What are the differences between ASHP and GSHP

Air source heat pumps and ground source heat pumps work along very similar lines. There are, however, some minor differences between them which can have fairly significant implications. With that in mind, here is a quick guide to the key differences between air source heat pumps and ground source heat pumps.

Understanding the technology

The key difference between air source heat pumps and ground source heat pumps is that the former draw heat from the air and the latter draw heat from the ground. This means that the former use fans and the latter use pipes laid under the ground.

Once the heat has been drawn in, the rest of the process is essentially the same for both forms of technology. The heat is absorbed into a liquid (called a refrigerant). This causes the liquid to evaporate, thus creating gas. The gas is compressed to raise the temperature further.

This warm gas is transferred to the home in the form of heating and/or hot water. As the heat is transferred to the home, the refrigerant cools and returns to its liquid form. The cycle then begins again.

The issue of efficiency

In the UK, air temperature rarely goes below -5°C or above +25°C. Modern air source heat pumps can work effectively in temperatures as low as -15°C. Ground temperature in the UK rarely goes below +4°C or above +18°C. This means that both air source and ground source heat pumps should be suitable for year-round use no matter where you live.

Both air source heat pumps and ground source heat pumps need electricity to operate. How much electricity they need depends on how hard the pump has to work. This in turn depends on how much ambient heat they have to work with. This means that the efficiency of air source heat pumps tends to be very high in summer and very low in winter.

Ground source heat pumps, by contrast, tend to have much less seasonal variation. This does not mean that they are necessarily more efficient overall. That would really depend on where you live. It does, however, mean that your energy needs (and hence bills) are more predictable.

The issue of upfront installation

Air source heat pumps are installed entirely above ground. This means that they are much simpler and therefore quicker to install than ground source heat pumps. In general, they are classed as permitted developments. In fact, they are often actively encouraged as part of the UK’s commitment to reaching net-zero by 2050 at the latest. If, however, you live in a conservation area, national park or listed building, you may be stopped from installing one.

Ground source heat pumps need to have piping installed underground. The most economical approach is to lay them horizontally. This usually requires 2.5-3 times more ground space than the floor area of your house. If you do not have this much available land, you can have the pipes laid vertically using boreholes but this will be more expensive.

The actual pump unit will be installed inside your home. Modern ground source heat pump units are about the size of an under-counter fridge and have a similar noise level.

The issue of maintenance and lifespan

In principle, both air source heat pumps and ground source heat pumps should run without any maintenance issues. In practice, the fact that air source heat pumps are exposed to the elements can make them vulnerable to weather damage. This is particularly likely in coastal locations where there is salt in the air.

As a rule of thumb, you can expect air source heat pumps to run for a good 10 years. Ground source pipe arrays by contrast can last for up to 100 years. The internal pumping units can be expected to last for at least 20 years.