All about ground source heat pumps

How does a ground source heat pump work?

All heat pumps work on a principle of heat transfer - moving heat from one place - an environmental source - to another.

Ground source heat pumps work by drawing heat from the earth and transferring it your home for use in heating your home and supplying hot water. To do this water is pumped through pipes laid in the ground (or water) to harvest the energy. This energy is then passed through a heat exchanger which extracts the heat ready for use in the home.

You might wonder how this can work when the ground is cold - or indeed frozen - in winter. The heat pump ensures that the fluid flowing through the pipes is always a few degrees cooler than the ground itself.  As heat will always try to reach equilibrium this causes heat energy to flow from the ground into the pipes. Even in cold weather the heat pump can thus extract plentiful energy.

Indeed vertical boreholes - which can go down to 150 metres deep - can be even warmer and more stable with a couple of degrees of extra heat conducted up from the sub surface of the earth.

Because the temperature of the ground is more stable and has a higher average winter temperatures than air temperature, ground source heat pumps are typically provide an even more efficient heating than air source heat pumps. This is because heat pumps are at their most efficient when there is small difference between your source and the water flow temperature you set your heat pump to deliver.

Ground source heat pumps are able to take this heat energy from the brine in the ground loop and "concentrate" it into a smaller amount of water at a higher temperature. This hot water is then available for domestic use and heating .

You can find more technical detail about how heat pumps work here.

Types of Ground Source Heat Pump systems

Ground source heat pumps can work with a variety of different types of collector loops which can be categorised as either open or closed loop systems. They can also use ground as a source of heat or a body of water (such systems are often called water source heat pumps but they use the same type of heat pump a ground source system).

Closed loop systems for ground source heat pumps

In closed loops the water in the collector loop is repeatedly circulated. Once the loop is filled with water, the water remains in the loop, gathering heat and having it extracted again over and over.

Closed loops can be laid horizontally in the ground in trenches which are then re-covered or laid into water such as a pond or river bed. Alternatively they can be placed in a borehole drilled vertically into the ground; these often pass through layers of earth, rock and water and extract heat from them all.

If you want to find out more about this kind of system and how much space they need please go to our page on ground source heat pump installations.

Open systems for ground source heat pumps

This kind of works by pumping water from a river, stream, well or even the sea into the collector pipes and expelling it again once the heat has been extracted. Check out our ground source heat pump installation page for more information.

How efficient are ground source heat pumps?

The are several measures for this! Let's look at what they mean

COP and SCOP for ground source heat pumps

The efficiency of heat pumps is usually expressed as a Coefficient of Performance (COP) and Seasonal Coefficient of Performance (SCOP). A COP is defined as the ratio of the amount of heat or cooling provided by the system to the amount of energy consumed by the system.

A COP of 3 means that for every unit (kW)of energy used as electricity a heat pump can produce 3 units - or kWs of energy for heating your home or hot water cylinder.

You'll find an indicative COP figure for the heat pump you install in the manufacturer's technical data. This is the performance measured under a set of certain standard conditions (as for miles per gallon figures given for a car so that you can compare various models with a level playing field).

Your heat pump controller will usually also tell you how your heat pump is performing in the moment once installed.

SCOP takes into account the varying environmental conditional that a heat pump system encounters throughout an entire heating or cooling season. Indicative SCOP figures are also often provided by manufacturers based on standard testing.

Unlike the instantaneous COP, which provides a snapshot of performance at a single moment, SCOP considers the system’s energy efficiency over the long term.

It is essential because real-world conditions are not static, and a heat pump’s efficiency changes in response.

SCOP helps us understand how well a system performs across different weather conditions and seasons. If your heat pump has monitoring software, you should be able to see a real-life SCOP figure for given time periods.

Ground source heat pumps typically have a COP and SCOP one point higher than an air source heat pump installed to heat the same property. If an air source heat pump can achieve a good COP of 3 (being 300% efficient and a respectable performance for an air source heat pump) then a similar sized ground source heat pump attached to the same heating system will score a COP of 4 (being 400% efficient and a good performance for a ground source heat pump).

This is because bodies of water and earth are far more stable in their average temperatures all year round. Essentially, earth and water remain warmer in winter than the air, which is when you want to extract the most energy for heating.

As previously mentioned, the ground at a 2m depth typically remains between 8 and 12 degrees in the UK with minimum recorded temperatures at 5.2 degrees and a maximum at 16 - (source: The British Geological Society Temperature and Thermal Properties (Basic) Georeport 2011 ).

As heat pumps are increasingly efficient the smaller the temperature difference between the source temperatures and the ultimate required flow temperature of the heating system, ground source heat pumps are thus almost always more efficient heating systems that air source heat pumps.

How ground source heat pump performance is affected by the thermal properties of the ground

"The rate at which heat is exchanged between the collector loop of the ground source heat pump and the ground is determined mainly by the thermal properties of the Earth. Thermal conductivity is the capacity of a material to conduct or transmit heat, whilst thermal diffusivity describes the rate at which heat is conducted through a medium. For a horizontal loop system in a shallow (1 to 2 m) trench then the properties of the superficial deposits are important, whilst for a vertical loop system it is the properties of the bedrock geology that are important." (The British Geological Society Temperature and Thermal Properties (Basic) Georeport 2011 ).

Some types of soil and rock give heat to your ground loop more efficiently than others. As water is a good conductor of heat wet soils are better than dry at heat transfer, for example. Moreover some kinds of rock and soil absorb heat from their surroundings more easily than others - some very dense rocks can conduct heat efficiently but then take a long time to heat back up again once an initial transfer of heat has occurred and thus may offer less energy as a heating season progresses, whilst others are sponge-like and transfer heat readily from surrounding substrate.

In Boreholes warm water flowing from the depths affect ground temperatures warmth whilst cooler regions can occur if water flows down from the surface.

These are just some of the ways that local ground conditions can affect the heat output of your heat pump and why the exact performance of every ground source heat pump is unique.

Your heat pump installer will survey the ground before installing your renewable heating system to give you an idea of the energy efficiency that can be expected from a ground loop system on your local geology.

Installation considerations for ground source heat pumps

This is a quick summary of the main points to consider for the installation of a ground source heating system - to find out more read our detailed article on ground source heat pump installation.

Space for heat pump ground loops

You will need 2-3 times the floor area of your property for a ground loops. Boreholes need considerably less space and so can be installed even in small urban gardens provided access for drilling rigs can be arranged.

However it can be a good idea - if you have room and budget - to consider oversizing your ground loop. Whilst the heat pump itself should always be sized as exactly as possible to meet the heat demand of the property to make it energy efficient (an oversized heat pump will cycle on and off more, which affects its longevity and increases running cost), the same is not true of your collector loop. 

Making a collector loop larger can future proof heat pump installations - making them ready for the more energy efficient heat pumps of the future, enhancing the energy efficiency of your current system and the protecting ground temperature. 

Space for indoor central heating system and hot water component

A domestic ground source heat pump is about the size of a fridge freezer and needs to be housed indoors. This can be in a utility room or dedicated plant room. You will also need space for a hot water cylinder (some ground source units have these built in) and possibly a buffer tank for the central heating system.

Heat emitters for your heat pump installation

Heat pumps work more efficiently with a lower flow temperature than gas or oil boilers. If you are building a  new property the obvious choice is underfloor heating to match your heat pump unit.  

For older properties this means that as well as replacing your boiler you may also wish to consider upgrading to new heat emitters which work well with the cooler temperatures from heat pump systems; typically those with a large surface area. This can be as simple as replacing existing radiators with larger models or it may be that you wish to install underfloor heating or fan coil radiators in some areas. 

Your heat pump installer will advise on what is necessary to ensure comfort levels based on the age of your home and its levels of insulation.

The costs of a ground source heat pump and the cost savings you can expect on heating bills.

Installation costs for a ground source heat pump

At the time of writing the average cost for a ground source heat pump installation is around £2000 per kW for a system with a horizontal ground loop and £2500 per kW for a vertical ground loop. It is worth remembering that once installed your ground loop should last a lifetime and so this is essentially a one- off cost like installing a central heating for the first time in a new home.

In the same way that you don't need to replace all your radiators when your gas or oil boiler reaches the end of its life, only the heat pump will need to replaced after 15-20 years of service.

Running costs for ground source heat pumps vs fossil fuel heating

Both ground and air source heat pumps are generally cheaper to run than boilers or an all-electric heating system.

As a rule of thumb a well designed renewable energy heating system can consume a third - or even less - energy compared to systems based on fossil fuels. Ground source heat pumps can outperform even air source heat pumps for savings .

What are the advantages and disadvantages of a ground source heat pump?

The advantages of a ground source heat pump

Ground source heat pumps are a highly energy efficient way to heat your home, lowering your carbon emissions without any need for compromise on comfort.

Running costs are low and, when paired with solar panels and battery storage, can give you almost complete independence from the grid and insulate you from fluctuations in the fossil fuels market and electricity prices, ensuring very low energy bills.

And if you decide to sell your home, these advantages can add value to your property.

Ground source heat pumps are extremely quiet in operation and have low maintenance requirements. Because a ground source heat pump works without a fan it is even quieter than other kinds of heat pumps that extract heat from the air.  

There are no planning requirements for ground source heat pumps.

Passive cooling as a benefit of a ground source heat pump

Ground source heat pumps have the ability to offer "passive" cooling in summer. In passive cooling, the heat transfer fluid (usually a water/glycol mixture called brine) is circulated through the heat exchanger without the compressor of the heat pump itself being in service.

As the ground temperature around the collector loop will be cooler than the air temperature in your home on a hot summer day, cool water from the loop can be pumped to the heat exchanger in the heat pump and for use in cooling your home.

The newly cooled water is then distributed to cooling fan coil units or underfloor heating. If you are using fan coil units, you’ll need one in each room that requires climate control.

Passive cooling by a ground source heat pump is an extremely low cost to run and has a low carbon emissions compared to air conditioning as it only requires a circulating pump to be run and does not require powering of the heat pump compressor.

Passive cooling uses the existing elements of your heat pump system, saving time, money, and space on additional equipment

Running costs are low – it only uses energy to circulate the brine through the heat pump system

What are the disadvantages of a ground source heat pump?

The initial investment is quite high .

You need at least some outdoor space for vertical boreholes, so this kind of heat pump unit are not usually an option for apartment dwellers.

Horizontal ground loops require quite substantial areas of land and are disruptive to any existing landscaping.

Government grants and incentives for ground source heat pumps

At the time of writing the national government and the devolved administrations are offering a number of grants and incentives for heat pumps in order to meet their net zero obligations.

For example, the Boiler Upgrade Scheme (or BUS) offers all homeowners in England and Wales up to £7500 towards the installation of a ground source heat pump where it replaces a boiler. The boiler upgrade scheme is open to domestic and small non-domestic properties and is expected to run until 2025. It replaced the renewable heat incentive or RHI as the main source of funding for a ground source heat pump.

Other schemes are also available to assist those on low or fixed incomes to lower their bills by switching to renewable heating systems.

To find out more visit our page on funding for heat pumps, which is kept up to date with the latest details of funding options.

Another good source of up to date independent information is the energy saving trust website. 

Choosing the Right System for Your Home

Ground source heat pumps are suitable for domestic commercial and applications. A small ground source heat pump installation can be powered by a domestic electricity supply on 230V. Large or commercial heat pump installations usually require a 400V, 3 phase electrical supply because of the amount of electrical energy required. Different types of heat pump are required for each situation and your installer will advise on correct models.

Whether you need a commercial or a domestic ground source heat pump will depend on how much heat you need to produce. Typically a domestic ground source heat pump can provide up to 15kW of heat on 230V. We have commercial units that go up to 87kW.

Ground source heat pump cascades

Having more than one ground source heat pump working in tandem is called having a heat pump cascade. Ground source heat pump cascades have a number of benefits. 

You can have one heat pump prioritized to provide underfloor heating whilst another will also respond to a call for hot water, for example. This means that the primary heat pump does not need to switch between the two and risk the temperature of the property dropping whilst the hot water tank is recharged. 

For large or critical systems heat pump cascades also build in redundancy. If you have two ground source heat pumps and one fails then the second can carry at least some of the load whilst the first heat pump is fixed. 

Cascaded heat pumps are also able to glean some further energy efficiencies and longevity benefits by balancing load in periods of high or low demand. 

One heat pump can turn off completely in periods of low demand for example, allowing the other heat pumps to run continually (which is efficient) rather than cycling on and off (which is inefficient and wears out your ground source heat pump more quickly).

In summary heat pump cascades provide cost and energy savings and build in a safety net for your building services, making them an ideal choice for large properties or those for whom reliability is a key concern. 

Hybrid systems with boilers

A ground source heat pump can also be run alongside a secondary heat source such as a gas boiler or an oil boiler. 

Where relatively new gas or oil boilers are already in place some property owners opt to retain it as a back up option, particularly on older properties where heat demand can peak at high levels during the coldest weather.

A gas boiler can also be used to cover periods of peak heat demand where increasing the size of the ground source heat pump would be difficult or go beyond budget.

Using a fossil fuel boiler to provide supplementary heat for just the worst of the cold weather during the year can mean being able to have a smaller heat pump to deliver the rest of the heating load.  

Keeping an oil or gas boiler is somewhat controversial as it does mean a continuing - even if much reduced - dependence on fossil fuels. 

There is an environmental argument that gas boilers should be removed and other electrical forms of heat used for peak demand to produce heat cleanly in this situation. However, this needs to be balanced against the carbon impact of throwing away functional equipment that is already in place. In addition the choice of integrating direct electrically driven heating devices needs careful consideration as it can also impact your heating bills. 

Conclusion

Ground source heat pumps, like all kinds of heat pumps, are a highly effective and efficient way to provide heating.

Your heat pump unit will be housed indoors if you choose this kind of heat pump and you will need to consider outside space for installation of a collector loop. 

Upfront costs for a first ground source heat pump can be higher than air source heat pumps but they can offset this cost in greater cost savings over time and the replacement costs of just the heat pump unit is comparable.

If you are interested in a ground source heat pumps for your building or refurbishment project it pays to think about space, building fabric and heating emitters early on to make the best accommodations possible so that your heat pump can work to the best of it's ability. 

A well planned and installed ground source heat pump system will provide comfort for your home for many years.