Ground source heat pumps are a greener alternative to conventional boilers, offering a more sustainable method of heating a home and providing hot water. These systems extract heat energy from the ground using a network of pipes buried beneath the surface (we’ll explore how they operate in greater detail below).
This guide covers everything you need to know about ground source heat pumps. Along the way, we’ll explain:
- What ground source heat pumps are;
- How these devices work;
- The various types that can be installed;
- The pros and cons;
- The costs of installing and running them;
- The grants available for homeowners;
- Planning permission requirements;
- The installation process;
- Typical maintenance requirements.
What is a ground source heat pump?
A ground source heat pump is an eco-friendly setup that can be used to provide hot water whilst heating radiators and underfloor heating systems. These devices are similar to air source heat pumps. Instead of drawing heat from the air, however, they use a buried network of pipes known as a ‘ground loop’ to extract heat energy from beneath the surface.
There are several different types of ground source heat pump, each of which uses a unique type of ground loop (more on this below).
How do ground source heat pumps work?
Ground source heat pumps operate using the following process:
To extract heat from the ground, a liquid mixture of cold water and antifreeze is pumped through a network of pipes buried beneath the surface. The ground around the pipes has a year-round temperature of between 8 and 12°C due to energy from the Sun. As the liquid travels through the pipes, it absorbs this low-grade heat from the surrounding earth.
The warmed antifreeze mixture then passes up out of the ground and into a heat exchanger known as an evaporator. The refrigerant liquid within the heat exchanger absorbs heat energy from the mixture, evaporating to form a gas. The cooled liquid antifreeze mixture then returns to the ground loop system to extract more heat. Meanwhile, the warmed refrigerant gas rises.
The warmed gas is fed into a compressor, which increases its pressure and causes its temperature to rise even further. Hot refrigerant gas then feeds into a second heat exchanger known as a condenser. Once inside, the gas heats up water contained in pipes – the heated water then travels into the home, where it can be used to power radiators and underfloor heating systems and provide hot water to showers and taps.
Having transferred its heat to the water, the refrigerant gas reverts to a liquid. This liquid passes through an expansion valve at the end of the cycle to reduce its pressure and lower its temperature. Finally, the liquid refrigerant returns to the evaporator, ready to be heated up once more by the antifreeze mixture from the ground loop system (and the cycle continues).
For a visual explainer of how ground source heat pumps work, take a look at this useful video.
Types of ground loop system
Ground source heat pumps can use four different types of ground loop systems for exchanging heat:
- Open loop
Let’s take a look at each of these varieties.
This is the standard type of ground loop system. A horizontal loop consists of a network of pipes buried in trenches around two metres beneath the ground, parallel to the surface – the antifreeze mixture travels through the pipes and draws heat from the adjacent earth.
Vertical ground loops are used in cases where garden space is limited as they cover a much smaller area. As the name suggests, the pipes are buried in a vertical position perpendicular to the surface, requiring a borehole of up to 200 metres deep – the pipes gather heat energy from the ground surrounding the borehole.
In some areas, you’ll find bodies of water underneath the ground. If there is a large enough underground lake available, then a pond/lake ground loop may be used. In this setup, the network of pipes goes down into the body of water at least three metres below the surface. The antifreeze mixture in the pipes absorbs heat energy from the water and carries it back to the heat exchanger.
Open loop systems are the least common type and operate differently from the other setups. Instead of using an antifreeze mixture carried in pipes, water from an underground lake or well is circulated directly through the ground source heat pump system to the heat exchanger.
Pros and cons of ground source heat pumps
One of the main benefits of ground source heat pumps is their efficiency – as the temperature underground remains relatively constant throughout the year, they can produce four times as much heat energy as is required to operate them (providing greater efficiency levels than air source heat pumps).
Other advantages to ground source heat pumps include:
- They provide hot water as well as heating your home;
- Your heat pump could provide a source of income through the Government’s Renewable Heat Incentive (RHI).
- Minimal maintenance is required;
- Depending on which fuel you switch from, a ground source heat pump could lower your fuel bills;
- If a renewable energy source is used to power them, they produce zero carbon emissions;
- They produce much less noise than conventional boilers and air source heat pumps;
- The components last much longer than other heating systems (up to 25 years).
These systems are not without their drawbacks. It’s important to be aware of the following disadvantages:
- The high initial installation costs can be prohibitive;
- Installing a ground source heat pump causes disruption to your garden (horizontal ground loops require a large area of earth to be dug up);
- The antifreeze mixture can be damaging to the environment, so the ground loop system must adhere to regulations and be installed securely to prevent this from happening;
- Ground source heat pumps provide heating at lower temperatures than conventional systems, meaning large radiators or underfloor heating systems are required to provide the same heating effect.
- Certain types of ground loop system may not be suitable for the area: horizontal systems require a lot of garden space, vertical systems need a certain type of bedrock, and water-based systems require a large underground body of water to be present.
The cost of ground source heat pumps
When considering the overall cost of a ground source heat pump, it’s important to consider both the price of installation and running the system.
Installing a ground source heat pump accounts for the bulk of the cost. The Energy Saving Trust estimates that a typical system will cost between £14,000 and £19,000 to install.
The two key factors when calculating the installation cost are the size of the home that needs to be heated and the area of garden space that’s available. As you’d expect, the more rooms that require heating, the greater the size of the ground loop and the higher the costs.
Horizontal ground loops are the cheapest option from an installation standpoint. However, they usually require at least half an acre of land. If a vertical ground loop is required due to limited space, the ground source heat pump borehole will cost two to three times more than the groundwork for a horizontal system.
Once the system is installed, a ground source heat pump still requires some energy input to pump the various heat-exchanging fluids around the system. That said, the running costs are typically lower than a conventional system if the home is insulated effectively.
For a typical well-insulated four-bedroom detached house, the Energy Saving Trust estimates the potential yearly savings when replacing the following types of system as:
- £530-£570 for an average LPG heating system;
- £400-£440 for a typical solid fuel heating system;
- £25-£30 for an average gas boiler system;
- £20-£30 for an average oil heating system;
- £1000-£1090 for a typical electric heating system.
Grants for ground source heat pumps
Grants are available for households heated via ground source heat pump systems through the Renewable Heat Incentive (RHI). The payments will be made over the course of seven years and vary depending on the size of the home:
- £11,580 for a one-bedroom semi-detached property;
- £17,774 for a two-bedroom semi-detached house;
- £25,180 for a detached house with three or more bedroom.
Use the Government’s RHI Calculator to get an estimate of how much you could receive based on the details of your property.
Planning permission requirements
Ground source heat pump planning permission requirements vary depending on your local authority. In most cases, these systems are classed as permitted development and will not require permission. That said, you must check with your local authority before installing a ground source heat pump.
Ground source heat pump installation process
The process of installing ground source heat pumps is as follows:
- The home and garden space will be assessed to determine the type and size of ground loop that is required.
- The underground area required for the loop will be excavated: for horizontal systems, this involves digging trenches two metres deep across the required area; for vertical systems, a deep borehole of between 15 and 200m will be created.
- The pipes will be installed in the loop fields.
- The existing heating system will be modified if necessary (this could include installing larger radiators or underfloor heating systems).
- The heat exchangers and heat pump will be installed, connecting the ground loop to the heating system.
In most cases, the maintenance requirements for ground source heat pumps are minimal. The manufacturer and installer will be brief you on this, but it’s likely that you’ll need to check the water pump, external pipes, and electronics once a year. A professional should also check over the system once every three to five years.
This guide has explained all of the key information about ground source heat pumps, from the different types that are available to the installation and running costs.