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. Typically consisting of coiled pipe, collectors of this nature are referred to as ‘slinkies’.

Collector options

Slinky Pipe

The favoured ground collector by Kensa, this pipe is coiled and then installed horizontally or on its vertical edge to provide maximum heat extraction from the minimum space available; it’s coiled design gives this configuration of pipe the name ‘slinky’.

The slinky pipe is 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 returfed, and the slinky coils extract heat energy from the earth all year round, for a lifetime of up to 100 years.

Every 1m of slinky trench contains 5m of pipe, which reduces trenching costs and makes the most of the available energy in every metre of ground.

For every 10m of slinky pipe 1kW of energy can be absorbed from the ground.

Straight Pipe

The same pipe as used by slinkies, but not coiled. This method requires more trenches and therefore will increase groundwork costs.

Slinky options


A horizontal slinky laid in a 1.2m deep by 1.2m wide trench – generally the most favoured option.


A vertical slinky laid in a narrow 2m deep trench by 300-400mm wide trench – care needs to be taken when installing vertical trenches to ensure the coiled loops are not crushed. Straight pipe installation can require up to 5 times the digging as that for slinkies.


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.

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.

Related Content

Videos: How to Unroll a Slinky Pipe

Watch as Darren Veal, Installation & Commissioning Engineer at Kensa Heat Pumps, explains the correct way to unroll coiled slinky pipe for a ground source heat pump installation.

Manuals: Slinky Unrolling Guide (Manual) V1.0

Slinky Unrolling Guidance Sheet Manual Version 1.0

Information Sheets: Slinky Manifolds (FS) V1

Slinky Manifolds Factsheet Version 1

Information Sheets: Ground Loop Installation (FS) V6

Ground Loop Installation Fact Sheet Version 6

Pre-coiled Slinky Pipe

  Kensa Heat Pumps manufactures three pre-coiled lengths of slinkies in 30m, 40m and 50m (indicated by a coloured cable tie) for supply with all Kensa ground source heat pumps; the length of slinky specified is the trench length that the slinky fits in when coiled, and not the straight ‘unrolled’ length of pipe. Each…

Case Studies: Woodcott, Warwickshire

Case study: Woodcott, Warwickshire The ground source heat pump heating Alistair Mackintosh’s luxurious modern new-build property is one of Kensa’s largest domestic installations to date.