District heating – an alternative solution

District heating networks are increasing in stature as the Government looks to shrink the country’s carbon footprint and reduce homeowner heating bills. In turn, this has placed the future of traditional fossil fuel heating systems in doubt. Taking this into account, Steve Richmond, Head of Marketing & Technical at REHAU Building Solutions’ division, discusses the latest innovative and efficient technology, and steps companies are taking to drive the new generation of district heating solutions.


The growing issue of climate change is making fossil fuel boilers a less attractive option for heating our homes, and refocusing priorities toward developing more efficient and sustainable heating systems that can also cut energy bills. This has led to a renewed focus on district heating systems, which supply buildings with heat and hot water from a central source, connected to individual buildings via a network of underground pre-insulated pipes. By supplying multiple buildings through this network, carbon emissions can be lowered while removing the need for maintaining individual boilers.

As it stands, the majority of district heating systems being installed in the UK are of the third generation, with water in circulation at 70 to 95ºC flow temperature. However, with the introduction of fourth-generation systems with water circulating at 40 to 60ºC, a transition to this newer design is underway. Because of the lower temperatures, heat loss in the network is reduced, allowing the system to meet consumer energy demand more efficiently.

Benefits of the fourth generation

Opportunities for smaller pipe sizes, combined with reduced heat loss and lower capital and operational costs, means fourth-generation systems are highly attractive to developers. Furthermore, lower installation and maintenance costs, alongside reduced homeowner bills, makes the technology an appealing option for housing associations and the public sector.

A further benefit of these fourth-generation systems is that they have been designed with a low-carbon heat source in mind. The lower temperatures made possible through these networks make them ideal for centralised air, water or ground source heat pumps, and lead to markedly lowering carbon emissions compared to their gas-powered counterparts. It is, therefore, clear to see the appeal of fourth-generation solutions as an efficient, sustainable and future-proof heating system.

Lowering carbon emissions

The need to transition to heating systems with lower carbon emissions is more prevalent than ever. This was underscored by last year’s investigations by the Committee for Climate Change (CCC), which included proposals for reducing emissions and improving energy efficiency in the heating market.

In the aftermath of this report, the Government has set policy for achieving a UK-wide net-zero carbon emissions rate by 2050, and has also established the Future Homes Standard for 2025, banning the installation of fossil fuel boilers in new-build homes. Against such a backdrop, district heating systems are becoming a preferred alternative solution.

However, developers must still carefully consider flow temperature within these networks, especially considering that the CIBSE/ADE CP1 – the code of practice setting out minimum requirements for a heat network – recommends a maximum flow temperature of 70ºC in new systems. Heat pumps have become even more of an attractive proposition following the publication of the new lower emissions factor for electricity in the 10th version of the Standard Assessment Procedure for Building Regulations (SAP10), which is out for consultation and comes into effect this year. In contrast to the value 0.519 kgCO2/kWH set out in the existing SAP 2012, SAP10 denotes a value of 0.233 to be adhered to.

Following this, October 2019’s Draft SAP 10.1 consultation provided an even lower value of 0.136, a 75% improvement in carbon savings of electricity used by a heat pump from the original SAP 2012 figures. This quick and developing trend toward grid decarbonisation is, therefore, setting the stage for a future of low-carbon heat pumps.

As well as this, the Heat Networks Investment Project (HNIP) have encouraged projects using commercial-scale heat pumps and very low or ambient temperature distribution systems through their scoring of projects applying for grants or loans.

Polymer – the way forward

Carbon and cost reductions are, therefore, key drivers behind the transition to district heating technology. Consequently, interest in modern, efficient heat networks, such as those using polymer pipework, has grown accordingly. To meet spiking consumer, developer and governmental demand, companies like REHAU are creating district heating solutions that efficiently provide hot water and space heating while also lowering emissions and heating costs.

Indeed, despite their relatively new involvement in district heating networks, many studies and Government reports have included the fact that polymer pipes have typically lower installation costs than traditional materials. Furthermore, the risk of corrosion associated with steel pipework is greatly diminished with a polymer system, along with the subsequent possibility system failure and costly repair work this would require to resolve. The reduced flow temperatures of fourth-generation networks instead enable a polymer pipework lifespan well in excess of 50 years, and with welding not required for pipe connections, installation is also swift and simple. The material’s flexibility offers further benefits for developers and contractors seeking freedom throughout the design process and on-site during installation.

REHAU’s pre-insulated PE-Xa pipework systems RAUVITHERM and RAUTHERMEX offer a reliable and proven solution suitable for district heating networks. RAUVITHERM’s combination of flexibility and low heat losses makes it an ideal choice for small- to medium-sized heat networks or house connections off a main spine. RAUTHERMEX uses a high-performance polyurethane foam, making it an ideal option for a large district heating network. The insulation afforded by the polyurethane rigid (PUR) foam ensures minimal heat losses across the network, further enhancing system efficiency. REHAU’s latest generation of large-diameter PP-R pipework now offers developers a real alternative to steel for a network’s main spine, allowing a 100% polymer network.

Fifth-generation heat networks also exist alongside fourth-generation networks, and use flow temperatures of around 10 to 30ºC – much lower than the 40 to 60ºC possible with their fourth-generation counterparts. Key to the design of these systems is the use of individual heat pumps in each building, boosting the temperature from a low-temperature heat source (e.g. boreholes). A benefit of this type of set-up is that heat losses are reduced across the network, but, in contrast to a fourth-generation network, individual heat pumps are typically required rather than a centralised one.

Polymer systems, such as those offered by REHAU, are becoming an increasingly appealing prospect to specifiers, developers and contractors. Indeed, they have an instrumental role to play in the district heating market’s transition traditional higher-temperature heat networks that are reliant on fossil fuels, to a lower-temperature set-up that uses renewable or waste energy sources. Reducing emissions continues to be a hot-button topic in the construction sector, and district heating is an effective option for those looking to improve sustainability while heating homes and buildings in an energy-efficient fashion.

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