Air Source Heat Pump or Ground Source Heat Pump – Which is Best?
We are often asked by our customers whether they should fit a ground source heat pump or air source heat pump. In our experience, in all of the projects we encounter the most cost effective solution over the system lifetime is the air source heat pump.
To explain why this is the case we will first discuss what is the same and what is different between the types of heat pump systems available.
What is the same?
All heat pump systems, whether air source heat pumps or ground source heat pumps work by transfering heat from outside into the property. Using refrigeration technology it is possible to extract heat even if the outside air or ground outside is colder than the building to be heated.
What is different?
1. Ground Source Heat Pump Systems
A ground source heat pump system is designed to extract heat energy from the ground and then using conventional refrigeration technology transfer this heat into the dwelling heating system.
Ground source heat pumps extract heat from the ground in one of two ways.
- Vertical Boreholes
Vertical boreholes are used when ground space is limited and can be up to 100m in depth. Multiple boreholes are usually created and a loop circuit containing anti freeze is pumped into and back out of the borehole pipes. The heat in the return loop from the bore holes is recovered and concentrated in the ground source heat pump unit which is an internal appliance and this heat is transferred into the heating system. Note that vertical borehole heat pumps are not the same as geothermal systems as the average temperature at the depths involved is around 10 Degrees C so no active heating is taking place.
- Horizontal Ground Collectors
Horizontal ground collectors are comprised of pipe loops buried at a depth of approximately 1.2m and spaced 0.75m apart. Most systems comprise of multiple loops, each one having a flow and return path. All the loops converge onto a buried header from where the circuits are combined into a single flow and return loop which is passed into the heat pump where the heat energy is recovered and concentrated. Some horizontal ground collectors are implemented with coiled pipes to save area. These are known as slinkies.
2. Air Source Heat Pumps
An air source heat pump absorbs heat from the surrounding air by sucking air across a heat exchanger with a large fan or fans. This heat is effectively concentrated in a refrigeration circuit via a cycle of evaporation and compression and then transferred into the property central heating system to warm radiators, underfloor heating or the hot water cylinder.
There are broadly two types of air source heat pump.
- Monoblock system
A monoblock heat pump system consists of an outdoor unit which contains the refrigeration circuit and heating system circuit. The heating flow and return pipework pass from the outdoor unit into the property and into the heating system and hot water cylinder by an arrangement of valves. The advantage of a monoblock system is that it is easy to install as no refrigerant handling is required. The main disadvantage is that you are generally limited as to where the outdoor unit can be located as it is important to keep the external pipe run as short as possible. It is also necessary to ensure that the whole heating circuit has antifreeze protection. This is to protect the heat pump heat exchanger in the case of a loss of system power during freezing temperatures. Adding antifreeze to a large heating system does add a significant amout to the installed cost of the system.
- Split system
A split system consists of an outdoor unit and indoor unit. A refrigerant circuit passes between the indoor and outdoor units. The heating circuit is connected to the indoor unit which contains a heat exchanger where heat from the refrigerant circuit is passed into the heating system. The split system gives added flexibility on location of the outdoor unit as the pipe run can be much longer. There is also no requirement to load antifreeze into the heating system which provides an installation cost saving.
Ground Source Plus Points
Borehole ground source heat pumps are more efficient that air source systems due to the fact that the ground temperature at depth is the same all year round. Typical Coefficient of Performance (COP) values range from 3.0 to 4.5 (where higher numbers are better).
Ground collector systems are slightly more efficient than air source systems but generally less efficient than borehole systems and performance can be compromised by ground conditions, annual sunshine levels and poor system design.
With a ground source system there is generally no external equipment on view.
Ground Source Negative Points
The installation cost is significantly greater than that of an equivalent output air source system due to the groundwork costs associated with providing the boreholes or trenching for horizontal ground collectors.
With horizontal ground collectors problems can arise over time due to ground condition changes which lead to large reductions in system performance. Rectification is costly and sometimes not easily implemented.
A large garden or field is required and once a ground loop is in place that land cannot be used for anything else and must be left clear. This includes the construction of any buildings or planting of trees.
Bore holes are also subject to an expensive geological survey to ensure that the ground is suitable.
Air Source Plus Points
Whilst being slightly less efficient than an equivelent ground source system air source heat pumps are still highly efficient with typical Coefficient of Performance (COP) values range from 3.0 to 3.5 (where higher numbers are better). During the summer months when being used for hot water preparation air source heat pumps are actually more efficient than ground source since the air temperature is higher than the ground temperature at the ground source collecter depths.
Air source systems are easy to install with no requirement for ground pipe work or boreholes.
Installation cost is much less than that of an equivalent output ground source system. Comparative installed costs range from between 25% to 50% of that of a ground source system.
In our opinion when taken over the full service life of a heat pump system, for the majority of domestic applications the small increase in system efficiency granted by a ground source heat pump system does not provide the cost savings necessary to negate the additional installation cost of a ground source system, whether that be a borehole or ground loop system.
A high efficiency air source heat pump system shall provide many years of low cost trouble free heating and hot water without the high initial outlay associated with a ground source system and the possible problems which can be encountered with ground loops and bore holes over time.
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