Hey there! When it comes to boilers, there are three main types: combi, system, and conventional. But what’s really important is finding the right size for your home, not the physical size, but the amount of heat it can produce for your heating and hot water.

So, how do you figure out the perfect size for your boiler? Well, it all depends on your house and your heating needs. If you live in a cosy bungalow by yourself, you won’t need a powerhouse boiler like a family living in a five-bedroom house with three showers.

But don’t make the mistake of going too big or too small. An oversized boiler will not only cost you more upfront but also run inefficiently. On the other hand, an undersized boiler will leave you with tepid radiators and not enough hot water.

That’s why it’s essential to calculate the right boiler size for your home, taking into account your current needs and any changes you foresee, like adding more occupants or a new bathroom.

Now, let’s talk about power input and output. When you look at a boiler’s specs, you’ll see two power ratings: input and output. Input tells you how much energy the boiler needs to work, while output indicates how much of that energy is turned into usable heat. Efficiency is the percentage of output from the input, and most modern boilers are around 90% efficient.

But here’s the good news: you don’t have to crunch numbers yourself. We’ve done all the hard work for you.

Now, let’s dive into the types of boilers:

  • Combi boilers: Perfect for smaller households, as they heat water instantly. Good water pressure is essential.
  • System boilers: These store hot water in a cylinder, allowing multiple taps and showers to run simultaneously. They work well with decent water pressure.
  • Heat-only boilers: They also store hot water but rely on gravity feed. Ideal for areas with low water pressure and homes with multiple occupants.

To determine the right boiler size, start by counting your radiators. More radiators mean you’ll need a more powerful boiler to keep them all toasty. As a rough guide, here’s what you’d need for combi, system, or heat-only boilers based on the number of radiators.

For combi boilers:

Up to 10 radiators: 24–27 kW

10 to 15 radiators: 28–34 kW

15 to 20 radiators: 32–40 kW

For system or heat-only boilers:

Up to 10 radiators: 12–15 kW

10 to 15 radiators: 18–24 kW

15 to 20 radiators: 30–40 kW

Remember, combi boilers provide instant hot water, while system and heat-only boilers take a bit more time.

Now, let’s look at some examples:

Single-occupant flat: For a 1-bedroom flat with 1 shower, a 26 kW Vogue Max Combi would do the trick.

Average family: If you have a 3-bedroom semi-detached house with 1 bathtub and 1 shower, consider a 24 kW Logic Max System.

Large family: Living in a 5-bedroom detached house with 2 bathtubs and 3 showers? Go big with a 32 kW Vogue Max System.

Lastly, while our recommendations are a great starting point, it’s always a good idea to consult a Gas Safe registered engineer. They can provide a precise recommendation based on your home’s specifics, ensuring optimal efficiency and effectiveness.

So, don’t just stick with the boiler you have; find the perfect fit for your home and stay cosy all year round!

If you need further assistance, check out our power calculator to make your decision even easier.


The size of the boiler you need for a 3-bedroom house can vary depending on factors like the number of radiators, bathrooms, and your hot water usage. As a general guideline, for a 3-bedroom house with one bathroom, a combi boiler with an output of around 24-27 kW should be sufficient. However, if your house has more radiators or higher hot water demand, you might need a larger boiler, such as a 28-34 kW model.

A 30 kW boiler can typically heat a medium-sized house with around 10-15 radiators. However, it’s essential to consider factors like insulation, the number of bathrooms, and hot water usage. If the house is well-insulated and has only one bathroom, a 30 kW boiler should provide adequate heating and hot water.

The size of a combi boiler is often determined by the number of radiators and the hot water demand. Here’s a rough guide for combi boiler sizes based on the number of radiators:

Up to 10 radiators: 24–27 kW

10 to 15 radiators: 28–34 kW

15 to 20 radiators: 32–40 kW

Keep in mind that these are approximate guidelines, and it’s essential to consider other factors like hot water demand and the number of bathrooms when selecting the right combi boiler size for your specific home.

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