Technical newsletters about emerging issues & our latest research

 

Introduction

Air Source Heat Pumps (ASHPs) can be an effective route to achieving carbon dioxide (CO2) reduction targets under Code  for Sustainable Homes Levels 3 and 4 and they can meet Part L1a of the Building  Regulations.

Whilst ASHPs can reduce absolute emissions of carbon dioxide when compared with conventional gas heating, there are a number of issues to keep in mind:

  • The British climate: The ASHP product selected must be proven to maintain performance at acceptable levels in British winters (low temperatures and high Relative Humidity (RH)). In these conditions, the external unit freezes.
  • Rating of systems: The heat pump must be correctly sized with respect to the heat losses of the dwelling so as to minimise frosting of the external unit and consequent loss of performance.
  • Heat distribution: Underfloor heating is ~50% better than enlarged radiators in terms of Coefficient of Performance (CoP).
  • CoP: This factor must be treated with caution as it is an instantaneous measurement and does not take account of varying external conditions throughout the year.

The Code is based on a relative comparison rather than absolute CO2 levels. The high electricity emission factors used within SAP coupled with the fuel factor adjustment for the Target Emission Rate (TER) make ASHPs cost effective at meeting Code requirements.

Very few manufacturers publish test data for British conditions of low temperatures and high RH conditions. This must be sought so as to prevent the installation of inappropriate ASHPs which would increase CO2 emissions and involve high fuel bills. There are ASHPs which are designed specifically for the British climate and their manufacturers have released performance data for these conditions. Such ASHPs have demonstrated that they are able to provide the level of performance required for real gains to be achieved in the UK.

 

ASHPs: The Technology Explained

This note is principally concerned with Air-to-Water heat pumps: that is those that take energy from external air and input this energy into a house with a wet heating system through radiators or underfloor heating. These are the systems most commonly specified by developers in order to meet Code targets; Air-to-Air Systems are also available.

The lower the temperature difference between the heat source (external air) and the heat sink (space heating circuit) the higher the CoP. With the majority of the energy for space heating coming from the environment, ASHPs are extremely energy efficient.

Manufacturers routinely advertise CoPs in excess of 4. However, the concept of a CoP must be treated with caution as it is an instantaneous measurement and does not take account of varying external conditions. CoPs from manufacturers are based on British Standard BS EN 14511 which specifies an outdoor air temperature (source temperature) of 7oC and a sink temperature (heating circuit) of 40-45oC.

ASHPs are commonly described as refrigerators in reverse. ASHPs rely on a refrigerant, which is a substance with a low boiling point and high heat of vaporisation/condensation (~latent heat) which readily changes state between a gas and a liquid by absorbing/releasing energy. These processes each take place in a heat exchanger, both of which make up a heat pump. The heat pump process is as follows:

  • Energy is absorbed from the heat source (in this case the external air) in the evaporator (heat exchanger) through the boiling of a liquid refrigerant to a gas. This process reduces the temperature of the utilised external air by ~5oC.
  • The resultant, comparatively high energy gas is then compressed to increase its temperature. The compressor is driven by electricity.
  • The compressed high energy gas is then taken to the second heat exchanger – the condenser. The condensation of the high energy gas to a lower energy liquid releases heat energy which is then used to power the central heating circuit (radiators or underfloor).
  • The liquid from the condenser is returned to the evaporator to continue the cycle.

Effectively, energy from the external air (high volume, low temperature) is upgraded for input to a dwelling (low volume, high temperature).

Installation issues

Developers are coming to grips with the installation of these systems. The following issues need to be overcome:

  • Split systems have both internal and external units. Internally they occupy space previously given over to gas heating systems. Space must also then be found for external coils. These are typically attached to exterior walls for flats, and installed on the patio of houses. Picture 1, below, shows an example of an external unit.
  • ASHPs work most efficiently providing ‘low grade’ heat. For this reason larger emitter surfaces are best, and connection to underfloor heating is recommended by many suppliers. The CoP of a system with radiators may be 50% lower than with underfloor heating.

The concept of a CoP must be treated with caution as it is an instantaneous measurement and does not take account of vaying external conditions throughout the year.

ASHPs can operate at external temperatures down to -20 oC, but they operate best when the temperarture difference between the heat source (exterior) and heat sink (dwelling) is low. This means their performance is reduced in winter months when it is most needed.

Temperature is not the only issue: Relative Humidity (RH) has a major influence on the operation of the ASHP at low ambient temperature as it leads to frosting:

  • An ASHP reduces the local temperature of the source air by ~5oC. Freezing of the ASHP therefore occurs when the external temperature falls below 5oC. This leads to frost build-up on the heat pump which reduces air flow and consequently performance. In extreme cases frosting can cause the shutdown of the system.
  • To defrost the external element the ASHP must be reversed, thereby diverting heat which would be used for space heating and causing a drop in performance. Correct sizing of the ASHP for to reduce the level of frosting is vital.
  • Frosting is a particular problem in Britain which has very humid winters. The CoP under these conditions is dramatically reduced and can be as low as 1 for some systems. Such performance is not acceptable as CO2 emissions and fuel bills will be substantially higher than when using gas.

For the appropriate use of ASHPs in Britain it is vital that performance is known and acceptable in British winter conditions. Many ASHPs are imported from Scandinavia, or further afield, and are not designed for British conditions.

Very few manufacturers publish test data for British conditions of low temperatures and high RH conditions. This must be sought so as to prevent the installation of inappropriate ASHPs which would increase CO2 emissions and involve high fuel bills. There are ASHPs which are designed specifically for the British climate and their manufacturers have released performance data for these conditions. Such ASHPs have demonstrated that they are able to provide the level of performance required for real gains to be achieved in the UK.