Challenges for the Transition to Low-Temperature Heat in the UK: A Review

Abstract

To reach net-zero emissions by 2050, buildings in the UK need to replace natural gas boilers with heat pumps and district heating. These technologies are efficient at reduced flow/return temperatures, typically 55/25 °C, while traditional heating systems are designed for 82/71 °C, and an oversized heating system can help this temperature transition. This paper reviews how heating systems have been sized over time in the UK and the degree of oversizing in existing buildings. It also reviews if lessons from other countries can be applied to the UK’s building stock. The results show that methods to size a heating system have not changed over time, but the modern level of comfort, the retrofit history of buildings and the use of margin lead to the heating system being generally oversized. It is not possible to identify a specific trend by age, use or archetype. Buildings in Scandinavia have a nascent readiness for low-temperature heat as they can use it for most of the year without retrofit. Limitations come primarily from the faults and malfunctions of such systems. In the UK, it is estimated that 10% of domestic buildings would be ready for a supply temperature of 55 °C during extreme external conditions and more buildings at part-load operation. Lessons from Scandinavia should be considered with caution. The building stock in the UK generally underperforms compared to other EU buildings, with heating systems in the UK operating at higher temperatures and with night set-back; the importance of providing a low-return temperature does not exist in the UK despite being beneficial for condensing boiler operation. Sweden and Denmark started to develop district heating technologies with limitations to supply temperatures some 40 years ago whereas the UK is only just starting to consider similar measures in 2021. Recommendations for policy makers in this context have been drawn from this review in the conclusions.

To Zone or Not to Zone When Upgrading a Wet Heating System from Gas to Heat Pump for Maximum Climate Impact: A UK View

“…It is not clear what the most common temperature control schemes are for stand-alone domestic heat pump systems in northern Europe, and the literature apparently has little to say directly. Informal discussions and in-passing comments suggest that on-off may be common [34] (supported by [6,35]), with weather compensation [26,[36][37][38] relatively common for newer installations, as already happens for combustion boilers. The UK's relevant standards body, MCS, "requires weather compensation to be available on all space heating systems but does not stipulate how this must be set up," [39], and "6.3.9.…”

“…UK residential heating, responsible for 10 to 20% of the UK's carbon footprint [1], must decarbonise as part of Net Zero goals to tackle climate change. Gas-fired radiator systems in circa 20 million UK dwellings standing now that will still be in use in 2050 are likely to be replaced with heat pump systems [1][2][3][4][5][6][7][8]. Most of those dwellings currently use TRVs (thermostatic radiator valves) to help avoid overheating, improve comfort and save energy, money and thus also carbon emissions, in compliance with building regulation and guidance [9,10].…”

Data-Driven Method for Optimized Supply Temperatures in Residential Buildings

“…There are several reasons for the oversized radiators in existing heating systems. First, advanced computer-aided design methods were not widely available until the late 1980s [13]. Second, the insulation of building envelopes has improved over the years with for example insulation of wall-cavities, floors, roofs and double-glazed windows.…”

“…For example, the number of heating degree days in the Netherlands has reduced by 20% over the past 5 decades [14]. Finally, design requirements and the assumption of intermittent operation (i.e., night setback) require oversized radiators [13]. The oversizing factor describes the ratio between available and required radiator output power.…”

Efficient District Heating in a Decarbonisation Perspective: A Case Study in Italy

“…As an example, estimations forecast-that the coefficient of performance of industrial waste-based heat pumps can rise from 4.2 to 7.1, thanks to a reduction in DH supply/return temperatures from 80 • C/45 • C to 55 • C/25 • C, while the cost of solar thermal technology can be decreased by about 30% [10,11]. In addition, Reguis et al [12] reviewed the DH networks operating temperatures that are commonly adopted in the UK and abroad, also analysing the pros and cons of low-temperature heat, as well as the adaptability of existing buildings to operate with lower temperatures. On the basis of the experience of Sweden and Denmark, they found that limited retrofitting to existing buildings is sufficient to allow the employment of low-temperature heat during the whole year.…”