A guide to ground source heat pumps and how they work
02 September 2020 | OVO Energy
Ground source heat pumps are a super important addition to the renewables family.
In a world where so many of us are hunting for greener, and more efficient ways to heat our homes, ground source heat pumps could be just the solution. Efficient, carbon-kicking, and low maintenance – they’re certainly an attractive option. So what are they, how do they work, and could they be a good fit for your home? Let’s dive in...
What are ground source heat pumps?
A ground source heat pump system absorbs heat from the ground, using pipes that are buried outside in your garden – and transports this heat into your home.
The earth takes in energy released from the sun all the time. Why not use this natural heat to our advantage? And better still, underground temperatures stay the same no matter what the season (around 11C in the UK, rain or shine). So you’re set all year round.
To give a bit of context, your ground source heat pump plays the same role as a boiler in a central heating system. But the brilliant bonus is that it uses ambient heat from the ground, rather than burning fuel to generate heat. And this natural heat can be used for your central heating.
Ground source heat pumps need electricity to run. But they take a small amount of electricity, to produce a large amount of heat. If this electricity is coming from renewable energy, your heating is basically carbon-free – and very efficient! Clever, right?
One important note: you'll need plenty of space for a system to be installed. Generally a garden that can fit digging machinery is good. But it really depends how big your home is, and how much heat you need.
How do ground source heat pumps work?
Here comes the detail. Ready? Let’s go!
- Heat from the ground is absorbed at low temperatures into a fluid (a mix of water and antifreeze) inside a loop of pipe (called a ground loop), which is buried underground in your garden.
- The ground loop's mix of water and antifreeze circulates at low temperatures (around 2C).
- Because the ground around it is at a higher temperature (10C), energy passes from the ground into the ground loop – warming up the water and antifreeze.
- This warmed-up liquid then comes back to the heat pump, and passes through a heat exchanger. A super-cold ‘refrigerant’ (-10C) also runs through the heat exchanger, so heat from the warmed-up ground loop liquid now passes into the refrigerant.
- As the refrigerant heats up, it turns from a liquid into a gas. The final step is to use some electricity to compress this gas. This then raises the temperature to around 50C!
- The heat is then transferred into the water used in your heating and hot water system.
Once the ground loops are buried in your garden, they aren’t intrusive. They usually sit horizontally, about a metre below the surface. But if you have a small garden, vertical holes can be made instead, to get heat from much further down (around 90–160 metres deep).
Both methods work, neither show on the surface, and your heat pump installer always has the know-how to design the best system for your space.
How are ground source heat pumps installed?
First of all, with a lot of digging! A typical 8kW output heat pump needs 200m of trench, with several loops – so overall you’d need 400sqm available.
There are several ways to lay these trenches: single trenches, coiled pipes, or laid out like underfloor heating. Ground source heat pumps can also go in vertical boreholes as mentioned before, but that does make the whole thing slightly pricier, as the equipment needed is more complex.
Usually, the trenches are dug close to your home. However, if you’re lucky enough to have a lot of surrounding land, your installer can scout out something further afield if the ground conditions are better. The heat pump itself goes in your home.
As you can guess, installation of a ground source heat pump – while ultimately a great long term plan – can be disruptive in the short term. That’s why they're often an appealing option for those building homes from scratch.
What are the main benefits of a ground source heat pump?
- Lower fuel bills -Especially if you replace standard electrical heating.
- Lower home carbon emissions – depending on the fuel you’re replacing.
- Heat pumps don’t just heat – they cool too. Perfect for hot summers.
- Heat pumps are safe - No combustion, no flues, no emission of potentially dangerous gases.
- They’re low maintenance compared to standard heating systems.
Money talk – how much does a ground source heat pump cost?
Ground source heat pumps are all very different in size and make up, so pinning down an average cost is tricky. The Energy Saving Trust (EST) estimates it can cost between £10,000 and £18,000 to install one in your home. Running costs depend on a few things, including the size of your home and how well insulated it is.
Installation costs also depend on what your outside space is like, the amount of heat you need, and the kind of trenches that suit your site (you’ll get this info via a site survey).
The UK Government’s Renewable Heat Incentive (RHI) scheme currently offers quarterly cash payments, just for installing a carbon-kicking green heating system at home. Find out more on our blog.
Maintenance tips for ground source heat pumps systems
Heat pump systems tend to go the distance, operating well for a good 20 years or more. But they do need regular maintenance. A yearly check by you, and a more in-depth check by a professional every 3–5 years should do the trick.
And with a warranty of 2 to 3 years for most heat pump systems (and workmanship warranties like Quality Assured National Warranties stretching to 10 years), you’re pretty much covered if something big goes wrong. Some manufacturers also offer extensions of warranty for a fee.
If all this is making you curious, here at OVO we’re running a Zero Carbon Heating trial, which includes installing Ground Source Heat Pumps. If you fancy being part of the trial and you’re eligible, you might be able to get your hands on one for free! Interested? Find out more about the Zero Carbon Heating Trial.
How efficient are ground source heat pumps?
Very! The only energy you pay for is electricity, which powers the compressor and the circulation pumps that move the heat from the ground into your home.
A well designed ground source heat pump installation will net you 3 or 4 times as much thermal energy (heat) than the electrical energy used to drive the system. And if you want to go that extra mile, you could get yourself a green electricity plan too.
Ground source heat pumps are an ideal partner for underfloor heating or ‘oversized’ radiators too. Why? Because they work best with heating systems already designed to run at a lower water delivery temperature than the temperature used in everyday radiator systems.
For every unit of electricity used by the heat pump, 3 to 4 units of heat are captured and transferred. This means:
A well installed ground source heat pump can be 300-400% efficient in terms of its use of electricity1 .
At this super efficient level, there’s 70% lower carbon dioxide emissions than a gas boiler heating system2 .
If your electricity is powered by renewable energy, then carbon emissions can be reduced to zero.
Your footprint is massively reduced and you become a carbon-kicking legend. Hurray!
Things to consider when installing ground source heat pumps
Is your garden ok for a ground loop?
Size wise, it doesn't have to be big. But the ground needs to be suitable for digging a trench or a borehole. And you’ll need to be able to get digging machinery back there too.
Is your home well insulated?
Good insulation is essential. Since ground source heat pumps work best when producing heat at a lower temperature than trad boilers, a well insulated (and draught-proof) home makes the heating system most effective.
What fuel are you replacing?
If it's electricity or coal, heat pumps will pay for themselves pretty quickly. However, they’re not the best option for homes using mains gas.
What type of heating system are you keen to use?
Ground source heat pumps work better with underfloor heating systems, or warm air heating. Radiator-based systems are less workable, because lower water temperatures are needed.
Are you getting a ground source heat pump system for a new build or renovation?
If you’re getting other building work done anyway, combining the installation with that will definitely reduce the cost. And be less of a headache for you too!
If you’re curious about changing the way you heat your home, why not read more about low carbon heating options?
1 2 Figures taken from the Ground Source Heat Pump Association.