Most heat pumps are not being installed correctly and do not perform as well as expected, finds a survey of 83 sites recently published.
Heat pumps are one of the government's answers to increasing energy efficiency in homes. If the Green Deal, or the Renewable Heat Incentive, go ahead, the advice behind government policy suggests that heat pumps are highly cost-effective for saving carbon per £ of money spent.
But this report confirms what the Low Carbon Kid has said before - they only do so under certain circumstances.
Heat pumps are a new and growing technology: during 2009, their installed base doubled with annual sales of around 14,000 units.
However, there have been very few field trials to determine whether they actually match up to expectations in real-life installations.
Moreover, their effects on reducing carbon emissions is widely misunderstood.
Now, the first study of installations in the UK has been published by the Energy Saving Trust. It provides a mixed picture.
How do they work?
The principle of heat pumps is the same as a fridge, but backwards: if it was operating in a fridge, heat, instead of being pumped out of the back of the fridge, would be pumped from the outside of the fridge, into it.
This would heat the fridge up because low temperature air from large volume would be concentrated into a higher temperature in a smaller volume.
If, instead of a fridge, we have a building or a room, the same principle applies on a larger scale.
Heat pumps can take heat from the ground, air or a nearby body of water if it’s available.
Many heat pumps are reversible, and can be used for cooling – except, of course, it would be better if the home could be cooled without using electricity (solar cooling is real, commercial, and one of the first uses of solar energy in 1876).
How do we judge a heat pump?
Heat pumps are judged by their coefficient of performance (CoP). This is the ratio of the amount of heat or coolth produced divided by the electricity consumption of the pump used to operate it.
So for example a heat pump with a CoP of 3 (or 3:1) will produce three times as much heating or cooling energy as the electrical energy it consumes.
The higher the COP, the better the performance. The COP depends on the difference between the temperature of the source and the final delivered temperature. The greater the difference, the lower the COP.
Therefore, heat pumps are far more effective if used for space heating which is delivered by radiant heating - underfloor heating or skirting board heating - which can be at a much lower temperature - for example 20°C - to achieve the same level of thermal comfort as central heating radiators kept at a much higher temperature. This depends on the overall heating and system design.
As for the temperature of the source, the ground, 10 feet or so under, has a relatively stable temperature in this country and rarely goes below freezing, whereas the air can go below freezing in the times that we need heating the most. This negatively affects the performance of air source heat pumps, reducing their efficiency severely. So in these situations in air source pumps are at a disadvantage.
So how did the heat pumps perform?
The 83 heat pump systems performed very differently. Many systems appeared to be installed incorrectly.
The air source products had a COP that varied between 1.2 and 3.3. The ground source ones were slightly better, varying between 1.3 and 3.6.
Overall system efficiency was approximately the same, but slightly less.
This compares to a European trial where the ground source heat pumps performed significantly better than air source.
The effect on carbon emissions
What the report does not discuss is how effective they are at reducing carbon emissions. This depends entirely on what form of heating for space or water the heat pump is replacing, either theoretically or in practice.
If the heating was previously supplied by electricity, and the electricity was not renewable, then the COP needs to be consistently over 3 to make any difference to carbon emissions.
This is because of the losses - typically over 70% - in efficiency between the burning of fossil fuel in the generating plant and the arrival of the electricity at the heat pump.
If the electricity is renewable, there is clearly a benefit.
They are most effective at reducing carbon emissions and heating bills if they are replacing heating fuels such as electricity, LPG and oil. They are less effective if they are replacing gas. In fact they may have a negative effect on carbon emissions, if the COP is below 3.
The main non-technical factors affecting the performance of the heat pumps are the system design, installation and customer behaviour.
The report concludes "it is essential that installation and system design meet the heat demand of the particular building".
It recommends improved consumer advice for the use of the controls, which many users found confusing. This was especially true of social housing tenants, if they are not actively involved in the choosing, installation and training process.
One problem with installation was that often there was "no single contractor responsible for installation, which might involve a ground works contractor, a plumber, a heat pump installer and an electrician. This meant that there was often no single point of responsibility for the whole installation".
The report concludes that the performance of heat pumps depends very much on installation and commissioning practices. It recommends a thorough review of installation guidelines and proper systematic training for installers.
The Energy Saving Trust is working with trade associations, manufacturers and the governments and the Microgeneration Certification Scheme to identify improvements in installation guidelines and training.
The industry response
"The heat pump industry is addressing these issues through major investment in training and support of the new National Occupational Standards published by Summit Skills earlier this year. Industry is also actively engaged in the successful development of a National Skills Academy," said Kelly Butler, BEAMA's marketing director of the British Electrotechnical and Allied Manufacturers Association, in response to the report.
"This year, an estimated 2,000 installers have been trained in heat pump design and installation. By 2020, under the new qualification framework, 8,000 installers will be trained to help install some one million heat pumps," Butler said.
The majority of field trial sites actually pre-date government's relatively new Microgeneration Certification Scheme (MCS), which is also supported by the heat pump industry.
This scheme certifies product and installer standards and currently has 357 products and 370 installers approved. More than 200 additional heat pump products are currently in the process of approval.
In addition, BEAMA says its Underfloor Heating Manufacturers Association will be seeking to publish guidance on the advantages of effective low temperature heating systems.
Heat pumps are a relatively new technology being rolled out quickly on a mass scale. If they are to have the effect on energy efficiency and carbon emissions that the government hopes, they certainly need to be installed and operated with much greater knowledge and sensitivity to how they work.