The installation of heat pumps needs to significantly improve if the technology's potential to cut carbon emissions is to be realised, according to the Energy Saving Trust (EST).
There were more than 10,000 heat pumps operating in the UK in 2008, according to Eurobserver's latest report on the technology. The last government said they could supply 4% of heat demand by 2020.
The devices work like fridges in reverse. They take heat from either the air or ground to warm a liquid. This is then compressed to produce a higher-temperature vapour, which is then condensed so that heat is released and can transfer to the water used to run a building's central heating system.
Earlier this month, the EST published results of a year-long field trial of 54 ground-source, and 29 air-source, pumps.
The trial included units installed under the Low Carbon Buildings Programme (LCBP) – the previous government's grant scheme for microrenewables, and the Carbon Emissions Reduction Target (CERT). CERT requires the big six energy companies to improve the energy efficiency of their customers' homes.
A "well-performing" heat pump should have a 'system efficiency ratio' of at least three, the report says. This means that for each unit of electricity needed to run a building's heating system, the heat pump will produce at least three units of heat. However, only four (14%) of the air-source, and nine (19%) of the ground-source, pumps, reached that (see figure).
"Many systems appeared to be installed incorrectly," the report says. This could include under- or over-sizing of units for a building's heat needs. The EST will investigate the cause of each pump's poor performance during the trial's second year. However, it has already decided there is a need to publish guidance for installers. This will be issued in the autumn.
Pumps could also be performing below expectations because of how they are operated by consumers, the report says. Many people found the controls hard to understand and the EST says they need to be "comprehensively reviewed".
The EST also intends to examine the impact of hot water production on heat pumps, which may have affected performance in some cases. A heat pump needs to be able to reach a higher temperature if it is to supply hot water as well as heat.
ENDS contacted several major manufacturers of heat pumps to ask them about the study's results. All stressed they took every effort to ensure installers were properly trained. Mitsubishi, which has installed more than 6,000 air source pumps in the UK, was typical. In a statement, it said: "We have always insisted that our installers attend product-specific training. We therefore welcome the EST's call for a training and standards roadmap and would also support the establishment of minimum compulsory standards."
However, none could explain why pre-existing pumps presumably installed by trained professionals had performed badly. They all simply pointed out that the EST report does not name any individual firms.
The poor performance of some pumps could also have wider implications for the UK's target to get 15% of its energy from renewables by 2020.
The European Commission is developing standards for when heat pumps count as renewable energy devices. Current proposals would require a system efficiency of 2.6, according to Roger Nordman of the Swedish SP Technical Institute, who peer-reviewed the EST study. Only 26% of the units in the trial met that level.
However, the Commission would probably simply require devices to meet the standard in laboratory tests rather than require auditors to monitor installations.
In spite of the problems experienced, the EST report is overwhelmingly positive about heat pumps' potential. A well-installed unit would save 280 kilograms of CO2 a year when replacing a gas boiler, it says, and 1 tonne of CO2 a year if replacing an oil heating system.
It "particularly recommends" heat pumps are installed in the 5 million UK homes not connected to the gas grid, and in new-build properties. If installed in all off-grid homes, heat pumps would save 10 million tonnes of CO2 a year – equivalent to almost 1.7% of UK greenhouse gas emissions.
It points out that the government does need to support devices using the proposed renewable heat incentive. Without this, payback could be as bad as 47 years if replacing gas central heating.