Boiler Size Guide

Determining which size boiler you need can be complicated. The first you will need to consider is how much heat and hot water your household requires. This will help you determine the correct boiler size and minimise the stress of getting a new boiler installed. At TMT Boilers, our heating engineers are highly trained and experienced in all aspects of boiler installation and boiler repairs.

Size of boiler installed

4 Things to Consider Before Choosing A New Boiler

It is important to choose the correct size of the boiler so you can have hot water and heating at your disposal, which will avoid high energy costs. Here are three things you need to consider before choosing a new boiler:

  1. The space available
  2. To keep your existing boiler or to change or upgrade it
  3. How much hot water do you need to heat your home (depending on how many rooms you have)
  4. Level of insulation and airtightness

Calculating Your Boiler Size

If you have a system or regular boiler, the size will depend on the heating requirements in your household. The boiler power is measured in kilowatts, which means the higher the kilowatt the greater the capacity of the boiler to meet your heating requirements. For Combination boilers (known as Combi), the size is larger as it includes hot water production. The heating output is usually around 18kW for combi boilers and 24kW for hot water. However, the vast majority of houses in the UK have a heat requirement of under 10kW. For heat-only and system boilers this is simple to accommodate as smaller boilers are obtainable. As a guide try measuring by 1.5kW per radiator in your home then add 3kW for your water cylinder.

If this appears daunting to you, make sure you talk to your heating engineer about this so they can work this out for you and make the right decision for you and your home. You can book a consultation with a Gas Safe registered engineer so that they can measure and install your boiler. This will ensure that you have the correct heating system for your needs and that it has been installed correctly.

System Boiler Sizing

Similar to heat-only boilers, the greatest capacity to install a boiler should be no more than 18kW for the majority of households. However, you must put in the hot water requirements for your cylinder. Even though your house might only need 6-8kW of heating, it might still require a larger output for hot water. A heating engineer can carry out a heat loss survey to determine the exact figures at peak times.

Heat Only Sizing

Realistically, the sizes of boilers are oversized for domestic properties. For the majority of households, a 6-8kW boiler will suffice, as you can get a rough idea of the sizing using the age and size of your property and insulation levels. However, a heating engineer can make exceptions.

Combi Boiler Sizing

Combi boilers, also referred to as combination boilers are the go-to choice for most people in the UK due to them being highly efficient and can reduce your heating bills. The size of your combi boiler is determined by the size of your family, the demands of hot water, and the number of bathrooms in your home. The average household has a maximum requirement of 6-8kW on a cold day. Only a few properties exceed 10kW. Remember that the kW outputs for combi boilers appear larger because they are measured for hot water. The correct sizing is crucial with this type of system.

Get Professional Advice On Boiler Sizing

Other factors can affect the boiler’s size, which makes the process rather complicated. To ensure you have the best-sized and correctly suited boiler for your home, contact one of our Gas Safe Registered engineers. You can also get in touch with us if you require any boiler repairs or servicing.

Wall mounted generic water boiler with LCD screens.

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