Air Source Heat Pumps have been around for a very long time, almost exclusively in the commercial market place to heat and cool large office buildings. The transition to the domestic market has occurred only relatively recently. Some mainstream manufacturers of commercial ASHP's have attempted to modify there existing range of commercial VRV (variable refrigerant volume) heat pumps to meet this new demand, all with varying amounts of success.

Others, have opted for bespoke pieces of equipment designed specifically for domestic use. These air source heat pumps are typically slightly more expensive, but are usually more robustly constructed utilising stainless steel and high grade plastics in the shell and external module heat exchanger, a very important consideration in coastal environments.

It should be noted that in coastal locations especially, with large amounts of airborne salt in the atmosphere, the air will very quickly attack and corrode external units with aluminium fin heat exchangers. Also, some of the modified units can be very bulky and often rather noisy.

Can ASHP be Installed Anywhere?

Where air source units score over ground source units, is, as they utilise the outside air for energy as the energy source, no slinky collectors or geothermal probes are required, making the installation process less disruptive and costly. The ASHP's and can also be more suited to retrofitting into existing fossil fuel installations and of use within tight urban locations where ground space is limited, or access difficult.

Air Source Heat Pumps come in all manner of shapes and sizes, however they all have one factor in common, they all comprise an outdoor module and an internal module. The outdoor unit is where the fan unit is located and the choice of location is critical. It must not be crammed into a tight space, or be close to a high garden fence, tall shrubs, or up against the house, as the air will simply short cycle out one side of the fan unit and back in the other side, which will reduce its efficiency. Likewise noise from the fan motor must be considered, in the dead of night or early on a Sunday morning when all is quiet, the fan noise can travel many meters.

Actually How Noisy are They?

Sound levels vary from manufacturer to manufacturer, below are the noise levels from Nibe F 2005-8 and F 2005 -10/11 units, these are high quality bespoke air source units designed for the marine and coastal environment.

Sound Level F 2005-8 F 2005-10/11

Sound Pressure level at 1m. Fan low/high speed dB(A) 55.9/60.5 56.8/59.3

Sound Pressure Level @ 4m. Fan low/high dB(A) 43.9/48.5 44.8/47.3

Sound Pressure Level @ 10m. Fan low/high dB(A) 35.9/40.5 36.8/39.3

Does the Outside Unit Need any Special Building Works?

Once a suitable location has been chosen, the external module can be installed onto a simple base of garden slabs, when in auto-defrost mode, several litres of water are ejected from the base of the external module fan coil / heat exchanger, so consideration must be given to where it will go. In Winter it could easily freeze over and be potentially dangerous. Usually a simple soak-away constructed of gravel chippings suffices. The pipework and power can then be extended to the indoor module.

The indoor modules usually consists of the hot water store, volume expansion tank with pumps and valves plus the system controller. The hydraulic layout and general installation set up of the indoor module varies greatly between manufacturers. All of the internal modules require space for commissioning and future access for servicing. Generally, Air Source systems need little general maintenance other than to ensure the external fan, louvre and heat exchanger unit is unobstructed by snow, and leaves are not allowed to build up and impede the air flow through the fan unit.

Are They as Efficient as Ground Source Heat Pumps?

The simple answer is no. The temperature a couple of metres below the soil surface is between 10 and 12 degrees Celsius all year irrespective of seasonal weather conditions, so the energy extraction rate is stable and more advantageous than that of the air, which often falls very low during the winter. This is also when the maximum extraction is required to keep the building warm. ASHP units loose a great deal of efficiency in Winter and also can use a fair amount of power to defrost the external heat exchanger that freezes up during the air / brine heat transfer process.

Taking all matters into consideration, a good quality purpose designed air to water heat pump should deliver an average annual COP (coefficient of performance) in the region of 2.81 to 3.45. This is about 60% less than the GSHP but is still favourable when compared to virtually all fossil fuel boilers.

If you require more information or a quotation please contact us.