Ground Source vs Air Source Heat Pumps: Key Differences Explained

Heat pumps are becoming increasingly popular in the UK as an energy efficient and low carbon alternative to traditional gas boilers for heating and hot water.

A heat pump works by extracting heat from either the air, ground or water and then uses a small amount of electricity to concentrate that heat and raise it to a higher temperature. This heat can then be used to warm up water for radiators, underfloor heating systems and hot water in your home.

Unlike gas and oil boilers which must burn fuel to generate heat, heat pumps simply move existing heat around. This makes them much more energy efficient – for every 1 kWh of electricity used to power the heat pump, between 3-4 kWh of heat energy can be supplied. This means much lower energy bills compared to conventional fossil fuel systems.

Heat pumps are also very low carbon. As most of the power on the UK grid comes from lower carbon sources like renewables and nuclear, using a heat pump can reduce carbon emissions from heating by 30-50% compared to gas. With the UK’s target to reach net zero emissions by 2050, heat pumps will play a crucial role in greening UK homes.

With rising energy prices and the need to decarbonize heat, heat pumps present an attractive option for households looking to upgrade their heating system. This blog post will explore the key differences between the two main types of heat pumps available.

How Heat Pumps Work

Heat pumps move heat from one place to another, instead of generating heat directly. They work on the basic principle that liquid absorbs heat when it evaporates into a gas, and gives off heat when a gas condenses back into a liquid.

Inside a heat pump there is a circulating refrigerant that goes through phase changes from liquid to gas and back again. On the outside of the house, the liquid refrigerant absorbs ambient heat from the air, ground, or water. It evaporates into a gas, which then passes into the heat pump.

Inside the heat pump, the now gaseous refrigerant goes through a compressor, raising its temperature. It then condenses back into a liquid, releasing heat. This heat is transferred via a heat exchanger into the heating and hot water circuits of the house.

The refrigerant, now cooled again, passes back outside to continue the cycle. The movement and phase changes of the refrigerant allow heat pumps to absorb “free” renewable heat and raise it to a higher temperature needed for heating the house.

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Ground Source vs Air Source

Ground source and air source heat pumps work in similar ways, but have some key differences.

• Ground source heat pumps use pipes buried underground to extract heat from the ground. Air source heat pumps extract heat directly from the outside air.
• Ground source heat pumps are more efficient as the ground temperature is more stable than air temperature. They can achieve coefficient of performance (COP) of 4-5 compared to 2-3 for air source.
• Ground source heat pumps have a higher upfront cost as installation requires digging to bury the pipes. Air source units just need an outdoor condenser fitted.
• Ground source heat pumps are ideal for new builds with space for ground loops. Air source may be the only option for existing homes without garden space.
• Air source performance drops as outside temperature decreases. Ground source is less affected by very cold weather.
• Both are very low carbon options compared to gas boilers. Ground source emits less CO2 as it uses less electricity.

In summary, ground source heat pumps are more efficient and eco-friendly, but have a higher initial cost. Air source units are a lower cost option suitable for many homes.

Upfront Costs

When it comes to upfront costs, ground source heat pumps tend to be more expensive than air source heat pumps.

• The equipment itself costs more. While the heat pump unit for an air source system runs around £3,000 to £5,000, the heat pump for a ground source system generally costs £8,000 to £12,000.
• Installation also costs more. An air source system requires minimal groundwork – just mounting the external unit on the outside wall. But a ground source system requires digging trenches and installing the piping underground. This excavation work costs around £5,000 to £10,000.
• In total, a ground source heat pump system can cost anywhere from £15,000 to £25,000 installed. An air source heat pump on the other hand may cost between £8,000 and £12,000 fully installed.

So when considering upfront price, air source heat pumps have a clear advantage. However, ground source heat pumps make up for the higher initial investment over time through greater efficiency and lower running costs.

Efficiency

Ground source heat pumps are generally more energy efficient than air source heat pumps. This is because the ground provides a more stable temperature for exchanging heat compared to the outside air.

The coefficient of performance (COP) for ground source heat pumps ranges from 3.0-5.0, meaning for every 1 kWh of electricity used, 3-5 kWh of heat is produced. In comparison, air source heat pumps typically have a COP of 2.0-3.5.

However, recent improvements in air source heat pump technology have significantly increased their efficiency. Variable speed compressors, larger heat exchangers, and other advances have allowed the latest air source models to achieve comparable COPs to ground source systems in some cases.

Overall, while ground source heat pumps still hold an edge for efficiency, high-performance air source systems are narrowing the gap. And air source heat pumps tend to have a lower upfront cost. So for some homes, the lower investment of air source can make it the better value despite slightly lower energy efficiency.

Ideal Installation Locations

When determining whether to install a ground source or air source heat pump, it’s important to consider the size and location of your property.

Generally, ground source heat pumps work better for larger properties, while air source heat pumps may be preferable for smaller homes, apartments or offices. This is because:

• Ground source heat pumps require excavation to bury the pipes in the ground either vertically or horizontally. This requires sufficient outdoor space on your property which is easier with larger land areas.
• The upfront installation cost of a ground source system is higher due to the ground loop. This cost can more easily be offset through energy savings in a larger property.
• A benefit of ground source heat pumps is they maintain their high efficiency even in colder climates, as the ground temperature stays stable year-round once you go beyond the frost line. This makes them well-suited for larger homes located in cold weather.
• Air source heat pumps are more compact, requiring just an outdoor unit installed on the wall or ground. This makes them better for small spaces where ground loops would be impractical.
• Air source heat pumps are also generally cheaper to install, making them a sensible option for smaller budgets. Their efficiency does decrease in very cold weather however.

So in summary, while ground source heat pumps require more space and installation work, they can be ideal for larger homes and properties. Whereas air source units are simpler to install and more cost-effective for smaller homes and offices.

Carbon Emissions

Both ground source and air source heat pumps have lower carbon emissions compared to gas boilers. This is because heat pumps use electricity, which in the UK is increasingly generated through renewable sources like wind and solar.

By switching from a gas boiler to a heat pump, a household can reduce its carbon emissions from heating by 1.3-1.7 tonnes per year. Over the lifetime of a heat pump, this can amount to significant carbon savings. The carbon savings will increase in the future as the UK grid continues to decarbonize and use more renewables.

So while ground source heat pumps edge out air source slightly on carbon emissions, both technologies are leagues ahead of fossil fuel systems. Homeowners who want to reduce their heating carbon footprint would do well to consider either type of heat pump.

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Maintenance

Both ground source and air source heat pumps require some maintenance to keep them operating efficiently. However, maintenance requirements are relatively minimal compared to other heating systems.

For ground source heat pumps, the ground loop should be checked annually for leaks or damage. The heat pump unit itself may require filter changes and cleaning of coils to maintain proper airflow. Any mechanical parts, like pumps or fans, may need occasional repair or replacement as they wear out over time. Proper water quality in the ground loop is also important to prevent corrosion. Overall, annual inspections by a qualified technician are recommended.

Air source heat pumps have fewer maintenance needs than their ground source counterparts, as they lack the underground loop system. The heat pump filters should be changed as needed, based on the manufacturer’s recommendations, often 1-2 times per year. Outdoor coils should be cleaned of debris and indoor coils should be cleaned if they get dusty to maintain proper airflow. Any mechanical parts and electrical connections should be checked to make sure they are in working order. Checking refrigerant levels and recharging refrigerant may be needed over time.

While routine maintenance does require a small time investment, it’s worth it to keep a heat pump running at peak efficiency and avoid more costly repairs down the road. Following the manufacturer’s maintenance guidelines can help prevent issues and maximize the system’s lifespan. With just periodic maintenance, both ground and air source heat pumps can provide many years of reliable operation.

Incentives and Rebates

The Boiler Upgrade Scheme is a £450 million program providing grants to replace old gas boilers with heat pumps. Grants up to £7,500 are available for air source and ground source heat pumps. Homeowners and small businesses are eligible. This covers roughly two-thirds the cost of a typical installation.

Check if you qualify for these and other incentives to help make a heat pump installation more affordable. The long-term energy savings plus available subsidies can make heat pumps a smart investment.