Should I get a borehole or a trench for my ground source heat pump?

Ground Source Heat Pumps (GSHP) are a great way to heat a modern home, especially if you are off the gas grid. They do require a bit of room in the garden though, so they aren’t for everyone!

How does a GSHP work?

Heat pumps are designed to move heat from one place to another, even when the source is colder than the room you are trying to heat! It may seem a little counter intuitive, but heat pumps work just like your fridge or freezer, but in reverse. In a GSHP, a refrigerant liquid is pumped around pipes laid in your garden, absorbing the latent heat in the ground. This heat is then released into your home after being amplified by a compressor.

GSHPs are great because the temperature of the ground is pretty much constant throughout the year, unlike an air source pump, which relies on there being some warmth in the air to operate efficiently. As such, GSHPs operate at a very similar level of efficiency all year round, and will always be more efficient than an air source equivalent.

The trickiest and most expensive part of a GSHP installation is laying the pipework under the ground. There are 2 ways to do this. You can lay the pipes in trenches running up and down the garden (horizontal collectors), or you can drill a borehole and sink the pipe way down in the ground.

Clearly, laying trenches is much cheaper than boreholes. Boreholes cost a small fortune to dig, and reinforcement will be required to prevent the hole from collapsing on itself. The cost of this is often £10-15k. However, there are some benefits of having the borehole.

Boreholes only take up a fraction of the room that trenches do, so if you only have a small garden, this is the only option for you. Further, it means you won’t have to dig up half your garden! You might be surprised just how much space the GSHP takes up when you have to run the pipes through trenches. It really will destroy the garden. The good news is that once it is laid and finished, you won’t be able to tell that it’s there, and you can plant your garden again. Be aware though that this land will no longer be usable for trees, driveways or ponds. The area needs to be free from large roots and must be porous.

Also note that to drill a borehole a very large piece of drilling machinery is required. If your garden is inaccessible to vehicles then it may not be possible to get the drill into the garden.

Local geology can make a difference

For horizontal collectors, the ground needs to hold water well to be most effective. Even at 1.5m deep which is the typical depth for a trench, the temperature can vary season to season, and the geological conditions can make the difference. Your installer should be able to advise if your property is suitable for a horizontal collector.

Because boreholes go deeper, the temperature is very consistent, so you won’t have any variation in efficiency at all between seasons. This makes borehole heat pumps great for cooling in summer, as well as for heating in winter.

Which is better: boreholes or trenches?

This is really a tricky one to give a definitive answer to. It really will depend on the heating and cooling requirements of the property, the geology and local conditions, room available, costing and aesthetic considerations in the garden.

Whichever route you go down, though, ground source heat pumps are is a great way to heat your home. Remember that there is a very generous RHI grant available from the government for this type of system, and you will definitely see considerable savings on your heating costs when compared to other off grid heating like oil boilers.

Could I save money with a ground source heat pump?

Technology Expected Savings with GSHP/yr
Gas Boiler (G Rated) £600
Gas Boiler (A Rated) £100
Storage Heaters (Older) £800
Storage Heaters (Modern) £500
Oil boiler (G Rated) £400
Oil boiler (A Rated) Increase of £50
LPG Boiler (G Rated) Increase of £1,100
LPG Boiler (G Rated) Increase of £400

*Figures approximated based on an average house. Savings will vary.

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