I joined the FairHeat graduate scheme in 2018 and I am currently working in the New Build team, having just completed my final of four, 6-month rotations. A key part of each placement is the research project, which offers graduates a unique opportunity to contribute to internal and external business development. My latest project focused on assessing the technical and financial impact of externally accessible HIUs on heat networks, and the potential savings this strategy could enable for developers and operators are summarised below.

The heat network industry norm is for Heat Interface Units (HIUs) to be located within dwellings. This limits access to the units and is an impediment to performance, as operators as typically responsible for their maintenance. In addition, single risers and long runs of lateral pipework from risers to dwellings are prevalent and result in a number of downsides (excessive pipework elevating heat losses, access restrictions and coordination challenges).

One solution to these issues is the use of externally accessible HIUs, ideally supplied directly off a riser with no lateral pipework.

This approach also has significant implications for network flow temperature constraints. The risk of exposing residents to high pressure pipework in dwellings is typically a key factor when determining the requirement for hydraulic separation. By specifying external HIUs, all pipework is removed from dwellings, meaning that entire heat networks can be operated at the same temperature, regardless of height. As the flow temperature of heat networks decreases towards 55 °C and heat pump technology becomes more widespread, measures to minimise flow temperature will become even more important in ensuring high generation and distribution efficiency.

External HIUs (and multiple risers) have been specified in the past, but are not currently favoured by the industry due to the significant changes required to the architectural design norm. Furthermore, given the UK property sales model, the reduction in available dwelling area to facilitate external HIUs would result in a reduction in sales revenue.

For my graduate project, I carried out a techno-economic lifecycle assessment, comparing external HIUs served from multiple risers to typical networks with 1 riser and internal HIUs.

The first part of my project centred around discussions with architects and manufacturers, to determine the key constraints, considerations and inputs (minimum dwelling area requirements, equipment and installation costs, regional variations in house prices etc). Using this information, I put together a lifecycle analysis tool considering 3 main factors: impact to revenue from property sales, construction costs, and operation and maintenance costs.

The impact that external HIUs have on these factors when compared to the current configuration have been analysed over a 30-year network lifespan using 2 case studies, and the results of this are shown in Figure 1.

These case studies are:

The results show that removing dwelling heat network pipework enables a reduction in total network length of 50-55%. Moving from the conventional 1 riser configuration to multiple risers increases this reduction to c. 65%. This corresponds to a £700-1200/dwelling CAPEX saving depending on the network configuration and sizing, even when considering the extra costs associated with building the cupboards (doors, locks, connecting M&E services from cupboard to dwelling).

Reducing the network length also impacts operating costs, as heat losses can be reduced by up to 60-70% for a multiple riser network, depending on pipe sizing. For a well sized network with multiple risers, the losses could be as low as 40 W/dwelling. As a comparison, heat losses from well performing heat networks with the conventional configuration are typically c.100 W/dwelling. In addition to this, multiple risers enable the use of a top of riser keep-warm, meaning that HIUs can cool when not in use and reduce heat losses by a further c.35 W/dwelling.

Operational savings are not only found through a reduction in heat losses, as easier HIU accessibility enabled by external HIUs allows for greatly simplified maintenance and servicing programmes. The majority of HIUs do not require dwelling access during a standard service, and enabling communal access to HIUs greatly reduces the time and cost that would typically be spent organising these logistics.

Figure 1: Regional variation in 30 year NPV (3.5% discount rate) with changing price of sellable area (£/m2). House price data taken from 2016 Office for National Statistics study.

The results of the feasibility study show that external HIUs enable a saving over the lifespan of a heat network in all but the most expensive areas of London. The main difference between the case studies is due to the impact of pipe sizing. The larger the pipes are sized, the greater the CAPEX and heat loss saving per metre as the network length is reduced. However, the actual CAPEX and heat losses are higher when comparing Case Study 2 to Case Study 1, so accurate pipe sizing is still important!

When considering the reduction in heat losses and resultant reduction in carbon emissions, externally accessible HIUs should be specified in all New Builds going forwards. This conversation is gaining momentum with developers, as one has already implemented external HIUs as standard and several others are reviewing their strategy.

Uptake of external HIUs is also likely to increase when considering the New London Plan and Future Homes Standard, which will increase the requirement (and financial incentive) to minimise carbon emissions.

The benefits to operators are even clearer, as this approach enables maintenance and servicing to be a far cheaper and easier process, in addition to the efficiency gains. As such this strategy should also be encouraged by all heat network operators, regardless of whether they are involved during the development phase or just adopting the network.