Soil: Slinkies & Straight Pipe

Ground source heat pumps work by extracting the heat from surface soil through a network of ground arrays consisting of slinky pipes buried in trenches.

Going underground

1m below ground the temperature remains a fairly constant 8-10°C all year round. This makes it an excellent renewable heat source for ground source heat pumps; indeed, surface soil was the first heat source used to work with ground source heat pumps, hence their name.

To extract the heat from surface soil, a network of ground collectors (or ground arrays), consisting of a series of pipes, is buried in the ground in trenches to a depth of 1 – 2m. Kensa favours using coils of pipe, which we refer to as ‘slinkies’.

Collector options


The difference is that coiled slinky pipe is laid in shorter, wider trenches. These will need to be spaced 5m apart to accommodate the excavated soil before the trenches are back filled.

Kensa favours slinkies, as when installed horizontally, they provide maximum heat extraction from the minimum trenching, as every 1m of slinky trench contains 5m of pipe, making the most of the available energy in every metre of ground.

Straight Pipe

Straight pipe can be laid in narrower trenches, however this means that the trenches will ultimately be longer. Trenches can be dug closer together, but space for the spoil will be limited.

Slinkies vs straight pipe

It’s an age old argument; which is better – coiled pipe (slinkies) or straight pipe?

In reality, both are equally effective and easy to install if you have the correct excavation equipment. The amount of pipe and the ground area needed are the same, as any ground array should be sized appropriately to meet the required heat load. For every 50m of pipe – 1kW of energy can be absorbed from the ground.

The pipe should be sandwiched between a fine layer of sand to ensure maximum contact and heat extraction, and then covered with soil to fill the trench. The trench is then re-turfed, and the pipes extract heat energy from the earth all year round, for a lifetime of up to 100 years.


Slinky advice

Watch Kensa’s Technical Support team demonstrate how to unroll and install a slinky.

Slinky tips

  • If the ground contains sharp flints or large clods of clay the trench may need to be back filled with sand to provide a close contact between the pipe and ground.
  • A minimum distance of 5 metres between trench centres should be maintained.  The edge of any trench should be at least 2.5 metres inside any property line and 1.5m away from any buildings.
  • Trenches do not have to be straight as long as the 5m separation distance is maintained.
  • Slinky trenches can also be used as soakaways providing that the trenches are at least half backfilled with good quality spoil before pea gravel is used.
Ground Source Piping

Heads up

Kensa slinkies are supplied with a header pipe, typically 25m long. This allows the slinky trench to be terminated a little way from the building. The header pipes from all the slinkies can join together into a single 1.2m deep header trench.

The distance between the slinkies and the manifold is fixed (i.e. approximately 25m, the header length).

Distances greater than this can be achieved by un-coiling coils in the coiled section of the slinky. It is important that there is a minimum separation distance of 1 metre between adjacent pipes for uncoiled sections.

Please note if one coil is unrolled at the front of the slinky’s coiled section then one coil needs to be uncoiled from the rear of the coiled section to provide enough return pipe.


How are slinkies made?

Kensa manufactures three different coiled lengths of slinkies; 30m, 40m or 50m (the length specified is the trench length that the slinky fits in when coiled, and not the straight ‘unrolled’ length of pipe).

  • Each slinky length is made from a straight header pipe, a coiled section (which is the energy absorbing part of the slinky) and a straight return pipe.
  • Slinkies are manufactured using black HDP (High Density Polyethylene) pipe, with a pressure rating of PE80.
  • All slinkies are leak tested by Kensa before they leave the premises, and this is indicated by the coloured cable tie on the return pipe.
Ground Source Installation Guide

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