Ground source heat pump: how it works, cost & grants

By William Morris

Content editor

While ground source heat pumps represent less than 2% of all heating systems in the UK, they do have some undeniable advantages that make them much more attractive than a gas boiler. Let’s see how this technology works to save people money while providing sustainable heating year-round. Is it a gamechanger, or is it just the luxury Tesla of renewable heating?

How do ground source heat pumps work?

Ground source heat pumps work by exchanging heat with the ground under which they are buried. Through this principle, a highly efficient form of residential central heating and cooling can be implemented.

Though they are called heat pumps, these devices work year-round: during the hotter months, the ground source heat pump takes away heat from the home and dissipates it underground while in the colder months, latent geothermal heat is transferred into the home. This occurs because beyond a certain depth (generally 7m or 23ft) the ground remains at a constant temperature that equates to the average ambient yearly temperature for that specific location, generally anywhere between 7 to 12 ºC in the UK.

Did you know?Ground source heat pumps work thanks to the ground being cooler than air in the summer and relatively warm in the winter. The difference between ground and air temperatures is key to transferring heat through the system, providing heating or cooling for the home depending on the season.

While ground source heat pumps are all the rage nowadays, they were actually invented in the mid-19th century by an Irish scientist and an Austrian engineer. Over the years different designs have been developed in order to adapt ground source heat pumps to their specific sites and increase their efficiency

Direct Exchange

This is a closed-loop system, where one part of the apparatus is buried underground and another part of the apparatus houses refrigerant to provide cooling.

In the winter, with ground temperatures warmer than the ambient air temperatures, the refrigerant absorbs heat from the ground and, in doing so, it evaporates, bringing that heat into the home. In the summer, with the ground being cooler than the air, the system works in the opposite direction and cools the home.

This system does not require a secondary liquid or anti-freeze to operate, which simplifies the design and day-to-day operation. Copper is often the material of choice for the tubes because of its excellent thermal properties and established use in climate control systems.

Dual loop closed system

This design adds an additional circuit containing a mixture of water and antifreeze to the system. The refrigerant circuit, in this case, is contained within the appliance itself, while the watery antifreeze runs through the ground loop. With this system, the relevant heat exchange occurs between the refrigerant and antifreeze loops, but within the appliance cabinet instead of underground.

Vertical, diagonal, horizontal installations

Depending on the type of ground source heat pump design and its location, you will have one of many possible underground circuit implementations:

Horizontal

Trenches are dug under the frost line to accommodate pipes laid flat in a coiled formation if the home has sufficient space for this type of ground source heat pump. If pipes are not installed fairly deep in the earth, then they will be more likely to be affected by any change in ground temperature. Therefore the cost of digging deeper can be offset by lower operating costs and better efficiency in the long run.

Diagonal and vertical

These installation types are for direct exchange type ground source heat pumps. Instead of digging, drilling will be the preferred procedure which brings the added advantage of a smaller footprint when compared to horizontal trenching. The downside is that drilling is more expensive, however, the silver lining is that less pipe needs to be laid down.

Radial

A radial installation is somewhat of a middle ground between horizontal and vertical implementations both in terms of cost and layout. This technique allows horizontal installations to be carried out without having to dig up a large area. With this technique, a household could choose between closed-loop and direct exchange installations because the horizontal directional drilling is compatible with both copper and polyethylene pipes.

How much does a ground source heat pump cost to install?

Ground source heat pumps of every type are significant long term investments no matter what type you go with. Based on estimates from the Energy Saving Trust, the average cost to install a heat pump is between £14,000 to £19,000.

It’s worth mentioning that the above range of estimates does not include moving to a floor heating system or widening air ducts within the home to facilitate heating and cooling performance and indoor comfort. There's no doubt that a ground source heat pump's cost in terms of installation will put off a great number of people, but it's important to consider that in the long term it will save you money.

How long do ground source heat pumps last?Unlike boilers, which require replacement or extensive repairs every decade or so, most ground source heat pump installations are guaranteed for 25 years, require little to no maintenance if correctly installed, and have projected operational lifespans in excess of 50 years for the ground loop itself.

Where to buy?

Ground source heat pump installations vary greatly in complexity depending on the location where they will be installed and the type of pump. A household looking into this kind of sustainable heating and cooling will need to not only find a heat pump distributor but also an installer and potentially a heating engineer to carry out any home modifications down the line.

The good news is that distributors generally work with specific installers in order to offer a turnkey service for residential households. Failing that, the GSHPA (Ground Source Heat Pump Association) is a trade body that represents the industry and sets its standards.

However, you can choose from a wide range of heat pump manufacturers, from household brands like Samsung and Vaillant to longstanding Nordic specialists such as Nibe while the installer handles the ground loop specifics.

Ground Source Heat Pump Grants 2020

While the UK government has recently phased out some of the more common renewable incentives, such as the feed-in tariff last year, ground source heat pump users are in luck - they have three main schemes to benefit from:

Renewable Heat Incentive (RHI)

The UK government is trying to encourage more British homes to opt for renewable heat instead of the more common gas and electric boilers. Starting out in 2011, eligible households have been getting cashback on a quarterly basis for the first seven years of operating a ground source heat pump.

At 20.89p/kWh, an approved average three-bedroom home could easily get back £1,800 every year for the next seven years, which comes to £12,600, a great saving by anyone's standards.

Home Energy Scotland Loan

There is no denying that the upfront installation cost for even a modest heat pump set up can make your eyes water. However, if you’re Scottish you should have no fear! Your local authorities have come up with an enticing loan programme that offers interest-free loans of up to £10,000 for eligible households putting a heat pump in their property.

That kind of leg up is nothing to sneeze at because it puts most heat pump installations at the same level as a bog standard boiler installation. In this case, however, you get significantly more reliable and efficient home heating system that is also cheaper to run.

Assignment of Rights (AoR)

If you don’t live in Scotland, a possible alternative to offset costs (if you don’t have the savings or disposable income) is to have a private investor fund the set up of a ground source heat system. Naturally, that investor then gets to claim the Renewable Heat Payment you would be eligible for in order to get a return on investment.

Are there cheaper alternatives?

Electric and gas boilers are the most common alternatives - and potentially the only legitimate alternative if you live in a flat unless your building is looking at communal heat pump installation for the whole building.

For rural households, the only remotely sustainable heating option would be a wood chip compatible furnace, which is also eligible for the RHI. Oil-fired furnaces are a disaster for the environment, with a colossal carbon footprint that comes not only from burning fossil fuels but also from the need to have fuel delivered to your home.

Ground source heat pump systems really are in a league of their own when looking at every possible metric, including:

1. Lifetime energy cost
2. Energy savings
3. Reliability
4. Efficiency

How big is a ground source heat pump?

The scale of the heat pump is contingent on the following factors:

1. Home size and type: number of rooms, detached, semi-detached or terraced
2. Average energy consumption dedicated to heating and hot water
3. Type of terrain surrounding the home: soil humidity and rocky layers affect efficiency and installation layout

A general rule of thumb for horizontal ground source systems (the most popular type) is to multiply the heated floor square footage of the house by 2 or 3 times to estimate the amount of land needed.

Vertical ground source installations will save you space but you still require several boreholes that will need to be sufficiently spaced by about 27 feet (8.2m) from each other.

Necessary Permits

While ground source heat pumps are seen as permitted developments in residential contexts. It’s always worth checking with the relevant local authority and documentation depending on where you live:

• England - General Permitted Development Order (GPDO)
• Scotland - Town and Country Planning (General Permitted Development)
• Wales - The Town and Country Planning (General Permitted Development)

Ground source heat pump advantages and disadvantages

In this section, we're going to take you through the basic pros and cons related to ground source heat pumps. Let's start with the positive:

What are the advantages of ground source heat pumps?

While geothermal heat pumps are rated through industry-accepted measures such as the seasonal coefficient of performance, we consider their main advantage to be their efficiency:

• Standard boiler efficiency: 60% to 90%
• Ground source heat pump efficiency: 300% to 500%

(according to Smart Renewable Heat)

While standard central heating and boilers cannot exceed 100% efficiency because of the thermodynamic limits of their design, heat pump efficiency numbers go through the roof thanks to the geothermal exchange principle powering them.

A few more potential benefits for households with heat pumps are:

• Considerably lower energy bills year-round
• Energy independence
• No need for boilers or air conditioners
• Better reliability
• Reduced carbon footprint

For households that already have solar panels installed, a ground source heat pump could allow them to be off the grid, especially if the home has some form of battery storage. Pump systems can offload most of the heavy lifting when it comes to household energy use by taking care of two of the three most energy-intensive tasks, namely heating and hot water. Solar energy could then comfortably take on lighting, electronics and cooking during the day and with added home battery storage during the night.

Ground source heat pumps are designed to have minimal environmental impact even during a spell of low temperature, unlike HVAC and air conditioners. With ground loops often rated to run for decades without intervention and heat pumps needing only usual checkups, the homeowner is left with a highly efficient system that is guaranteed to remain trouble-free for years to come.

What are the disadvantages of ground source heat pumps?

Despite our lauding of ground source heat pumps and all the benefits they bring, we know that they're neither the best option nor a viable one for absolutely everyone. Here are some of the drawbacks to the technology:

• Installation is expensive.
• It is location specific and not equally distributed, and tends to lose energy when travelling longer distances.
• If you use an open-loop rather than a closed-loop system, it requires large quantities of water to keep working.

For many, the first point will be enough to rule a ground source heat pump out. It's all very well making savings later, but if you don't have the money lying around to fork out the considerable installation costs then the matter is simple.

Energy distribution shouldn't be an issue, however, if you're installing for a more or less normal-sized home. It's more of a problem for larger buildings, and makes it unlikely that anyone living in a flat would benefit from one for now - flats, however, are suited to air source heat pumps.

Is a ground source heat pump worth it? Our verdict

Here at The Switch, we believe that ground source heat pumps are an ideal heating and hot water solution for most new homes. They do, however, represent an investment that is not realistic for absolutely everyone, as replacing a traditional boiler system to install a heat pump like this is, for now, a particularly expensive process.

For rural or remote homes which are not connected to the national grid, ground source heat pumps are seriously good option, allowing total energy independence at a low cost in the long-term.