurate idea of what to buy.

How many bathrooms can you run with a combi boiler?

You need to measure it against the size of your house and number of bathrooms. A small flat or house with one bathroom can run perfectly well with a 24-27kW combi boiler. A large house with 3 or more bathrooms will suit a system boiler with an unvented cylinder.

What happens if your central heating boiler is undersized?

If your central heating boiler is undersized for your property, your property won’t have any heating and hot water properly circulating around your home. Your radiators, and hot water cylinder won’t receive enough hot water. This means your family (or tenants) won’t be warm or happy during the colder months.

Do you need a hot water tank for a combi boiler?

Combi boilers heat water directly from the mains when you turn on a tap. With a combi boiler you don’t need a hot water cylinder or a cold water tank in the loft space, roof or basement. For a 4-person family or an average-sized home, combi boilers are usually the most efficient option.

Can a combi boiler run more than one shower?

Warning: Combi boilers don’t fare well when there’s more than one shower running at the same time. If yours is a busy household or you’re a landlord of a multiple occupancy property, consider a system boiler with an unvented cylinder.

What is the best combi boiler size?

There are three main groups of combi boiler sizes, all of which work best for various property and household sizes: 24kw-27kw. 28kw-34kw. 35kw-42kw.

How fast does a boiler provide hot water?

Please note that the boiler will only be able to provide you will hot water as fast as it receives the cold water from the mains. So for example, if you have 10 litres per minutes going into your boiler and you purchase a bigger boiler which can provide 16 litres per minute of hot water, you will only be able to receive 10 litres of hot water per minute at a maximum. So, you might be better looking at a smaller boiler to save yourself some money.

What is a combi boiler?

A hugely popular choice (and a favourite for many BOXT customers), combination boilers, also known as combi boilers, heat hot water as and when it’s needed, so no hot water is stored. They are a combination of a water heater and a central heating boiler in one single system, and they work by staying on standby and heating water almost instantly when you require it. However, combi boilers can only support one outlet at a time, so can either be used for heating or hot water, but not both at the same time.

What is a conventional boiler?

Conventional boilers. Also known as 'Standard', 'Regular' or 'Traditional' boiler. No matter what you choose to call them they are all a traditional heating system. Conventional boilers use hot and cold water storage tanks to heat your water. A conventional boiler uses fuel, such as gas or oil, to warm up the water, ...

How does a boiler work?

A conventional boiler uses fuel, such as gas or oil, to warm up the water, which is then transported to the hot water storage tank through the use of a pump. When your heating or water is turned on, this hot water travels to your taps.

How does the number of radiators affect the water in a house?

Number of Radiators – The more radiators in a home, the more water will be needed to keep them warm, and the longer they’ll take to heat up.

Can you choose a boiler at Boxt?

Here at BOXT, we help you to find the right boiler for your property, without all the extra fuss. We’ll simply ask you a few easy questions and give you a choice of fitted boilers at a fixed price. You can then choose your new boiler and installation date online, without the hassle of pushy salespeople!

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.

What to consider when looking for a new boiler?

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. ...

What are you heating?

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

How many radiators does a 24 kW boiler have?

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

Why does my boiler need to work harder?

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.

How does a boiler increase the flow rate?

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.

What does a boiler do?

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.

What does boiler size mean?

Okay. To get straight to the point, the boiler size means the kilowatts (kW) needed. Not the physical size or different shapes.

Why Is the Right Size So Important for a Boiler?

We all know that finding a suitable size boiler is essential. That’s why you’re here.

Why do combi boilers use more kW?

Combi boilers use more kW because they need more power for instant heating.

How does a system boiler work?

A system boiler works like a conventional gas boiler, minus the cold water tank.

Why is a conventional boiler more expensive?

A conventional boiler is more expensive and more complicated to install because of more traditional and old-fashioned pipework.

What are the concerns of a traditional gas boiler?

One of the concerns with a traditional gas boiler is that it requires and wastes a lot of energy.

Can a combi boiler be used with a strong water pressure?

Not suitable for simultaneous use and strong water pressure. If you have higher heat demands, make sure to carefully find the appropriate size combi boiler.

What size boiler do I need for a 3 bedroom house?

The best size boiler for a 3 bedroom house is again the Viessmann Vitodens 050-W combi boiler.

Why is it important to choose a boiler size?

Choosing a suitable size boiler is extremely crucial for two main reasons: Does not meet the heating and hot water requirements of your home if you undercut it. Opting for a larger kW than you need will be a waste of money.

What happens if my boiler is too small?

If your boiler is too small ofr your home then you may find that you are requiring hot water but it is not running as hot as it can, or should be.

What boiler is best for 4 bedrooms?

For another option, the Viessmann Vitodens 100-W combi or system boiler would be a great fit for a home with 4 bedrooms.

How many radiators does a 30kw boiler run?

A 30kW boiler will typically run 15 radiators with ease. The boilers that we recommend for this range would be the Viessmann Vitodens 050-W combi boiler. Get an online quote on this boiler by clicking here.

How many bathrooms can a combi boiler have?

It is a combi boiler that stores the water within the unit enabling it to suit homes with up to 5 bathrooms.

What is the main factor in a boiler?

The number of radiators are the main factor with regular boilers as well as the number of bathrooms that you have. Both of these give us an idea of the heat only boiler size that you will require.


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