Menu

What are Slinkies & Straight Pipes?

Buried in trenches at depths of 1.2m, slinky pipes (also known as slinkies) and straight pipes are a common form of ground array. They collect energy for a ground source heat pump, which upgrades and delivers heating and hot water to a building’s heating distribution system.

Of all ground array collectors, slinkies are the most common for individual property projects.

Slinky vs straight pipe

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.

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.

Get Kensa’s full support

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.

Brick Cottage ground source heat pump case study: front on view of slinky trenches

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 are more space-efficient heat collectors than straight pipes. Due to their coiled design, they can extract more heat with minimum trenching. 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.

Proving to be the most cost-effective collectors for ground arrays, slinkies reduce the amount of digging required for trenches compared to straight pipe. This is why Kensa manufactures a range of slinky pipes to complement our heat pump range and make ordering and installation even easier.

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.

 

kensa-ground-source-heat-pumps-straight-pipe

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.

How much does it cost to install slinky pipes?

Here are some costed examples of ground source heating projects featuring slinky pipes.

Example: Single self-build home

This example illustrates the cost of installing a ground source heat pump with slinkies vs. an LPG boiler. It could cost £16,000 and earn £26,000 through a 7-year income from the Domestic Renewable Heat Incentive (RHI).

Type of system 13 kW Evo ground source heat pumpVs. LPG boiler
Type of ground arraySlinky pipes1-
Land area required1,000 sq/m-
Appliance & infrastructure costs*£16,000£1,500
Domestic RHI income (over 7 years)**£26,000Not eligible
Running costs (over 20 years)*** £10,000£15,000
Servicing costs (over 20 years)****£0£700
Total net cost (after 20 years)*****£0£17,200 cost
Lifetime expectancy: Appliance Up to 20 yearsUp to 10 years
Lifetime expectancy: InfrastructureUp to 100 years-
Carbon saved (over 20 years)28 tonnes 0 tonnes
Local NOx emissions******0kg NOx/y1.31kg NOx/y

 

Example: Multiple buildings

This example illustrates the cost of installing individual heat pumps connected by a Shared Ground Loop Array of slinkies into four homes. It could cost £64,000 and earn £185,000 through a 20-year income from the Non-Domestic RHI. 

Type of system 4 x 13kW Evo heat pumpsVs. LPG boiler
Type of ground arrayShared Ground Loop Array of slinky pipes1-
Land area required3,500 sq/m -
Appliance & infrastructure costs*£64,000£15,000
Non-Domestic RHI income (over 20 years)**£185,000Not eligible
Running costs (over 20 years)*** £125,000£187,500
Servicing costs (over 20 years)****£0£2,000
Total net cost (after 20 years)*****£4,000 cost£204,500 cost
Lifetime expectancy: Appliance Up to 20 yearsUp to 10 years
Lifetime expectancy: InfrastructureUp to 100 years-
Carbon saved (over 20 years)300 tonnes 0 tonnes
Local NOx emissions******0kg NOx/y5.24kg NOx/y

 

Compare the costs to other ground arrays

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.

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.

Submit your plans for advice

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.

View Kensa’s slinky pipes

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.

view Kensa’s straight pipes

Pricing table small print



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)

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


Slinky Pipe For Heat Pumps

Pre-coiled slinkies Kensa Heat Pumps manufactures pre-coiled slinkies on-site for supply with all Kensa ground source heat pumps, enabling ease of installation. Slinky pipe features Three pre-coiled slinky pipe length options: 30m, 40m and 50m (indicated by a coloured cable tie). Each slinky features a straight header pipe (typically 25m long), a pre-coiled section (30m,…


Case Studies: Woodcott, Warwickshire

Ground Source Review: 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.


GET A QUOTE0345 222 4328