Optimising the efficiency of heat networks

As the UK aims to reach net zero, heat networks are increasingly seen as a critical technology to enable the transition from gas boilers to low-carbon heating solutions.

How heat networks work

Instead of each building having its own heating system, a central plant, such as a combined heat and power plant (CHP), waste incineration plant or biomass facility, produces hot water or steam, which is distributed through underground insulated pipes to nearby residential, commercial or industrial buildings.

Each building that is connected to the network uses a heat exchanger to facilitate the heat transfer from the central heat network to a secondary circuit in an individual building or dwelling. Individual units on the heat network each use a heat interface unit (HIU) that manages the heating and hot water delivery and regulates temperature and flow. It doesn’t generate heat itself, but receives heated water from the central heat network, using the heat exchangers to transfer this heat to the local system.

Types of heat network

There are currently three main types of heat network:

• Third-generation (3GDH): Often fuelled by a central natural gas boiler or CHP energy centre. Some gain energy from waste and heat pumps. Systems typically operate at 70-90°C for supply temperatures and 40-50°C for return.
• Fourth-generation (4GDH): Designed to operate at lower temperatures than 3GDH, making them more compatible with renewable energy sources and low-carbon heating technologies. 4GDH systems operate at significantly lower temperatures than previous generations to supply temperatures of about 50-60°C and returns of 20-30°C.
• Fifth-generation (5GDH): Operate at very low temperatures (10-30°C) and need additional hot water immersion heating. 5GDH networks use ultra-low-temperature heat loops, relying on small heat pumps in each building or unit to raise temperatures to the required level. They also allow for bidirectional heat transfer, enabling buildings to share excess heat and meet both heating and cooling needs.

Benefits of heat networks

The scale of heat generation compared with individual boilers means that heat networks are typically more energy efficient. This reduces the use of fuel and lowers carbon emissions, even where conventional fuel sources – such as natural gas – are used.

Heat networks also allow easier and more effective deployment of renewable energy. It is not usually possible to utilise wind or geothermal energy for individual properties or in dense urban areas, and solar thermal can be challenging to implement on multi-storey developments. However, the centralised nature of heat networks makes these technologies more widely viable – expanding the use of low or zero carbon energy.

Heat networks offer flexibility to utilise various energy sources, even sources of heat that are generally considered waste, such as by-product heat from industrial processes and power generation.

Heat networks can also reduce heating costs by using economies of scale and tapping into local energy resources. Bulk purchasing, shared infrastructure and centralised maintenance can offer substantial cost savings.

Regulation and compliance

The Chartered Institution of Building Services Engineers (CIBSE) is directing designers, specifiers and installers towards 4GDH and 5GDH heat networks, depending on the application. 4GDH networks are considered better suited for larger residential schemes as they can use different heat sources. 5GDH schemes are ideal for multi-use buildings such as retail and residential, as they operate as both a producer and consumer of heat and cooling.

CIBSE has developed the CP1 Code of Practice for Heat Networks (2020) which offers comprehensive technical standards for the design, installation, commissioning, operation, maintenance and monitoring of heat networks in the UK. While not a legal requirement, CP1 provides the minimum standards to ensure that any heat network meets the necessary energy performance targets and it is widely regarded as the benchmark for regulation.

The Heat Network Technical Assurance Scheme (HNTAS), due to launch in 2025, builds on the CP1 to ensure a minimum level of performance, reliability and efficiency for heat networks as a means of reducing carbon emissions, as well as reducing the cost of heat supplied to consumers. It will include standardised processes for demonstrating compliance with technical standards, such as pipe insulation and water flow temperatures, for all new and existing heat networks.

Optimisation and heat loss reduction

Optimising the performance of a heat network is essential for maximising energy efficiency, reducing costs, and minimising environmental impact.

For any heat network, whatever its scale, it is important to select a pipework system that will ensure maximum efficiency. This will depend on various factors such as thermal performance, flow rate, pipe size and the overall project needs.

The CP1 outlines the minimum thermal insulation thicknesses for different pipe diameters as well as guidance on velocity-based pipe sizing. It also states that the calculated total annual heat loss from the primary network should be less than 10%. Designers must justify losses greater than 10% and the total primary network heat losses should not be more than 20% of the heat supplied by the energy centre.

It is clear that the thermal performance of the pipework is key to the success of the heat network system, so it is crucial to use a pipe system with excellent insulation properties. Insulated pipes that have a reduced overall diameter are more flexible and can therefore be bent around obstacles and directed around the site without the need to spend time and labour on joints and fittings to accommodate such requirements.

By following CIBSE guidance and remaining up to date on the required standards regarding the operation and maintenance of all components of the scheme, heating engineers can ensure heat network systems perform as required.

FIND OUT MORE

GF Building Flow Solutions has released its latest CPD, Minimise the heat and energy losses of heat networks. To register interest or book, visit uponor.com/en-gb/specification/cpd-page.