Ground source or air source
Regardless of whether you install an air source or ground source heating system, you'll get a highly efficient unit. Which one would be best for your house?
Mass deployment & net-zero ambitions
In any heat pump rollout program that utilizes large numbers of air source heat pumps, backup generating, and storage capacity will be much higher than for ground source heat pumps. In cold weather, air source heat pumps’ efficiency is compromised due to a drop in air temperature, which coincides with a rise in heat demand.
On the coldest day of the year, if every home in the UK were equipped with air source heat pumps, it would require additional power grid capacity equivalent to 2,400 x 10MW wind turbines or 8 x 3GW nuclear power plants compared to homes that were heated with ground source heat pumps.1
Temperatures under the surface of the ground (and the water) remain fairly stable all year round, regardless of the time of day. Consequently, the ground source heat pump’s source temperature can be up to 15°C warmer than the cold air coming into an air source heat pump on a cold day. When the source temperature is higher, the ground source heat pump has to do less work to generate the temperatures needed for home heating; this means less electricity is consumed, leading to greater efficiency.
Electrification of heat will require less generator capacity if ground source heat pumps are deployed in mass. Due to the fact that ground source heat pumps can run when electricity demand is low and prices are low, as well as when renewable electricity sources, such as wind and solar, are more available, which means that the electricity supplied contains the lowest amount of carbon.
- Based on the following conditions:
24 million homes with 6kW average heat loss
Coldest day efficiency for air source vs. ground source heat pumps – ASHP: 200% , GSHP: 300%
In light of its constant exposure to the elements, air source heat pumps typically have a life expectancy of just 10 years. Moreover, there are some places where air source is not recommended at all, such as near the sea, where saltwater causes internal components to rust and stop working.
In contrast to air source units, ground source units are enclosed systems that are installed safely inside the property, protecting the assets from environmental damage and preventing theft. In addition to needing very little maintenance, the unit itself has a design life of 20 years, while the buried ground array it connects to has a lifespan of more than 100 years.
Upfront price & running costs
In comparison to ground source heating systems, air source heat pumps are typically cheaper, since you only need to pay for the unit itself. In contrast, ground source units are more expensive due to the need for groundwork and ground arrays. Nonetheless, air source heating could end up costing end-users more in the long run due to the lower efficiency of air source heat pumps and the need to pay for more electricity to bring colder air up to temperature.
A second disadvantage of air source heat pumps is their ineffectiveness with time-of-use tariffs – which provide customers with a dynamic pricing system based on demand on the electricity grid. By avoiding running appliances during peak hours, these innovative tariffs allow users to save money. In the case of an air source heat pump, however, this is not relevant because the air temperature is often significantly cooler at 2 am – when the tariff is likely to be cheaper – than at 2 pm, so an air source heat pump will perform at its least efficient at these times.
Since ground source heat pumps are buried at sufficient depth and insulated from such fluctuations, they perform well regardless of the external temperature, and they can even be run overnight when electricity prices are at their lowest.
Initially, an air source heat pump seems more attractive due to its lower upfront cost. Due to lower efficiency, any initial savings could be quickly negated by higher running costs.
Due to their exposure to the elements, air source units have a shorter lifespan. The Boiler Upgrade Scheme also provides them with less grant funding than ground source heating. In terms of internal heating distribution, such as radiators and underfloor heating, air source and ground source heating share similar costs. Overall, ground source technology is extremely efficient and pays off in the long run.
Differences in Efficiency
In air source heat pumps, heat is generated from air and then pumped into a building at higher temperatures. Air source heat pumps vary in efficiency according to the season and time of day. The air – the unit’s source of energy – is at its coldest in winter, so the unit will need to use more electricity to function efficiently when you need heating the most.
It can be difficult to determine the true efficiency of air source heat pumps. Normally, their performance is based on an inlet temperature of 7°C, which is unrealistic since the air temperature fluctuates. In contrast, ground source heat pumps are tested with an inlet temperature of 0°C – representative of real-life climate conditions.
In winter, the average ground temperature will always be significantly warmer than the average air temperature since the ground can maintain temperatures of 10-12°C all year. Consequently, a ground source heat pump can have a source temperature up to 15°C warmer than an air source heat pump on the coldest day.
A ground source heat pump uses less electricity, since it doesn’t have to work as hard to upgrade the source’s energy into usable heat for space heating and hot water – making it a more efficient heating and hot water system.
Ground source heat pumps are also more efficient than air source heat pumps in terms of operational time. The use of smart controls and time-of-use tariffs enables ground source heat pumps to participate in load shifting, where electricity consumption can be shifted to low-cost or low-carbon hours.