Which is better - a ground source or an air source heat pump?
When it comes to specifying heat pumps, we are often asked which is better: a ground source or an air source heat pump. In actual fact neither are ‘better’. The decision comes down to design factors on a specific project, which will drive the specification to ensure the demands required by the building and occupants are met.
The ground source heat pump option
The two most important factors when choosing the right heat pump for your project are location and building type. Ground source heat pumps (GSHPs) require a network of ground loops to be buried in trenches or placed in boreholes to absorb energy from the ground, a buffer tank to smooth out the energy loads and a water cylinder to store the hot water to be used in the building. This means that the design must allow space for these to be included and generally requires early consideration to allow for the groundworks and infrastructure.
Once installed, however, ground source heat pumps can offer better overall operating efficiencies than air source alternatives due to the increased and more consistent temperature of the ground it is drawing energy from.
The air source heat pump option
Air source heat pump (ASHP) installations require less space compared to ground source as the energy is collected by the integral air coil in the heat pump. As ASHPs only need an external electrical and mains water connection, they offer lower installation costs and can be installed anywhere with an adequate air flow.
This makes them a popular choice for tall buildings where they can be situated on the roof, reducing internal plant room sizes and keeping them out of sight. As with ground source heat pumps they require a buffer tank to enable output during defrost cycles, which must be considered during the building’s design.
Considerations when specifying the number of heat pumps required
Once the building design requirements, and to some extent the designer themselves, show preference to a type of heat pump, there are some further considerations which will impact the final size and number of units:
1. Heat loss
2. Solar gains
3. Size of space to be heated
4. Hot water demand
6. Plant room and riser space available
These factors affect the natural heat lost (or generated) by a building and considering them will help to ensure that units are not under sized, which could impact on the building’s performance and occupants’ comfort. Selecting oversized units, on the other hand, could mean excess units are specified, incurring unnecessary expenditure and inefficient operation.
When choosing between ground source and air source, it therefore comes down to a project’s requirements and potential restrictions. Both systems can offer an increase in energy efficiency and have the potential to reduce the running costs of a building compared to other available systems.