Air source heat pumps (ASHP) work on the same principal as a ground source heat pumps (GSHP), however, the medium from which heat is extracted is the external air rather than the ground. ASHPs will generally have a lower seasonal CoP compared to GSHPs due to the generally lower air temperatures compared to the ground. They may require additional backup in extremely cold conditions and this facility is normally provided by electric heating built into the system. Its requirement will be limited, but it affords peace of mind to the end user.
ASHPs can either be internal modules, ideally installed in a garage, basement or utility room (normally linked to whole house ventilation systems), or external modules, normally next to a wall of the building they serve. The location of an external unit is important for performance as exposure to high winds can cause defrosting problems. It is therefore advisable to create a fence or hedge around the unit to protect it from high winds. In addition, the fans and compressors do make a small noise (typically imperceptible and akin to an oil/gas boiler firing up), so it is worth considering locating the plant away from windows and adjacent buildings. It is also possible to reduce this noise by mounting the unit on a noise-absorbing base. Manufacturers should display the noise levels of the units to enable an informed selection.
A typical system comprises three key components; the absorber, the boiler and the cylinder, although fully external, all-in-one units and other variations are also available.
The weather-proofed outdoor absorber unit can be located up to 30-70 meters away from the indoor hydro-box boiler. It contains the heart of the heat pump system, absorbing low grade heat from the ambient air, upgrading it to high grade heat, which is transferred via refrigeration pipe-work to the hydro-box boiler. Absorbers will be between 80 – 120cm high, 90cm wide and 30 – 40cm deep. Acoustically, they tend to have noise levels of around 50dB, but are normally mounted on rubber matting to counter any issues of disturbance.
The indoor hydro-box boiler is a wall-mounted compact unit that can be located anywhere in the house. It does not require flues or ventilation. Its main function is to transfer the absorbed heat from the outdoor unit to the house heating and hot water system. Dimensions are typically 90 – 120cm high, 500 – 650cm wide and 350 – 600cm deep. The domestic hot water tank is the final key component and is normally complete with recirculation connection, 1.2m tank, tank heater overload sensor and controls.