Getting up-to-speed with domestic heat pumps

This page contains information that may be helpful to know prior to a site visit and quotation from an installer. It is not a complete list and should be considered only as a start.

1. Coefficient of Performance (COP)

The Coefficient of Performance (COP) is the useful heat energy delivered divided by the electrical energy you pay to run the heat pump. When the temperature difference is small, it is relatively easy to move heat from the colder outside air to the inside. When the temperature difference is large then it becomes more difficult. For an air source heat pump that problem gets even harder with ice build up on the outside unit. Most will have an automatic de-icing mode that keeps the unit ice free. This will take more energy to run too. The variation in Coefficient of Performance with outside air temperature is shown for an example air-to-air air source heat pump in Figure 1.

Example Heat Pump CoP change with Temperature
Figure 1. Change in Coefficient of Performance with Outside Air Temperature for an example Air-Source Heat Pump

Manufacturers will report a seasonally-adjusted coefficient of performance which, for the example unit in Figure 1, is just above 4. This number takes into account the variation in outside temperature through the year to give a single representative number of performance. It is clear that the number will depend on the climatic zone so check that it is the appropriate number for the location.

For air-to-air heat pumps, the coefficient of performance depends on the performance of both the outside and inside units. Internal high COP units may be available and may be worthwhile.

Be aware that manufacturers may have specific models designed for different climates too. This may be worth checking as well.

2. Global Warming Potential (GWP) of Refrigerants

While the refrigerants used in modern heat pumps are no longer ozone depleting CFCs (chlorofluorocarbons), they are often HFCs (hydrofluorocarbons) and so a small quantity released would contribute a relatively large warming effect on the environment. This is why it is particularly important to ensure heat pumps and associated equipment are installed, maintained and replaced by F-Gas registered businesses (

Global Warming Potential (GWP) indicates how much damage is done by comparing it to carbon dioxide over one hundred years. The larger the number the more the heating effect is. Refrigerants used today include R32 (GWP 675), R410a (a 50:50 mix of R32 and R125, GWP 2088) and R134a (GWP 1430) [1]. Some very new units use R744 which is actually carbon dioxide, so this has a GWP of 1. There is a commitment to phase out the use of refrigerants with very high GWP, so it is likely that heat pumps will ultimately use something like R744. Automotive air-conditioning has moved from R134a (GWP 1430) to R1234yf (GWP 4) in recent years.

3. Noise

Noise is a consideration for Air Source Heat Pumps. Noise is strongly influenced by the speed of the air exiting the fans into the room or outside. Therefore quiet operation is with lower fan speeds and lower air speeds. A model with a larger fan is likely to move more air and so the air speed can be reduced giving a lower noise operation. Look for low noise models with large fans if noise could be an issue. The technical specifications will include noise measurements.

4. Outside Unit Positioning

Outside unit positioning is a consideration for Air Source Heat Pumps.

  • Consider locating in a position that is not going to affect the appearance of the property – consider along the side or at the back of the property. Lower down is less visible than higher up on the wall.
  • Consider the electrical cable run from the distribution board to the the external unit.
  • Consider locating so that the fan is not directly impinging on neighbouring properties (ideally some sound barrier such as a fence or vegetation between the unit and the neighbouring property).
  • Consider that the front, sides and back of the unit should be clear of obstruction to allow efficient airflow round the outside unit.
  • Consider how to ensure condensates drips away from the property (a slight slope on the concrete base if the unit is to be on the ground)
  • Consider how ice may form under the unit in winter. Best to slope the concrete and use a concrete mix that is ice resistant
  • Consider the distance of the unit from the property. While most pipes are well-insulated, there is the possibility of some heat loss before that heat gets to where it is intended. May need extra insulation if the pipe run is long.

5. Inside Unit Positioning

Inside unit positioning is a consideration for air-to-air Air Source Heat Pumps.

Multi-split units can be placed in multiple rooms in a property to distribute heat directly to the rooms that are to be heated. Multi-splits work well with open plan living and fewer units may be needed.

  • Consider that the unit should face the area being heated and be fairly central to it (although many will allow for the unit to be offset to the side).
  • Consider that the inside unit will need power and the refrigerant circuit (pipes) connected to the outside unit.
  • Consider that the inside unit will need a way of allowing condensate to drain away.
  • If located on an external wall, the power, refrigerant and condensate pipes can exit on to the exterior of the property easily and then the power and refrigerant pipes run along the exterior wall to the unit. Consider that if the outside unit is offset from the interior unit then there will be pipes visible on the exterior and this may not be aesthetically pleasing. It could be covered under plastic or perhaps even wooden ducting. Alternatively the pipe run could be kept on the inside of the wall if this is better. Installers may have ideas on how to hide the pipes.
  • If located on an interior wall then the installation can be a little trickier. The internal wall would need to be able to support the unit (a block wall works nicely) and the unit condensate pipe needs to either run on a slight downward slope outside or connect to a drain inside the property. To hide the cable and pipes then some new plasterboard may be required. It may be possible to connect to an existing pipe run often boxed in the corner of a room. Installers may have ideas or you could look online for how people hide pipes.
  • In my example I had one unit on an inside (block) wall and had the pipes running through the kitchen to get there. The pipes are hidden by running them above the kitchen cupboards. The condensate pipe gently slopes down and manages to just pass above a doorway running along the top frame before linking up with a sink drain in the downstairs bathroom.
  • Consider the risk of winter ice build up on any condensate pipes exiting outside the property (this is still a common problem with condensing gas boilers). This might mean connection to a drain and insulating the pipe, or shortening the pipe to not give opportunity for drips to freeze within it. Installers may have good experience with what works.

Click here to see how to calculate the cost of a heat pump.

Click here to see how to calculate the carbon dioxide emissions of a heat pump.

[1] “Report of the Conference of the Parties on its nineteenth session, held in Warsaw from 11 to 23 November 2013”, Warsaw COP19, United Nations Framework Convention on Climate Change, January 2014,

  • Air-to-water Air Source Heat Pump behind vegetation
  • Air-to-air Air Source Heat Pump outside unit
  • Air-to-air Air Source Heat Pump inside unit
  • Air-to-water Air Source Heat Pump fan