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Sizing The Ground Loops For a Ground Source Heat Pump

A ground loop is the collector pipe laid in the ground which has has fluid pumped through it to absorb the heat from the surrounding earth. How much of this ground loop a given system requires depends on two factors:

  1. Heat demand of the building
  2. Ground type

Heat Demand of the Building

Let us first consider the heat demand of the building. This is assessed by one of our geo-technicians either on site or from detailed architectural drawings. During a 'heat loss survey' our technicians measure every heat loss element of the building. This includes all openings such as windows and doors, external walls, room dimensions and neighbouring room temperatures. Insulation levels are also recorded as these make a large difference to the heat losses of a building.

Using this information we are able to calculate the property's peak load using our modelling software. We define the peak load of the building as energy required to heat it to British Heating Standards (or specified desired design temperatures) during peak winter outside temperatures. We obtain our external temperature information from the MET geographical weather database. Heat pumps are sensitive to the external temperature so will never heat the building more than necessary. This peak heat load informs the size of the heat pump. A heat pump matches the heat losses of a building, heating it at a constant temperature as opposed to the bursting heating effect felt by a traditional fossil fuel boiler.

To give you a sense of heat loads, an average 3-bed domestic property built in the 1990s-2000s will have a heat load of circa 11kW. The older or larger a building is, the higher the heat demand. Our projects range from well insulated bungalows with a heat load of 3kW to large commercial properties with a heat load of 180kW.

Ground Type

We now know how much energy we wish to extract from the ground. The next factor to consider is the thermal properties of the ground itself.

Different lithologies have different thermal conductivities, either aiding or hindering the absorption, retention and transfer of heat. For instance, limestone is a better conductor than sandstone. The presence of water from a nearby aquifer or water table is also an important factor as water is an excellent conductor thus will improve the thermogeological conditions and ensure movement of heat through the site.

If you are using horizontal ground collectors there are seasonal variations to account for. Vertically laid ground collectors (boreholes) are not influenced by seasons and are in contact with more compacted, saturated rock so will present the most efficient systems.

Let us take an example of a 12kW 4-bed detached house using trenches. If this house is situated on clays it will require circa 700m of pipe work. The best way to install this 700m would be in 7 x 50m trenches (as collector pipe is laid down either side of each trench and looped back, 100m of pipe fits in a 50m trench). the trenches must be 1m apart to avoid thermal interference. As a result, this system will require an area of 15 x 50m.

Our ground loop modelling software considers the above factors as well as many additional components such as pipe spacing, thermal conductivity of fill and thermal borehole resistance. This allows us to determine the exact length of ground loop required in order to run the most efficient systems and avoid under/over sizing issues which have been problematic for GSHPs in the past.

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