Our winter series on ways to keep homes warm without killing the planet continues with an assessment of ground and air source heat pumps. PAD Studio shares the fruits of its experience
Both ground and air source heat pumps (GSHP & ASHP) employ heat exchange as the means of creating energy; the ASHP uses energy from the air, and the GSHP uses solar energy stored below the ground.
In the cold damp UK climate, an ASHP can suffer energy losses in the winter as it struggles to extract heat from the cold air and prevent the heat pump from icing up, whereas the temperature below ground is relatively stable and more efficient.
Depending on the energy efficiency of the building (which determines the amount of input heat required) the energy bills of a house that uses a GSHP should be 70-80% lower than one heated traditionally. Alongside the monetary saving in fuel bills is a vital reduction in carbon emissions and, as the energy is generated on site transference losses at source are minimised.
The GSHP is more expensive in capital costs but is almost maintenance free, has no more noise pollution than a domestic washing machine – and when we were installing it at our project for a down-sizing couple with green ambitions, The Gardener’s Cottage, the government’s Renewable Heat Incentive repaid the capital cost of the installation and heat pump over a period of 10 years.
Vertical GSHP systems sit in a vertical bore hole near the house. Boreholes can go down 80m-120m depending on soil conditions but the disruption to the site is minimal (which makes them an excellent solution for existing buildings). Conversely, horizontal GSHP systems require a series of trenches which are only around 1.2m deep – these are cheaper to install and work very well if you have a lot of level land, but they do require a lot of space as the trenches need to be spaced out to ensure maximum efficiency.
On average the installation cost of a vertical GHSP is around £25-50,000 and a horizontal system around £15-25,000. An ASHP of the same capacity is around £6-10K. All heat pumps still require electricity to run them (which, like those at Gardener’s Cottage, can be generated via photovoltaic panels). For every unit of energy inputted, around four is generated via the heat pump (this is called the coefficient of performance; CoP) and this will drop slightly over the lifespan of a heat pump. The CoP of a GSHP is slightly higher over its lifespan than an ASHP and in our experience over the last 10 years of using both, they tend to have a longer lifespan and fewer maintenance issues.
Wendy Perring is director, PAD Studio