Preparing for heat network zoning DISTRICT HEATING/HEAT NETWORKS


With the anticipated rollout of heat network zoning, thermal substations will be a vital component in connecting buildings to the distribution networks. Ian Robinson, technical manager for Heat Network Solutions at Baxi, discusses the design and installation considerations


H


eat networks are a critical part of the UK’s roadmap to accelerate Net Zero. While supplying less than 3% of the UK’s heat today, the government aims to increase this figure to 20% by 2050. One of the ways by which it aims to achieve this growth efficiently and cost-effectively is through the introduction of heat network zones in designated geographical areas. We still await the government’s response to the consultation on the details of this initiative. The first six towns and cities to develop heat network zones were announced last year. Buildings within these zones will all require connections to the heat networks, using building interfaces such as thermal substations and/or heat interface units (HIUs). The scale is huge.


The role of substations


What could zoning mean for existing buildings in designated heat network zones? Most large non-domestic building stock and buildings that


Figure 3 Communal LTHW Duty Assist Substation


Combinations of different substation types will usually be required in most buildings and can be fitted on one base.


Legacy stock buildings


Figure 1 Primary School Heating and Hot Water Schematic


are already communally heated will need to connect once it is cost effective to do so. This includes schools, hotels, office blocks and leisure centres as well as residential tower blocks and mixed-use developments.


Substation types


Thermal substations can be divided into three basic categories: heating (LTHW), domestic hot water (DHW) and cooling (CHW). Typically available with either one or two plate heat exchangers, they can be configured as duty, duty/standby, or, more commonly, duty assist with each heat exchanger capable of a minimum of 50% of the duty. They can also be connected to thermal storage vessels such as primary stores, DHW cylinders and chilled water vessels.


Let’s consider a primary school within a designated heat network zone that is currently heated by three wall hung boilers (300 kW total output) with a 500 litre indirect cylinder meeting the building’s hot water needs. The system can be replaced with a 250 kW LTHW and 110 kW instantaneous DHW substation. In this case the hot water services are kept hot with trace heating. A schematic overview is shown in Figure 1. Now let’s consider a secondary school heated by three floor standing boilers (1,200 kW output) which also heat two 800 litre indirect cylinders. We could replace this with a new 900 kW LTHW substation and 300 kW stored DHW substation with two 800 litre DHW buffers and bring the secondary return into the upper part of the DHW store (see Figure 2).


Communally heated buildings within a designated heat network zone, such as apartment blocks, will require an LTHW substation. One example might be an apartment block with 34 flats, heated with a hybrid system involving heat pumps (120 kW) and boilers (135kW) linked to a 1,000 litre primary store. All the heat generating plant can be swapped out using a twin-plate LTHW duty/assist substation with each plate heat exchanger capable of a minimum of 50% of the total duty (see Fig 3). This would feed the HIUs already in the individual apartments.


Considerations when specifying substations


When specifying equipment such as substations, a number of factors should be considered at the outset.


First, it’s important to understand the demands the building will have on the network, the base and peak loads and the turn down requirements. Maximising the operating temperature and pressure differentials will optimise heat network performance. If connecting to an existing building, understand the condition of the system and ensure that any necessary improvements, such as flushing through the pipework, are carried out. What are the system back-up requirements? Buildings such as apartment blocks will require reliable heating and hot water supply for residents, so select the appropriate configuration – whether single duty, duty assist, or duty standby. Experienced manufacturers and suppliers will be able to advise.


Figure 2 Secondary School Heating and Stored Hot Water Schematic 8 BUILDING SERVICES & ENVIRONMENTAL ENGINEER MARCH 2025


Location, location


Identify where the equipment will be sited and any restrictions or limitations that might influence selection of equipment. Are there any noise limitations that will impact the choice of location? Is the area dry and free from frost? Is there adequate space or do access restrictions require the unit to be broken down for entry? Are there any special delivery requirements that might require craning, road closures or night-time deliveries? Can the floor or structure support the weight of the equipment?


Connection requirements


Understanding how the equipment is intended to be brought on-line is also critical. Is the intention to flip the switch and bring it on-line in one go? In buildings requiring constant heat supply and where equipment must be removed to make space for the new substation, a temporary plantroom may be a more suitable option to allow for decommissioning of the existing system. It’s worth noting too that larger projects such as multiple apartment blocks will likely require a phased-in approach, typically using multiple smaller substations rather than one larger unit.


Operation and maintenance


Maintaining efficient operation is key, so factor in any system connectivity requirements to monitor system performance as well as the necessary fault diagnostics and service and call-out support. An early awareness of the metering and billing requirements for the site – will ensure the equipment is compatible with the M&B partner?


Accelerating heat network connection


Finally, understand the budget – be aware of any financial constraints from the outset and design accordingly. Specialist manufacturers like Baxi can provide bespoke, modular or batch produced substations to replace the plant previously serving the building. However, standardising the design where possible will reduce costs and accelerate connection. Zoning is coming. At Baxi, we look forward to


providing design support and manufacture of thermal substations as part of our end-to-end solution service for your heat network.


For more information, visit: https://www.baxi. co.uk/commercial/our-solutions/integrated- solutions/heat-networks


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