Evaluation of the UK Government’s Renewable Heat Premium Payment (RHPP) scheme

Overview:

Multiple research approaches to empirically determining the performance of domestic heat pumps in the largest European field trial to date (699 dwellings in total). The full reports from the RHPP project can be found here.

Aims:

Heat pump technical performance – Determine the median performance defined in terms of the SPF, and its variation.Determine the performance by group (e.g. air source heat pumps vs ground source heat pumps and other classifications) and explore possible reasons for variations in performance.

MCS Compliance – Understand the effect of the Microgeneration Certification Scheme (MCS) on heat pump installation, commissioning and performance.

Sociotechnical case studies – Explore householder satisfaction across a number of metrics, and its relation to the technology and its actual performance through socio-technical case study research.

Recommendations – Recommendations for future heat pump trial design.

Key Findings:

Heat pump technical performance:

• Data for statistical analysis was generated by physical monitoring systems in each dwelling installed and operated under the supervision of the BRE. This data was complemented by metadata, provided by BRE and the Microgeneration Certification Scheme (MCS).
• A wide distribution of SPFs was observed, thought to be partly due to the effect of metering error and partly to real differences in heat pump performance. The latter in turn relate to differences in heat pump technology (for example, ground source and air source), and to differences between dwellings, patterns of use, and occupants’ lifestyles.
• The statistical analysis showed higher SPFs of ground source compared to air source heat pumps, and of air source heat pumps with underfloor heating, compared to those with radiators.
• In addition to the statistical analysis, detailed case studies of 21 heat pumps were undertaken. These showed additional factors at work. Among these was the effect of heat pump load factor, which appeared to have a strong effect in a sub-group of three similarly sized dwellings with identical heat pumps.
• A large proportion of air source heat pumps displayed high frequency cycling behaviour, with periods of 10-minutely (on-to-on) cycles. This problem did not significantly affect ground source heat pumps.
• Winter space heating flow temperatures were generally below 50°C.
• Some sites showed excessive use of immersion heaters for domestic hot water heating, and there were several clear examples of poor control related to immersion heaters.
• The RHPP field trial did not reveal the performance of heat pumps during a period of very cold weather.

MCS Compliance

• The Microgeneration Certification Scheme (MCS) publishes guidelines for the design and installation of heat pumps systems in dwellings. Datasheets completed by heat pump installers through the MCS scheme were made available to the project team. These were compared against schematics provided by the BRE and the physical monitored data itself.
• Heat pump sizing for design conditions was found to be either poorly understood or poorly expressed. However, a majority of heat pumps appear to be adequately sized when compared to peak measured load.
• A comparison of installers’ estimates of annual heat demand with measured values indicates a relatively poor correlation. Calculating annual energy use by integrating the technical complexities of heat pump whole-system performance and occupant operational preference is a complex socio-technical challenge.
• Radiator sizing analysis indicates that ‘star rating oversize factors’ as described in the Heat Emitter Guide (HEG) published by the Microgeneration Certification Scheme (MCS) may be inadequately understood or ignored due to practical and aesthetic considerations of size and location.
• Correlations between the Heat Emitter Guide (HEG) SPF and measured space heating SPF are poor, with the observed SPF’s being significantly lower than the HEG values.
• Weather compensation was used for 64% of installations.

Further insights from sociotechnical case studies

• For a sub-sample of 10 site visits, the quality of planning and pipe insulation was assessed. Three heat pump systems were found to be poorly planned/insulated, one was classed as “intermediate” and the remainder were “of good quality”.
• In eighteen out of twenty-one cases, occupants were satisfied or very satisfied with their heat pumps and preferred them to their previous heating system.
• But the case studies revealed the rich complexity of the notion of satisfaction, which included the level of thermal comfort felt, running costs, ease of use, environmental impact, technical integrity, noise levels and controllability of the system.
• There was a high amount of heterogeneity in the way that case study households used and controlled their heat pump.
• Despite the high levels of householder satisfaction, at least 10 out of the 21 cases had experienced some significant problem since installation.
• Cross-checking of monitored data with interview data yielded helpful insights into physical monitoring installations and the monitored data

Recommendations

• The complexity of energy technologies in the domestic setting requires multiple research methodologies, rather than a purely statistical approach.
• Socio-technical, mixed methods case studies are likely to add significant value to any field trial of new energy technologies. Conversely, the value of case studies is likely to be significantly enhanced by the availability of high quality physical monitoring and energy bill data.;
• There is a trade-off between redundancy, complexity and cost (including cost of quality assurance) in any physical monitoring project.
• Data collection systems need to be trialled and thoroughly understood before being deployed at scale.
• Richer metadata and more thorough commissioning of metering equipment is required to maximise the value of field trials of domestic energy technologies.
• Data on internal temperature and previous energy use is essential to understanding the performance of heating technologies in the field.

Impact, Influence and Outreach:

Two industry engagement events were held to enable the heat pump industry to comment on the work. Final Project Reports were reviewed by two internal reviewers, and by UK representatives of the domestic heat pump industry.

The processed data have been stored on the UK Data Archive.