How to find out what size boiler you need for your home

3 June 2024

When you’re considering a boiler installation, you’ll notice that there’s quite a wide selection of options to choose from, not least the size and type of boiler. It might seem obvious that a bigger home needs a more powerful boiler, and that is generally true. But other things factor into the equation, too. The type of boiler, how many people live in the home, the number of radiators, baths and showers – they all count.

The important thing is to get the right size – not too big, and not too small. Going either way can cost you in terms of running efficiency, as well as the price of the unit itself. So although there’s always going to be a little wiggle room, it’s useful to get it right first time.

In reality, you shouldn’t have to worry about the maths too much. When we come to assess your property ahead of a boiler installation, we’ll know exactly what physical features to look for, and what questions to ask. But to give you a heads up, these are the main things we’re looking for.

Number of radiators

The number of radiators in your home is probably the most important  indicator of the size of the boiler you need. That’s because they’re used the most often, and will often all be used simultaneously. Generally, the more radiators you have, the higher the boiler output required to heat them. Here’s a rough idea:

  • Up to 10 radiators: 18–24 kW
  • 10 to 15 radiators: 24–30 kW
  • 15 or more radiators: 30–35 kW or more

Type of boiler

The choice between a combi, system or traditional boiler impacts the required size:

  • Combi boilers are compact units that do not use external tanks and provide heat and direct hot water on demand, making them ideal for small to medium-sized homes with one bathroom (or maybe two).
  • System boilers require a cylinder for storing hot water. They are more suitable for homes with multiple bathrooms, where water can be used simultaneously without a significant drop in pressure.
  • Traditional boilers, also known as conventional or regular boilers, need both a cylinder and a tank. They can deliver water to several outlets at once without losing pressure but require space for a separate tank.

Number of bathrooms and showers

The number of bathrooms and showers in your home plays a significant role in determining the appropriate size and type of boiler. Here’s how bathrooms and showers affect your boiler choice:

Water flow demand

Homes with multiple bathrooms that are used simultaneously require boilers that can handle high water flow demands without a drop in water pressure or temperature. For instance, if two showers are often running at the same time each morning, the boiler and its accompanying systems (like water cylinders) need to sustain an adequate flow rate to supply all outlets efficiently.

Type of showers

The type of showers installed can impact the hot water demand significantly. Electric showers, for instance, heat water directly from the mains, so don’t impact the boiler’s workload at all. In contrast, mixer and power showers draw hot water directly from the boiler-fed system, increasing the demand on the boiler to heat and pump water.

Need for instantaneous hot water

Combi boilers provide hot water on demand and eliminate the need for a storage tank. However, their ability to supply hot water is limited by how much water can be heated instantaneously. This makes combi boilers more suited to homes with fewer bathrooms or where boiler-fed showers aren’t used simultaneously. 

Storage systems for hot water

For homes with multiple bathrooms, a system boiler with a hot water cylinder or a traditional boiler can be more appropriate. These boilers heat water and store it in a cylinder, making it available from multiple outlets at once without loss of pressure or temperature. 

The size of the hot water cylinder should be chosen based on the number of bathrooms and the likelihood of simultaneous use. The hot water stored in the cylinder will run out and need to be topped up with new hot water from the boiler. If you have high demand and simultaneous use, you’ll need a bigger cylinder. However, the boiler itself might not need to be much bigger if you manage your usage carefully, for example heating up overnight and keeping the water hot with an immersion heater.

Peak usage times

Understanding peak hot water usage times in your household can also inform your choice. If several family members need to shower within the same hour, a larger system or traditional boiler with a larger cylinder is crucial to prevent running out of hot water. Combi boilers are not usually the best option for large households unless the occupants spread out their hot water demand.

Hot water usage

Bathtubs, frequent showers, and appliances like dishwashers (and washing machines that use hot water), all determine your system’s capacity. For heavy usage, a system or traditional boiler with a larger capacity hot water cylinder may be necessary.

Home size and number of rooms

The larger your home, the larger the capacity your boiler will need to have to heat it effectively. As a rule of thumb, you should consider higher kW output as the number of rooms increases. If your boiler needs to feed a cylinder, radiators or showers high up, such as in a third floor or loft, you might need a more powerful boiler.

Insulation quality and climate

Well-insulated homes retain heat better, so may require less heating power compared to poorly insulated ones. Homes with upgraded windows, doors and insulation in walls, roofs and floors will maintain heat more effectively, potentially reducing the size of the boiler needed. The local climate also plays a part. Colder areas might require more powerful boilers to achieve comfortable living conditions.

Talk to the experts

Selecting the right boiler size involves a detailed understanding of your home’s heating and hot water needs based on all these factors. It’s always worthwhile to hire a heating professional who can assess your specific circumstances and provide recommendations tailored to your home’s requirements. Getting it right first time can save you thousands of pounds in efficiency and replacement costs.