When it’s time to replace your old boiler, there are a few questions that come up. One of the most important to answer is – ‘what size boiler do I need to heat my home?’ or ‘what kW output does my boiler need?’. As with many important questions, the answer depends on a few different factors – which we will look at in this blog.
Making sure you have the correct size boiler fitted will ensure your home is properly heated and provides adequate hot water for your needs.
A modern combi boiler will last up to 15 years, so it’s important you choose the right size boiler for your property.
How is boiler size measured?
Boiler size is measured in kilowatts (kW), boiler size isn’t referring to the physical dimensions of the boiler. A kilowatt is a unit that is used to measure how much energy is output by the boiler in the form of heat. As a general rule, the more heat and hot water you require for your home, the higher the kW boiler you will need.
Should I oversize my boiler?
In the past, the tendency was for heating engineers to advise customers to choose a higher kW boiler than they needed to compensate for heat loss, as insulation in older homes wasn’t as efficient. Due to the increased efficiency in modern-day boilers and insulation, this is no longer advised as you would be wasting energy
Sizing for hot water
The hot water flow rate of a combi boiler is expressed as ‘litres per minute’ (LPM), this tells you how quickly a combi boiler can heat hot water. Combi boilers can vary from as little as 9 LPM to as much as 25 LPM flow rate.
The majority of UK households have a combi boiler installed as they provide heating and hot water without the need for an additional storage tank to heat water. If, however, you have more than one shower or bath running at the same time you may wish to consider a conventional or system boiler, which stores hot water in a tank rather than heating the water as it is required.
The higher the flow rate you require, for filling baths more quickly or to have a more powerful shower, the higher the output combi-boiler you will need. System and conventional boilers are more efficient for larger homes but they are more expensive to install and require the storage space for a separate hot water storage tank.
Number of radiators
The number of radiators you have in your property will have a direct impact on which kW boiler you will require to heat them all and ensure your home is the right temperature.
A small flat with only a few radiators will only require a relatively low kW boiler (12-24 kW), whereas an average 3-4 bedroom house with around ten radiators would require a medium size combi (24-30kW). If you have a large house with as many as twenty radiators, a large combi boiler (35-42 kW) may be needed to provide sufficient heat. With a house of this size however, it would be more likely that you would have a system or conventional boiler in place.
How well insulated your home is will also have an impact on the size of boiler you need to heat your home. If you have poor loft/cavity insulation and single glazed windows then you would need a more powerful boiler to compensate for the heat loss in your property.
Newer build homes will be more efficient in terms of insulation and therefore require less power to heat the home sufficiently.
We have created a table to help you estimate what size kW boiler would be best suited to your home, this is intended as a rough guide, speak to a heating engineer for a more complete estimation of boiler size requirements
Combi Boiler kW Size Guide
System Boiler kW Size Guide
Different fuels vary in terms of efficiency and less efficient fuel types will require a more powerful boiler. The most efficient fuel type for boilers in the U.K is gas, however, not all houses in the U.K are connected to the mains gas supply with more rural areas choosing LPG or oil.
Older homes may not have the same water pressure as newer homes and as such may require a different boiler. Combi boilers must have enough power to heat the water passing through them if the water pressure is high the kW output needs to be higher to match.
Modern combi boilers are usually built to be compatible with alternative energy sources. If your boiler is utilising energy from alternative energy then it won’t need to be as powerful.
In the same way a car requires a service to keep it running well, a boiler is much the same. It will perform at it’s most efficient when it’s serviced regularly and kept in top condition.
Hopefully, you have more of an idea of the factors involved in deciding what size boiler you require for your home to ensure it is warm and fulfils all your heating and hot water requirements.
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