Retrofit23 spotlight: Heat pumps, pipework and district heating


18 April 2024
REHAU Retrofit 231



In the final article in the series panel expert at REHAU Retrofit 231 event, Steve Richmond, Head of Marketing & Technical at REHAU Building Solutions, looks at retrofitting district heating systems.

According to data from Energy Systems Catapult, one in five homes are set to be heated via heat networks by 2050, as the UK looks to meet its net zero obligations1.

Considering the pace of innovation within the heat pump space, with higher- temperature models becoming increasingly efficient, this figure might be considered conservative – especially when accounting for non-residential building stock.

This is in part down to the technology’s adaptability. District heating can be used for multiple applications, such as new build low-density housing developments, multiple residential tower blocks or mixed use residential and commercial buildings.


Multiple waste heat sources

District heating can also reduce emissions through the use of otherwise wasted heat.

The future is undoubtedly exciting for this technology and its applicability is not limited to newbuilds. Approximately 80% of current properties are forecasted to still be in use by 2050, so further thought must be given to how the benefits of heat networks can be applied to existing building stock2.

The sheer scale of waste heat sources from existing infrastructure makes this need even more pressing. For example, low-grade heat emitted from buildings, water treatment works and sewers can be harnessed, alongside heat from food refrigeration and IT equipment within commercial buildings. Even heat from braking, lighting and passengers within train tunnels, via ventilation shafts, may be reused in these systems. Similarly, high-temperature heat from transformer coils within electrical substations, industrial sources such as chemical, clinical waste and food production, and rejected heat from power stations can be utilised.


Powering development

These possibilities have also driven the development of newer, more efficient systems that supersede existing third- generation network designs. These previous schemes, which were designed for flow temperatures of 80-90°C, often use a fossil fuel-based source. From this centralised point, heat can be efficiently distributed across a network of buildings, reducing carbon emissions in comparison to the gas boilers commonly used in current building stock.

The rise of fourth-generation district heating models has further expanded what is possible from an efficiency and sustainability standpoint and is fast becoming the common standard for new networks. Referring to systems with flow temperatures of around 60-70°C, these schemes are ideal for use with low-carbon sources such as centralised heat pumps or waste heat.

There is also a fifth generation of district heating networks. These schemes are markedly different, using very low temperature or ambient loops to provide heating and cooling. To ensure this design’s viability, individual heat pumps are needed for each building to boost the temperature to the required conditions.


Generational Change

Given the scale of uptake necessary to decarbonise new and existing building stock, this variety of solutions is to be expected, and each heat network project will have unique circumstances and requirements. Despite this, the direction of travel is clear third-generation networks often reliant on fossil fuels are no longer being specified for new projects. For futureproofed, low-carbon heat, greater uptake of heat pump-based fourth or fifth-generation networks is required, especially given rising fossil fuel costs and the subsequent rise in fuel poverty and energy insecurity.

Industry thinking is also increasingly aligned with this conclusion. For instance, the National Infrastructure Commission, the executive agency responsible for advising the Government on the UK’s infrastructure challenges, has said hydrogen should be ruled out for home heating applications3. Previously regarded as a viable option because it can be delivered using existing boilers and gas pipes, the commission advised against hydrogen’s residential use on efficiency and cost grounds.

Though the dust has not yet settled on the debate and what may happen in the commercial building space, these comments demonstrate the increasing and continued viability of district heating schemes and heat pumps. With fossil fuels being phased out in 80% of off-gas domestic buildings by 2035 and gas boiler sales banned in 80% of houses by the same year, heat pump professionals should look at district heating as an immediately available and effective option to achieve urgent decarbonisation goals.

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Furthermore, incentive schemes such as the Green Heat Network Fund, which provides £270m of support for low-carbon projects in England from 2022 to 2027, highlights a willingness to engage with the technology at scale. Similarly, the Department for Energy Security and Net Zero’s (DESNZ) Heat Network Zoning plans identify heat generation sources and how they can be added into existing systems via pipework to meet community heating needs, demonstrating how heat network rollouts can continue at larger scales4.


Exploring the practicalities

Now that it is clear that heat networks are essential to the UK’s decarbonisation journey, the practical concerns must be addressed such as supplying existing properties and buildings. One major consideration is the installation of pipework running from a centralised source in fourth- generation networks, or to individual homes for fifth-generation schemes.

In occupied buildings, retrofit pipework installation must be completed with minimal disruption to those living, working or using affected properties. Laying pipework in trenches is the first major step that must be navigated during the installation process. Steel sticks have traditionally been the material of choice for heat network piping, but they are heavier than polymer alternatives, less flexible and require expansion loops, leading to more jointing. Class 1 welders are required for steel pipework, carrying out hot works that may impede other contractors on-site, as well as requiring wider trenches.

Further issues can arise during the jointing process, which is key to any successful district heating scheme due to the large number of tee connections required for each building or house served. To be successfully installed, a below- ground joint requires effective insulation to reduce heat loss, which is where district heating shrouds are used.


Ensuring long-term performance

Despite trained installation teams being used, steel welding does not guarantee success. In fact, the German District Heating Association cites on-site welding as the reason behind 75% of pipe failures5. If this occurs, costly replacement or remedial work may be required, which often involves digging up roads and gardens. As steel carrier pipes can be corroded through water ingress from impaired joints, so leak detection is required.

By contrast, lighter polymer pipework such as PE-Xa coils, combined with mechanical jointing methods, can lead to quicker, high-quality installations capable of being carried out by either civil or mechanical contractors without leak detection equipment required. It also does not require specialist welding so can be completed in a narrower trench, meaning less initial groundwork and disruption before laying pipework.

The popularity of these polymer-based pipework solutions has increased with the advent of fourth and fifth-generation systems, which run at markedly lower temperatures than their predecessors.

While steel was commonly used for third-generation systems running at over 90°C, this higher heat tolerance is no longer required. Consequently, heat pump professionals are advised to consider newer materials for piping, especially as the uptake of newer low carbon schemes continues.

In conclusion, decarbonising heating for the nation’s existing building stock will be a massive undertaking. The list of viable technologies that can be used in the transition to a net zero future is small one, and fourth and fifth-generation heat networks hold a unique appeal among them. Yet to ensure a high-quality and efficient installation, heat pump professionals must consider multiple factors, chief among them material and jointing choices. By working together with supply chain leaders such as REHAU, these sector stakeholders can better navigate these considerations and put themselves at the forefront of an expected boom time in the low-carbon heating space.


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  5. Statistics taken from German District Heating Association in EuroHeat and Power, 2013