Need some advice? - 01293 821 345 - consult@iso.co.uk

When planning a retrofit heat pump project, calculating the required size of a ground source heat pump system for your home is not as straight forward as you might think.

With ground source heat pumps, there is no ‘one size fits all’ solution. The variation in insulation levels of existing houses in the UK is huge, so calculating how much heat you need to generate for the house depends on how much heat the house loses through its floor, walls, windows and roof. This figure is represented as an average heat loss (Watts) per square meter. As new build houses are built to a minimum insulation standard, we can assume that figure is going to be somewhere around 30W/m2. However, as existing houses have been built with different insulation levels, that figure could be anywhere between 35W/m2 and 100W/m2.

For example, a 200m2 new-build property may only require a heat load of 30W/m2, 200 x 30 = 6,000 watts or 6 kilowatts (6kW). Conversely, a 200m2 Victorian home may require 65W/m2 or more, which comes to 13kW.

When retrofitting ground source heat pumps, it is tempting to estimate the heat load of the building by looking at the size of the existing boiler system. However, as fossil fuel boilers are less expensive and the installers are usually worried about a peak heating requirement of starting at 15/16°C in the morning and achieving 21°C in 20mins, the boilers are usually twice as large as they need to be. So this does not provide an accurate estimate of what the property may require to keep warm.

The heat loss per metre squared can only be determined accurately by carrying out a formal heat loss survey of the building. During this process an energy surveyor will look at the fabric of the house, the external wall area, window sizing, glazing type and air changes amongst many other variables to determine an average W/m2 figure for an individual property. Only then can the required heat pump size be accurately assessed and designed.

Why not just put a bigger one in and have done?

Heat pumps, particularly ground source heat pumps need to be accurately sized to give you the kind of efficiency that will save running costs and see you a fast return on investment. If the heat pump is too big, it will need to turn itself on and off again a lot as the thermostat calls for heat. This is when the efficiency will suffer as starting the compressor takes more energy than running it does. This ‘short cycling’ will also increase the risk of breakdown due to the stresses put upon the compressor when they start up.

Isoenergy has experience working with lots of different types of buildings and we can normally help you assess what size heat pump you require based on what we have seen before. This information is normally enough to provide you with an outline proposal including the cost benefit analysis of replacing your current heating system with a heat pump. We would then conduct a formal heat loss survey as the first stage of the installation to confirm the required heat pump size.

If you would like to know what size ground source heat pump you need for your house, get in touch on 01293 821 345.

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