What size boiler do I need and what do kW and kWh mean?

What size boiler do I need – come the colder months, lots of people will be asking themselves this question. There are also other considerations, such as what type of boiler is necessary. If you’re stumped for an answer, fear not – this page will explain exactly how boiler sizing works and what the associated terms mean.

What size boiler do I need?

The best boiler size, or to give its correct term, boiler output, depends on several factors. The main considerations are the number of radiators, the number of bathrooms, and how many people live in the home. Some of these factors will also affect the type of boiler that is suitable for the property. For example, a combi boiler probably won’t be suitable in a house with two or more bathrooms. This is because hot water is not stored in advance but heated on demand from the mains. If two people shower at the same time, the consequence of dividing the mains this way will be two equally disappointing showers.

Calculating the size

In order to determine the appropriate size of boiler for a household, a competent heating engineer will need to calculate the appropriate heat output needed in every room in the home. There are numerous factors which affect how much energy is required to heat a room to the suitable temperature. These include – but are not limited to:

  • The dimensions of the room
  • Whether or not the windows are double glazed
  • Whether or not the room faces north, and if the room is upstairs or on the ground floor. (A room on the first floor will benefit from heat rising from the room below it).
  • The purpose of the room – bedrooms may be one or two degrees cooler than the living room; bathrooms may be one or two degrees higher.

If the necessary heat output is not calculated correctly and a boiler with insufficient power is selected, the home won’t be warm enough and won’t have enough hot water to keep up with demand. A boiler with excessive power will waste energy, resulting in unnecessarily expensive heating bills and damage to the environment.

While the necessary boiler size must be calculated on a case-by-case basis, most properties fall into one of three ranges.

Boilers for 1 and 2 bedroom flats and houses

For these properties, a 24-27 kW combi boiler is adequate in most cases, assuming a property with one bathroom, 2-3 people, and up to 10 radiators.

Boilers for 3 and 4 bedroom houses

For medium to large properties with 3-4 bedrooms, one bathroom, or up to 15 radiators, a 28-34 kW boiler should be suitable. However, if there is more than one bathroom, you should consider a regular or system boiler which integrates a hot water cylinder. This will help ensure that there is enough hot water to keep up with demand.

Boilers for big houses and houses with 4+ bedrooms

For large properties such as those with 4 or more bedrooms, a 35-42 kW regular or system boiler is necessary, especially if the property has more than one bathroom.

It is important to remember that these ranges are a guideline rather than a rule. This guide assumes average size rooms and therefore average-sized radiators. The best boiler size should always be calculated by a qualified heating engineer.

What does kW mean?

Boiler output is measured in kW or kilowatts. A watt is a measurement of energy in relation to time, i.e., how much energy can be delivered per second. One watt of energy equals one joule of energy per second. At maximum output, a 24 kW boiler therefore delivers 24,000 joules of energy every second of operation.

So what does kWh mean?

The term kWh stands for kilowatt-hour. No doubt you will have seen this term on your energy bill. At first it may sound confusing to express a quantity of energy with a measurement that we normally associate with measuring time, but it’s actually quite straightforward. Here’s how it works:

  • There are 3600 seconds in an hour.
  • If a 24 kilowatt boiler running at full power delivers 24,000 joules of energy per second, that’s 86,400,000 joules of energy per hour. (3600 seconds x 24,000 joules).
  • 86,400,000 – eighty-six million, four hundred thousand – is a big number. Instead of having to write it out like that, it can simply be expressed as 24 kW/h.

So to continue with the same boiler:

  • If it was operating at full power for 2 hours, it would therefore deliver 48 kW/h of energy or 172,800,000 joules.
  • If it was operating at full power for half an hour, it would therefore deliver 12 kW/h of energy or 43,200,000 joules.

Bearing in mind that energy is measured in joules, if we assume that one hour of boiler use produces such a large figure, then try to imagine how many joules of energy your home uses every month – the number would be huge. It’s much easier for energy companies to express this figure as kilowatt hours. (And while we are at, if the average UK home uses 4000 kWh per month, then that figure would be 14,400,000,000 joules. That’s fourteen billion and four hundred million!)

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