What is an Air Source Heat Pump (ASHP)?
Air Source Heat Pumps absorb heat from the outside air to heat buildings. They work even when external air temperatures are below freezing (to approx. -15⁰C). ASHP’s can be set up to deliver heat via water for underfloor heating & radiators (“Air-to-Water”) or via air almost like a reverse air-conditioning system (“Air-to-Air”).
Why Install an ASHP?
ASHP’s are installed against the outside of a building, usually at ground level and as they are approximately the same size as a standard external air-conditioning unit they provide a very flexible solution for homes with limited roof space (for solar) or garden (for Ground Source Heat Pumps). Also, as they are quite unobtrusive they provide an attractive alternative to Wind Turbines and larger solar installations. In comparison with GSHP’s there is much less disruption as there is no need to excavate any adjacent land.
It should be noted that ASHP’s (as with GSHP’s) require electricity to power the pump and are therefore not strictly 100% “renewable” if the electricity is sourced from a standard utility tariff. However, for every kW of electricity used to power the pump, 3 to 4 kW of heat are generated, making ASHP’s an efficient way of heating your home (i.e. up to 400% efficiency). In addition, if electricity to power the pump is generated by a Photovoltaic system on the roof or a Wind Turbine the system can be made fully renewable. Even without this step, ‘green tariff’ electricity can now be sourced from a number of utilities (usually at a premium) in order to achieve the same outcome.
How does an ASHP work?
An ASHP is often described as a ‘refrigerator in reverse’. This is because it uses a refrigerant liquid which is warmed by heat from the outside air and which is then compressed and passed through a heat exchanger in order to heat water or air. Air-to-Water systems heat water which is then used to circulate through under floor heating or radiators. Note that ASHP’s produce hot water that is a lower temperature than a normal boiler would produce (approximately 35⁰C -45⁰C), which means that they are more suited to under floor heating than radiators. It should also be noted that larger radiators are often recommended due to this lower water temperature. However, in many cases, the existing boiler is retained with the water generated by the ASHP being used to preheat the water to 45% and the boiler generating the final 20° – 30°, meaning that the existing radiators are retained. These are described as “bivalent” systems as they use two energy sources.
Air-to-Air systems produce heat that is circulated by fans, in a similar way to conventional air-conditioning.
Will it work for me?
In order to install an ASHP you will need external wall space (approx 3-4m2 for a typical unit) to locate the evaporator coil and its housing. As with most Cleantech solutions, ASHP’s work at a lower temperature than a conventional boiler system & its important that the property has been properly insulated and draught-proofed. In common with Ground Source Heat Pumps it is important the the ASHP is correctly sized to the building in question and that as many heat-loss prevention measures as possible have been carried out to the fabric of the building prior to installation. Typically these would include increasing loft insulation to a minimum of 270mm depth and cavity wall insulation.
Without these modifications and especially in older properties, the benefits of heat pumps can be questionable. Further, replacing or supplementing an existing oil fed, electric or LPG heating system with a heat pump will be far more advantageous than in a house on the gas grid.
Finally, heat pumps do require changes in owner behaviour compared to conventional systems. As they run at lower temperatures and take longer to bring a room up to temperature, it is more efficient to have the heating on much longer than with a conventional system. This requires more planning than with a conventional system as is something that most of us are unused to. Failure to change you heating habits will however negate many of the financial and environmental benefits.