For starters, boiler size doesn’t actually mean its physical size. Rather, it refers to the boiler’s output in kilowatts (kW), which measures how much energy the boiler produces in the form of heat. The ‘bigger’ the boiler, the higher the kW.
A general rule of thumb: the bigger your house and the more radiators, hot taps, and showers you have, the more powerful the boiler you’ll need – therefore a bigger boiler will be more suitable.
When choosing the correct boiler size for your home, you should consider how many bedrooms, bathrooms and radiators you have, as well as your existing boiler. You should also think about the type of boiler you want.
How many radiators, bedrooms and bathrooms are in your home, and how many people are living there? The more heat and hot water your household requires, the bigger the boiler you’ll need.
If your home is well insulated, you can get a smaller, more energy-efficient boiler – as you won’t need such a high output of heat.
Combi boilers are more suitable for smaller properties, while system boilers are better for larger ones with greater demand for hot water. Electric-only boilers are another compact and efficient option, as they only heat up water (no radiators).
Most UK homes use gas boilers (such as combi boilers), which are compact and energy efficient. If your home isn’t connected to a mains gas supply, you may need a bigger oil-fired boiler instead, or a smaller electric-only boiler.
Combi boilers are the most popular boiler type in the UK, due to their smaller physical size (no need for additional units or bits of equipment) and better energy efficiency.
As with deciding on any boiler, you’ll need to think about its size (power output in kW) and the number of radiators, bathrooms and bedrooms in your home.
Smaller houses typically require a 24-30 kW combi boiler, while bigger properties would need a larger 30-35 kW boiler.
It’ll use more power and produce excess energy, which is bad for the environment
Your energy bills will be higher, due to all the excess energy generated
It can perform poorly and break down quicker
Your radiators won’t feel warm enough
You may often run out of hot water
Your home might feel too cold in winter
If you buy a boiler that’s a much higher capacity than you need, you’ll be wasting money and energy. As well as being more expensive in the first place, an oversized boiler will cost you more in the day-to-day running of it.
You’ll be paying higher energy bills, and your boiler will be producing excess fuel that can be damaging to the environment. Not to mention, an oversized boiler will run ‘on and off’ for short periods of time, which can result in it breaking down quicker.
A boiler that’s too small will struggle meet your household’s needs. You might run out of hot water often, and find that your radiators aren’t warm enough.
For a larger four-bedroom house with more than twenty radiators, consider a combi boiler with an output between 35-43 kW.
The average three-bedroom house in the UK will typically only need a small or medium combi boiler between 29-35 kW.
It also depends on the type of property you’re living in. For example, flats or bungalows only require a boiler size on the lower end of the scale (around 29 kW), while a detached house will likely need something bigger (around 35 kW).