What is a slinky?

A slinky is a network of coiled pipe buried in the ground in horizontal or vertical trenches to a depth of 1 – 2m to form a ground collector for the ground source heat pump.

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.


Watch our video 'How ground source heat pumps work'

Three slinky steps:


1 day

The sizing of the slinky should be performed by a Kensa Approved Installer, MCS accredited installer, or by Kensa direct. Sizing of the slinky will match the heat needs of the property (not the heat pump) – careful sizing is required to ensure too much heat is not extracted, causing freezing, or too little heat extracted, causing an underperforming ground source heat pump system and higher bills.


2 – 3 days

The digging of slinky trenches will need to be performed by a suitable groundworker or persons with experience and access to digging machinery.


1 – 2 days

Slinkies can be laid by all persons and do not require any specialist training, however if training is needed take a look at Kensa’s guide to slinky installation using the link below.


Slinky trench choices


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 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.

How slinkies are made

Kensa manufactures 3 different coiled lengths of slinkies; 30m, 40m or 50m.  Please note the length specified is the trench length that the slinky fits in when coiled, and not the straight ‘unrolled’ length of pipe used for the slinky.

  • 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 our premises, and this is indicated by the coloured cable tie on the return pipe.

Heads up

The slinkies are supplied with a header pipe, which is usually about 25 metres 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 1.2 m 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.