With so many boiler options and types available, determining which one is best for your property can be difficult. If installed correctly, the boiler you select can last up to fifteen years and will be responsible for meeting your heating and hot water needs throughout the year.
The ideal boiler will differ depending on your property’s heating and hot water requirements, so it’s essential that the right boiler is installed to meet your needs while not wasting money on your energy bills.
When answering the question “what size boiler do I need?” There are several factors to consider. Fortunately, we’ve put together this guide to help you answer that question.
Introduction to Boiler Sizing
When it comes to heating your home, selecting the proper size boiler is important. A boiler that is too small will be unable to meet demand, leaving you with a colder home and no hot water, whereas a boiler that is too large will be inefficient and may cost more to operate.
The size of your home, the number of radiators, and the type of fuel you intend to use are all factors that can influence your decision when sizing a boiler. In this article, we’ll look at the various methods for sizing a boiler, how to measure heat loss, and what factors to consider when deciding between the different types of boilers.
Why is it Important to choose the right size boiler for your property?
When we discuss the size of a boiler, we are not referring to its physical dimensions. Instead, we’re discussing output in kilowatts (kW), which is the unit used to measure how much energy the boiler generates in the form of heat. Another important factor that comes into play is the type of system, so it is important to correctly size the boiler according to the system type. E.g. the correct size for a system boiler isn’t the same size if that boiler is changed to a combi.
Boiler Sizing Methodologies
To size a boiler, three methods are used: rules of thumb, heat loss calculations, and manufacturer guidelines. Although rules of thumb are a quick and easy way to get a rough idea of the size of boiler you need, they are not always accurate. Some say that you need 1 kW of heat output for every 10 square metres of floor space, but this doesn’t include factors such as insulation and the number of windows.
Heat loss calculations on the other hand provide a more accurate way to size a boiler. They involve measuring the amount of heat that is lost through the walls, roof, and windows of your home as well as through doors and other gaps. There are a number of software tools and online calculators that can help you with this process but it’s also possible to do it manually using a formula that takes into account factors like the U values of the different materials and the size of the room.
Manufacturers also provide guidelines for sizing their boilers based on the size of your home the number of radiators and the hot water demand. These guidelines can be a useful starting point but it’s important to remember that they are designed to promote their own products and may not be the most accurate or efficient solution for your property and your specific hot water and heating requirements.
How to Measure Heat Loss
Measuring heat loss is an important part of sizing a boiler, there are three main factors that contribute to heat loss: insulation, air leakage, and windows. To measure the level of insulation in your home, you can check the thickness of the walls and roof as well as the quality of any insulation material that has been installed. You can also use a thermal imaging camera to identify areas where heat is escaping.
Air leakage is another important factor that can contribute to heat loss this occurs when warm air escapes through gaps around doors and windows as well as through cracks in walls floors and ceilings. To measure air leakage a blower door test is used which involves sealing off your home and using a fan to create negative pressure. This will help to identify any areas where air is escaping and give you an idea of how much heat is being lost.
Finally, windows can be a significant source of heat loss particularly if they are old draughty or poorly insulated. To measure the impact of windows on heat loss you can check the U-value of the glazing material and the frame as well as the overall size and orientation of the window.
Types of Boilers and Their Sizing Considerations
There are three main types of boilers:
- Combi boilers
- system boilers
- Regular (conventional) boilers
Each type has its own strengths and weaknesses, and the size of the boiler you need will depend on a number of factors.
Combi boilers are a popular choice because they don’t require a separate hot water cylinder which needs additional space in your home. They are also more efficient because they heat water on demand rather than storing it in a hot water cylinder which will lose heat over time. But combi boilers have a limited output and may struggle to cope with high demand for simultaneous hot water from different outlets .
They’re typically suited to smaller homes with one or two bathrooms and if you have a larger property with multiple bathrooms you may need to consider a different type of boiler. When sizing a combi boiler you’ll need to take into account the number of radiators you have and the hot water demand. As a rule of thumb, a small combi boiler with an output of around 24kW should be sufficient for a small property with one bathroom, while a larger property with multiple bathrooms may require an output of 30kW or more.
Combi boiler recommended sizes: Rule of thumb*
| Number of bedrooms | Total No. of radiators | Number of Bathrooms | Recommended Size |
| 1-2 | 0-10 | 1 | 24 Kw |
| 3-4 | 10+ | 1-2 | 28-34 Kw |
| 4+ | 20+ | 3+ | 35-43 Kw** |
** Changing to a system boiler and unvented cylinder maybe a more suitable option
System boilers are an option that can be suitable for larger properties, they have a hot water storage cylinder but this is usually smaller than the one used with a regular boiler. This means system boilers are more space saving than a regular boiler but still provide enough hot water to meet the needs of a larger household. System boilers are also easy to install and maintain making them a popular choice for property owners. When sizing a system boiler you’ll need to take into account the size of your property, the number of radiators, and the hot water demand. A system boiler with an output of around 18-24kW should be sufficient for a smaller property while a larger property may require an output of 30kW or more.
Regular boilers also known as traditional or conventional boilers are typically used in older properties. They require a hot water storage cylinder, a Feed and Expansion tank (F&E) and a cold water tank which can take up a lot of space. They can be very reliable due to their simplicity and can provide a high level of hot water and heating output. It is important to note a lot of the components of a regular boiler are located externally and whilst these components can be easier to service and repair, they are not individually covered by the manufacturers warranties.. When sizing a regular boiler, you’ll need to take into account the size of your property, the number of radiators, and the hot water demand. A regular boiler with an output of around 12-15kW should be sufficient for a smaller property, while a larger property may require an output of 18kW or more.
System and regular boiler recommended sizes: Rule of thumb*
| Number of bedrooms | Total No. of radiators | Number of Bathrooms | Recommended Size |
| 1-2 | 0-10 | 1 | 9-15 Kw |
| 3-4 | 10+ | 1-2 | 18-26 Kw |
| 4+ | 20+ | 3+ | 27-40 Kw |
* Please bear in mind that these are only general guidelines for what size boiler would fit within specific properties. Before making any decisions, we always recommend consulting with a professional heating engineer.. Why not get in touch with our team and they will be able to give you a free no-obligation quote after collecting the right information from your property.
Considering Fuel Efficiency when sizing your boiler
When sizing a boiler, fuel efficiency is an important factor to consider, different fuels have varying efficiency ratings which can influence the size of the boiler required. Gas boilers for example are typically more efficient than oil boilers, so you may be able to select a smaller boiler if you go with gas. Similarly, biomass boilers can be very efficient but they require a lot of space for fuel storage, which must be considered when sizing the boiler.
It’s also worth noting that while more expensive to install energy-efficient boilers can save you money in the long run by lowering your energy bills. Look for a boiler with a high efficiency rating (this is expressed as a percentage) and the higher the percentage the more efficient the boiler. Most boilers have an efficiency rating of around 90%, which means that only 10% of the energy is wasted.
Boiler Sizing Case Study and efficiency, avoid oversizing
In the retrofit heating industry (replacing or upgrading systems in existing properties) it is common practice for heating engineers to replace boilers with like for like sizes and output. Whilst this is fairly common it isn’t always the best practice to increase efficiency or to lower running costs and carbon footprint.
Mrs. Smith lives in a three-bedroom semi-detached house which she shares with her husband and two children. Her current boiler is over 15 years old and has become inefficient, resulting in higher energy bills. The boiler is a 30KW combi boiler and she likes the on demand hot water feature. She is now considering replacing it with a newer, more efficient model that would help her save money and reduce her carbon footprint.
Mrs. Smith’s house has 9 radiators, no thermostatic radiator valves, no room thermostat, one bathroom and 50mm thickness of loft insulation. She is looking for a boiler that would provide sufficient heating and hot water to meet her family’s needs. To get expert advice on the right boiler size she has contacted two local installers, who arrange a visit to her property.
Installer A carries out a quick inspection on the boiler and recommends a like for like replacement with a 30KW combi boiler and a room thermostat to centrally control the heating.
Installer B carries out a more detailed site survey and notes down the number of radiators, bathrooms and even looks through to the loft to take a note of the insulation level. After the survey Installer B recommends downsizing the combi boiler to a 24KW model. This type of boiler provides both heating and hot water on demand and is suitable for properties with one bathroom and up to 10 radiators. He also recommends installing TRVs, a room thermostat and increasing the insulation in the loft to 350mm. This would help retain heat in the house and further increase the efficiency of the new boiler.
Installer B also notices that there is no magnetic filter fitted onto the system and heating system itself has never been drained or serviced, therefore he recommends a full power flush to remove debris and blockages form the system and recommends fitting a magnetic filter to catch future build up within the heating system. A clear and free flowing central heating system can work much more efficiently and compared to a system with blocked pipework and radiators, can have a significant impact on running costs.
Whilst there is a small saving to be made from downsizing (or correctly sizing) the boiler, the quote provided by installer B is likely to be a few hundred pounds more expensive than the quote from installer A. It however would provide a much more efficient system that will not only significantly save on running costs and reduce heating bills but will also prolong the life of the boiler to reduce future repair costs.
How to Choose a Boiler Installer
When choosing a boiler installer do your homework, look for a reputable company with positive reviews and ask for references if possible. Also check that the installer is Gas Safe registered, this is a legal requirement in the UK for anyone performing gas work. You should also obtain quotes from several installers in order to compare prices and services.
When selecting an installer don’t simply go with the cheapest option. A reputable installer will provide a detailed quote that includes all necessary work such as boiler installation, commissioning, and disposal of the old boiler as well supplementary works as detailed above. In addition to registering the boiler and the manufacturer’s warranty, they should also provide a guarantee for their work giving you peace of mind in the event that something goes wrong.
Final Thoughts
Choosing the right boiler size is critical for keeping your home comfortable and your energy bills under control. To determine the proper size, first select the appropriate type of boiler for your requirements, whether combi, system, or regular. Then consider your property’s size, including the number of radiators, bedrooms, and bathrooms. Combi boilers have a higher power output, making them suitable for the majority of homes. Seek professional assistance to ensure you select the correct size boiler for your specific needs.