What Size Boiler Do I Need?

Boiler size

While size isn’t everything, finding out the answer to ‘what size boiler do I need?’ is up at the top of the list when you are replacing your boiler. It involves a bit of thinking and research into the heating and hot water needs of your home. For the record – Boiler size relates to the output rating(not the actual physical side of the boiler).

In short – Output rating is the power your boiler can provide to your heating system. A higher output rating is suitable for homes with more radiators.

It’s not easy to define exactly the size of boiler you will need as our requirements will differ, but, with some basic information we can help you find a boiler that will do the trick and keep you with plenty of hot water and warmth.

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

Why is boiler size so important?

Your new replacement boiler needs to make your home nice and comfortable whilst keeping your energy bills as low as possible. No-one likes paying more than they need too for their gas and electricity!

So on this occasion, (boiler) size is very much important and we need to get it right and work out the correct output rating of your new boiler.

Output rating is measured in kilowatts(kW). If you get a new boiler with an output rating that is too low it won’t be able to meet your homes demand and you’ll end up cold and miserable. On the other hand, an output rating that’s too high will waste fuel and you’ll be burning money on extortionate energy bills. – Not good. Yep, not to over-egg the pudding but getting boiler sizing correct is essential.

What type of boiler do I need?

Before we get to deciding on the size of your new boiler, we have to find out the type of boiler you’re going to need. In the UK, most of our homes are powered by natural gas but for those not connected to the National Grid, boilers run on oil or LPG – fuels that can be stored inside.

There are 3 different boiler types – Combi, System and Regular. You can get all of these models to run on gas, oil or LPG.

Combi boilers

Combi boilers are very popular in the UK. Cost-effective and compact in size, combi boilers are ideally suited for small to medium sized homes with 1 or 2 bathrooms. They take water directly from the mains and provide central heating and hot water on demand. There is no need for any additional tanks or cylinders saving you valuable storage space.

System boilers

System boilers are similar to combi boilers as they also take water directly from the mains They also heat the central system directly but with a difference. Unlike a combi boiler, a system boiler needs a hot water cylinder for storage. While this will take up some additional space, it does mean they are good for homes with a higher demand for hot water.

Regular boilers

Regular boilers are the older statesmen of the heating system world. Commonly found in older, more traditional homes, 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 gets heated and then circulated to the central heating system or hot water cylinder. 

You should only consider a regular boiler if it’s a direct replacement for your existing heating system. The complexity of the pipework will lead to a headache you can do without.

If you have any doubt about the most suitable boiler for your home and heating needs, a heating engineer will be able to recommend the right one. You can get an idea of what you need based on the size of your home and amount of hot water you need.

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

    Boiler-calculator
    Boiler-calculator

Boiler size calculator


Ok, let’s get on to the reason why you’re here and see if we can work out the right size for your new boiler.

There’s a few things to consider to get boiler sizing right. Once we’ve done this we can put together a boiler size calculator.

  • 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

Naturally, the size of your property and the specifics of the home are the key identifiers for what size boiler you will need. The good news is that we have a selection of guides to help you find the most suitable boiler for the size of your property:

Our guide will give you a decent idea of the ideal boiler size but it’s always worthwhile to get a professional to provide you with an assessment and quote. This just means a qualified heating engineer visiting your home to look at your heating system.

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 brands are 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

Adding up the number of radiators around your home will help you understand 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.

Up to 109-18 kW

Number of Radiators Recommended Regular Boiler Size
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

    Boiler-Comparison-Tool
    Boiler-Comparison-Tool

What size hot water cylinder do I need?

If you’ve decided to have a combi boiler installed this next bit isn’t for you as you won’t need a hot water cylinder. (Remember, they deliver heat and hot water on demand). This is not the case for system and regular boilers and you’ll need to store your hot water in a cylinder. Getting the right sized cylinder for homeowners with system and regular boilers is just as important as getting it right for your boiler.

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.

Homes with lots of people living in them and with more bathrooms are naturally going 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 regular (conventional) 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.

Direct or Indirect?

Being direct or indirect refers to the water in your cylinder being heated by the boiler or an immersion heater. Immersion heaters are electric heaters that sit within the cylinder and can be powered by solar thermal panels. Which will help keep those energy bills down. If you want the boiler to heat the water in the cylinder then it’s an indirect cylinder for you.

Have a look at the table below. This detail is used by heating professionals and you’ll notice that direct cylinders need to be larger because of the room 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 new addition to the world of home heating and have quickly soared to the top of the boiler popularity charts. It’s easy to see why – regular boilers need to be installed with tanks and cylinders that take up lots of valuable space whereas the combi boiler is a single standalone unit and also very cost-effective.

If you are considering changing from a regular boiler to a combi boiler you do need to make sure that it will be able to meet your demand for hot water. Extra space is one thing, but not at the expense of a cold house with no hot water for an evening soak in the bath. Having said that, unless your hot water needs are considerably high, a combi boiler should be enough to 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.

There’s a good chance that you’ll be without heating and hot water for a few days so the middle of winter is probably not the best time to make the change. Hire an engineer in the summer to avoid unnecessary uncomfortable nights. Heating engineers are less busy in the summer as well so you may be able to save some money on the installation compared to the emergency winter period.

It’s not an easy job so if possible, arrange a regular-to-combi boiler swap as part of  a bigger renovation. Your pipework will need to be completely rerouted to connect the combi boiler directly to the mains and water outlets so please be prepared for some disruption.

Would a system boiler be a better option?

If you live in a home with a number of bathrooms a combi boiler isn’t likely to provide you with enough hot water. Combi boilers also have a weaker flow rate so if you want water from multiple taps at the same time you’ll notice the limitations of the combi boiler. Thanks to the hot water stored in the cylinder, regular and system boilers can provide enough hot water for multiple baths, showers and taps at the same time.

A benefit of replacing a big old regular boiler with a combi is again the freeing up of space in your home. While system boilers need a cylinder, they do connect directly to the mains so you can get rid of the tank in the loft. It’s worth noting that you’ll need to swap your unvented cylinder with a vented cylinder to install a system boiler.

When to replace a regular boiler with a new regular boiler

Some of us do not like change! And that’s ok when replacing your boiler –  if you are happy with your regular boiler, simply install a new regular boiler.

It’s true that regular boilers are the oldest model but that doesn’t mean they’ve been left behind. Modern regular boilers have improved massively and have higher levels of performance, reliability and efficiency. Remember, a change in boiler type could mean rerouting pipework. There’s nothing wrong with a like-for-like swap. It will certainly mean a lot less hassle.

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

Boiler sizing and efficiency tips

There’s no such thing as TOO much research when it comes to boiler sizing. Ok, it’s not everyone’s idea of a great night in, luckily for you we’ve put some extra tips together so save you some time and help you to get boiler sizing right.

Existing boiler size

A good place to start is finding out the size of your boiler in your home at the moment. But wait! – Don’t assume that’s it! You may need to still make a size change. Since you last replaced your boiler, your home’s water demands may have changed. You could have had children, added an extra bathroom or you may be thinking of extending 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

We know that regular boilers get their water supply from a cold water storage tank in the loft while combi and system boilers are fed directly from the mains. If you’re replacing an older system with a combi or system boiler, the size you need will also be influenced by the flow rate/pressure of the water coming from the mains. It’s important you choose a boiler big enough to heat the quantity of water passing through it.

To get an idea of your boilers flow rate you just need a bucket with a measurement scale. Run the cold water tap for 60 seconds and measure how much water you collect in litres. Boiler manufacturers will specify the maximum flow of water in litres/per minute so you’ll be able to check to see if your new boiler is a good match. The more water 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.

If you don’t fancy getting your bucket out, you can call a professional installer who will be able to measure the flow rate and temperature of your mains water for you.

Boiler efficiency tips

Replacing your boiler should increase the energy efficiency of your home which hopefully, will see your energy bills reduce. We’ve got some extra tips to help your boiler work as efficiently as possible.

Insulation

A lack of insulation in your home will result in you losing lots of heat through the roof, walls, floors and windows. This just means your poor boiler is going to have to work harder to keep you nice and cosy. Working harder unfortunately means higher energy bills. To help your boiler out, consider investing in additional 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 of us have a smartphone and smart television so why not get smart with your heating? Most boilers are now compatible and you’ll be able to adjust the temperature of your home when on the move. Smart technologies are an effective way to keep an eye on your energy use. We’ve compared a couple of the best smart thermostats here

Annual boiler servicing

Yes, it’s a pain but trust us on this one. Having your boiler serviced every year will keep it running to the best of its ability for much longer and should save you money in the long run. Catch any issues early on to do your best to avoid a boiler on the blink.

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.


Get FREE Boiler Quotes