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How Many Radiators Can A 24kW Boiler Run?


A 24kW combi boiler can run ten household radiators. On the other hand, a 24kW system boiler can run up to 15 radiators. As a rule of thumb, experts calculated that one radiator requires about 1.5kWs. Nonetheless, the answer isn’t that plain and simple. There are a lot of factors you need to consider when sizing your boiler.

As you know, there are mainly two types of boilers in the market, combination (combi) boilers and system boilers. Unknown to many, these two boilers have different power outputs dictated by how they operate.

Are you interested in learning more about boiler sizes and how to find the ideal boiler size for your home? Continue reading.

What is boiler size?

“Boiler size” doesn’t refer to the unit’s physical dimensions. Instead, it refers to the output power your boiler can discharge in one second. 1kW accounts for 1000 watts. The “bigger” the boiler, the more power it can produce. Therefore, a 24kW boiler means the boiler can discharge 24kWs in one second.

Generally, household boilers come in three output ranges depending on the property’s size and hot water demand. These are:

  • 24 to 27kW boilers for homes with up to 10 radiators.
  • 28 to 34kW boilers for homes with up to 15 radiators.
  • 35 to 40kW boilers for homes with up to 20 radiators.

Output difference between combi boilers and system boilers

Combination, aka combi boilers, are currently the most trending boilers in the market. Indeed, they are the most efficient boilers. However, most combi boilers are best suited for small houses (One person flats), not big households with several members.

On the other hand, system (heat only) boilers can comfortably supply hot water in large households (3 bedroom houses).


In large households, combi boilers will work harder than system boilers. This is because they produce hot water on demand. Therefore, it will turn on every time an individual needs hot water. On the other hand, system boilers have storage tanks for storing hot water. Thus, they only turn on usually around two times a day.

Will a “larger” boiler cost more to operate?

Most people assume the “larger” the boiler, the more money they’ll spend on heating expenses. That used to be the case with older boilers, but not any more. Modern boilers (both combi and system) are optimised to save electricity bills.

Of course, you’ll end up paying more if the house has more rooms and several occupants, compared to a house with fewer rooms and lower demand for hot water.

What to consider when calculating the boiler size for my house?

There are three main factors to consider when determining your home’s boiler size. These include:

  • The number of radiators.
  • The number of bathrooms.
  • The number of people living in the house.

Indeed, there are other important factors to consider, such as your home’s insulation and future renovations, among others. Boiler size calculators are great tools for determining the appropriate boiler size for your home. Nonetheless, it’s always best to consult a Gas Safe Registered engineer, and have him inspect your property for a more accurate “prescription.”

What is the best boiler size for a one-bedroom flat?

From what we’ve just described, a 24kW combi boiler would work fine for a one-bedroom flat with one or two showers and not more than ten radiators. On top of that, combi boilers are perfect for small households since they are compact (Don’t require water storage tanks).

What is the best boiler size for a three-bedroom house?

Three-bedroom houses are the most popular for average-sized households. A 24kW system boiler is perfect for a house with three bedrooms, several baths and showers, and less than ten radiators. Because of the storage tank, the boiler can supply hot water to several people simultaneously.

What if my boiler is too big?

Most people assume it’s okay and even advantageous to have a boiler rating that is well above their requirements. That’s a myth. If you’re not careful, you might end up spending more on heating costs. Additionally, the performance will be poor and unreliable.

In the worst-case scenario, the excess heat produced by the boiler might end up damaging your central heating system.

Here is what happens in the case of an “oversized” boiler:

After heating water in the morning, the water will be directed into storage tanks. Therefore, it will gradually cool as the day goes by. When you come back home in the evening and want to take a shower, your oversized boiler will end up using more energy than is necessary to heat the same volume of water. You end up spending more.

What if my boiler is too small?

An “undersized” boiler won’t be able to keep up with your heating requirements. The demand will overwhelm the boiler leading to a hot water shortage. It can be very frustrating if the boiler fails during the harsh winter. You’ll have to suffer cold showers and a freezing house.

Is it safe to add more radiators to my boiler?

Yes, you can add at most two radiators to your current boiler without experiencing any performance problems. Nonetheless, we advise you to first confirm with your boiler’s manufacturer. The user’s guide might offer this information.


Indeed, a 24kW combi boiler can handle only up to 10 radiators because it produces hot water on demand, and therefore works more frequently (harder) than a system boiler. On the other hand, a 24kW system boiler can handle up to 15 radiators because it has storage tanks to store hot water for future use (It doesn’t produce hot water on demand).

Are you still having doubts? Why not call a Gas Engineer and allow him to help you determine the best boiler size for your household?

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