Understanding Slinkies & Straight Pipes for Heat Pumps

What are Slinkies & Straight Pipes?

How do slinkies or straight pipes collect heat?

Slinkies or straight pipes collect heat from surface soil, about 1.2m below the ground. Maintaining a fairly constant temperature of 8-12°C all year round, the soil gains and replenishes most of its heat from solar energy.

Soil is a reliable source for heat pumps, and it was the first heat source used to work with ground source heat pumps – hence the name ‘ground source’. Nowadays many other sources can also be utilised by Kensa ground source heat pumps, such as water and rock.

See how a heat pump works

How much space do I need for them?

The length and number of slinky pipes required will depend on the heat demand of the property. Kensa manufactures three different lengths of slinky pipe: 30m, 40m and 50m long.

As a general guide, Kensa recommends 10m of trench with slinkies per kilowatt of the heat pump size. This means a 4kW heat pump will require one 40m long trench with slinky pipes buried in the ground for space heating. If domestic hot water is required, the length of the trench will increase slightly.

Slinkies vs. space-saving ground arrays
Also known as a horizontal ground array, a slinky trench takes up much more ground area than vertical arrays like boreholes, but it’s cheaper to install because there is no need for specialist drilling equipment.

Trenches are dug in the property’s surrounding space, such as the back garden.

Compare to other ground arrays

What if I don’t have the space to install them?

Surprisingly often, calculations for a ground array will show that the available garden area isn’t quite big enough to meet the heat load. One solution for this situation is to use boreholes, but this will typically add several thousands of pounds to the overall installation cost.

An alternative is spending some, or all, of this money on the building’s insulation to reduce the heat load. By reducing the heat load, the available garden area may well become viable as the heat source. It may even be possible to specify a smaller heat pump, further offsetting the insulation costs.

By adopting this solution, the energy bills will be significantly lower, the efficiency of the system will be higher and inevitably, its carbon footprint will be smaller.

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For your peace of mind, Kensa will help assess your ground array needs and provide detailed costs. To give us an idea of your project requirements, tell us about your plans for advice and a quote.

How do slinkies or straight pipes work?

The slinkies or straight pipes are buried & backfilled

To extract heat from surface soil, straight or slinky pipework is buried in trenches. The pipe is sandwiched between fine layers of sand to ensure optimal contact with the ground and maximum heat extraction. It is then covered with soil to fill the trench, which is backfilled and returfed. Once it’s complete and the ground has recovered, you won’t even know it’s there!

The pipework is connected to the heat pump to deliver heat energy

Header pipework in another trench connects the slinky or straight pipe to the ground source heat pump via a manifold. A mixture of glycol and water is then pumped around the buried pipework, absorbing energy in the ground and transferring this energy back to the heat pump.

The ground array extracts heat for up to 100 years

The pipes extract heat energy from the earth all year round, for a lifetime of up to 100 years. Complementing the sustainability of slinkies, Kensa’s ground source heat pumps have a design life of 20 years.

What is the difference between slinky & straight pipe?

Slinky pipes

Slinkies are one of the world’s most popular ground collectors for heat pumps. They are Kensa’s favoured method of surface soil heat extraction.

A slinky is a length of coiled plastic pipe which is buried in the ground. The coiled pipe is usually installed horizontally and laid in long, narrow trenches.

Each slinky trench typically measures 1.2m wide by 1.2m deep. The trenches are spaced 5m apart to enable the natural replenishment of energy in the ground and avoid over-extraction of heat. This spacing also accommodates the excavated soil before the trenches are then backfilled.

Slinkies do not reduce the area of land required compared to using straight pipe. However, the benefit of slinkies is a significant reduction in the amount of digging required compared to using straight pipe – slinkies reduce the trench length by a factor of around 5, so are quicker and lower in cost to install.

Every 1m of a slinky trench contains around 5m of pipe, so slinkies make the most of the energy in every metre of dug ground. This is why Kensa manufactures a range of slinky pipes to complement our heat pump range and make ordering and installation even easier.

Slinky trenches for a Kensa ground source heat pump installation

Straight pipes

Straight pipes are laid in narrower trenches. Without the benefit of coils like the slinkies, straight pipes require more digging. To be precise, they require up to five times the length of digging compared to slinky pipes. Straight pipe trenches can be dug closer together than slinky trenches, but space for the spoil during excavation works will be limited.

There is no difference in the amount of ground required for slinkies or straight pipe. However, due to the additional trenching costs, straight pipe ground arrays are more expensive than slinky ground arrays.

Both straight pipe and slinky pipes utilise the same energy source, so their efficiency and performance are effectively identical.

Slinkies vs. straight pipe for heat pumps

Which is better, a slinky pipe 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 ground area you need is the same, as any ground array is sized appropriately to meet the building’s required heat load. For every 60m of pipe, approximately 1 kilowatt of energy can typically be absorbed from the ground.

Kensa supplies both types of ground array, but we usually recommend slinkies as they are more affordable and tend to have reduced installation costs.

Key things to remember when installing slinkies or straight pipes

The distance between trenches & property

When digging trenches for slinkies, a minimum distance of 5m should be maintained between trench centres. The edge of any trench should be at least 2.5m inside any property line and 1.5m away from any buildings.

Groundwork can flex around garden space

Trenches do not have to be straight so long as the 5m separation distance between them is maintained.

Ground obstacles can get in the way

If the ground contains sharp flints or large clods of clay, the trench may need to be backfilled with sand to provide close contact between the pipe and ground.

Excess rainwater can be drained

Slinky trenches can also be used as soakaways, providing the trenches are at least half backfilled with good quality spoil before pea gravel is used.

Distance between manifold & slinkies can be adjusted

The distance between the slinkies and the manifold is fixed (i.e. approximately 25m, the header length). However, distances greater than this can be achieved by unravelling the coiled section of the slinky pipe. It is important that there is a minimum separation distance of 1m between adjacent pipes for uncoiled sections. 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 to Unroll a Ground Source Heat Pump Slinky Pipe

Need help with your project?

Kensa has decades of experience when it comes to designing and installing ground arrays. For advice on your project, tell us about your plans.

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Useful features of Kensa’s slinkies

Kensa slinkies are supplied with a header pipe, which is 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.

Kensa’s slinky pipes are manufactured in lengths of 30m, 40m and 50m. All of our slinky pipes are made out of high density polyethylene to support the ground array’s typical 100-year lifetime. They are each supplied pre-coiled.