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Ground Source Heat Pumps

Ground sourced heat pumps (GSHP) are often the more expensive renewable heating option but they bring the highly desirable benefits of being the most reliable and lowest maintenance solution.

This is demonstrated by the 2.5 million worldwide installations, with many now over 20 years old. Carbon Legacy’s involvement goes back to 1999 when David Hill, Managing Director, was the developer of the seventh UK GSHP installation for a new office building in Nottingham. This was a very successful world first that provided commercial heating and cooling.

Having been involved in over 150 domestic and commercial installations and with the benefit of 8 years’ worth of independent monitoring, we have the confidence to know what a well designed and installed GSHP can consistently deliver. Points to consider when thinking about a GSHP are:

Key Points
  • Do not be misled by manufacturer’s claims on performance. What you need to know is the yearly performance average or average co-efficient of performance (seasonally adjusted COP) for a building similar to yours.
  • An effective system can have an average COP of 3.5 and above. This means that for every one kWh of electricity used by the heat pump you will get 3.5kwh of useful heat into your building or hot water system.
  • There are 3 basic types of ground collectors; closed loop boreholes, closed loop trench slinkies and “open” ground/river/lake water sources. Closed loop either in bore holes or trenches are the most commonly used solution. Which will suite your situation will depend on site constraints and your budget. Open water solutions using river water or water extracted from boreholes can have very high efficiencies of over 5 COP, however they are dependent on Environment Agency approval and require a lot of up front and expensive investigation and tests therefore are usually only considered for larger commercial installations.
  • In-trench slinkies (buried water pipe coils) are by far the cheapest solution and prove to be very effective where there are large areas of open space/land available. This can be in large gardens, adjacent fields or even under car parks/playgrounds.
  • Where we can advise customers to build very low energy buildings we have successfully installed “in-foundation trench slinky ground collectors”.
  • Where existing buildings have little available external space then vertical bore holes are used. These provide a problem-free, efficient solution but do have a high initial cost. The other advantage is that almost any ground type can be utilised from hard to soft rocks even in suburban built up areas.
  • To work at their best efficiency heat pumps like to be connected to lower temperature heat emitters such as the Dimplex range of low temperature fan assisted radiators (SmartRads) or under floor heating. Removing an old boiler and directly replacing it with a heat pump will not work. Radiator sizing is absolutely crucial in the overall system design, and just connecting a heatpump to existing radiators without them being tested and sized accordingly, will make the entire system under-perform, resulting in nothing but headaches and frustration.
  • Some manufacturers will tell you that their high temperature heat pump (65 degrees output) can use your existing radiators; which it can, but only at the expense of efficiency and increased running costs (reduced COPs of 2-2.5) which then make the financial case very weak or even cost you more than your old system.
Horizontal Ground Collectors

If a large enough land area is available,horizontal ground collectors provide an effective method of extracting heat from the ground. The pipework is buried at a depth of approximately 1.2m and spaced 0.75m apart.

Vertical boreholes

If land space is limited the ground collectors can be installed vertically in a borehole, drilled up to 100m deep in the ground. Multiple boreholes are commonly used in large installations where very high levels of heat extraction are required.