Appendix A. Programme-averaged data definitions
Four typical housing unit archetypes have been identified, based on the unit schedules received and heating to date. Whilst there are numerous layout variations in the units across projects, the floor area and estimated heat demands do not vary in any discernible patterns for a unit with a given number of bedrooms. For example, the range of two-bed units includes flats, bungalows, terraces and semi-detached homes, but floor areas are largely similar across these variations. As such, the archetypes have been defined based on occupancy and the most common layout variations have been used in the description for each one.
Solar PV heating systems (individual)
There are no examples of solar PV and heat battery/electric boiler systems in one-bed flats amongst the projects considered. Therefore, no costs are presented for this first type. The reason for this is very likely that these systems need extensive solar PV capacity in order to be cost effective to tenants, and this cannot be accommodated on shared roofs in flatted buildings. Two similar capex values were presented for projects aligning with the second archetype, so these are high-confidence values and have been used to inform medium confidence values for larger archetypes.
Air source heat pumps (individual)
Five projects have provided capex for heating systems with individual ASHPs. Only the costs for the ASHP units have been accounted for in Table 16 (so additional technology costs, where applicable, have not been included). However, due to the wide variation in costs amongst these six projects, capex has been split into high and low estimates for small and/or rural developments, and large and/or urban developments. The wide range of data provided allows for medium-to-high confidence in the values presented.
Air source heat pumps (shared)
As shared ASHP systems have been found to be preferred only for flats, capex has only been presented for the one-bed archetype. Only one project has supplied a cost per unit, so the capex has been presented with low confidence.
Ground source heat pumps (shared)
Two projects in the evaluation have costed GSHPs. While the costs obtained are very similar, they are presented with medium confidence as site-specific factors can cause variations in GSHP that are not captured in these projects.
Biomass heat (shared)
Costs have only been collected for two types of biomass DHN – one which is an extension of an existing heat network, and a purpose-built one. As such, these two systems have been differentiated in Table 16. Additionally, one project costed a biomass DHN as a counterfactual, so this is included in the range for purpose-built DHNs. As with GSHP costs, these values are of low-to-medium confidence due to site-specific factors that may not be captured.
Gas boilers are the primary counterfactual heating system for the evaluation, although this can only be applied to projects located on the gas grid. For off-gas locations, LPG DHNs have been chosen as the key counterfactual as they were mentioned by several participating projects. Costs for direct electric systems have also been presented, although with the caveat that these cannot be directly compared with the costs of the other technologies, given that they incorporate electric radiators and therefore inherently include heat distribution costs, which fall outside of the capex definition for the other technologies.
Several stakeholders have advised, in agreement with our understanding of Scottish building standards, that fossil fuel and direct electric systems require additional energy efficiency measures to pass SAP. The most common approach to achieving this is to add sufficient solar PV capacity. Therefore, the counterfactual capex values in Table 16 include solar PV.
For the mains gas boiler option, values are presented with medium-to-high confidence because several capex values for boilers were collected, although only a handful shared the corresponding solar PV costs. LPG DHN costs are presented with low confidence as only one project provided capex values, and, as with other DHNs, these will be site-specific. Direct electric system costs are presented with low confidence as only one project provided capex values, which applied to one-bed terraces only.
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