Design and sizing practices of ground source heat pump systems in Finnish multi-apartment building retrofits

Ground source heat pumps (GSHPs) are increasingly used in cold-climate retrofits of multi-apartment buildings (MABs), yet empirical knowledge of their design and sizing remains limited. This study presents an exploratory analysis of a national survey of Finnish GSHP contractors, examining system design, sizing, configuration, and controllability in MAB retrofits. Respondents ( n = 7) were categorised by experience level and together reported 605 projects, covering about 21% of the Finnish GSHP-equipped MAB stock, although the distribution was markedly uneven. Results show that GSHPs were typically sized to cover 75–85% of peak heating demand, with backup and top-up heating almost exclusively electric. The electrical grid connection was increased in 70% of these projects, although earlier studies based on historical consumption data suggest that the need for such enlargements is uncertain. Control functionalities based on electricity use or power demand appeared in fewer than 10% of systems. Despite broadly similar design outcomes, calculation tools, input data, and sizing rationales varied widely between contractors, reflecting the absence of shared standards and the predominance of investment-cost considerations. Findings highlight a gap between modelling-based optimisation and contractor practice and indicate directions for more consistent design guidance and assessment of grid-level impacts of GSHP retrofits in cold-climate MABs. Practical application This exploratory survey clarifies how GSHP retrofits in Finnish MABs are typically designed and sized. GSHPs were most often sized for partial capacity (75–85% of peak heating demand), while systems generally rely on electric backup and top-up heating. Grid connections were frequently enlarged, although earlier studies based on historical consumption data suggest that the need for such upgrades is uncertain. Control functionalities based on electricity use or power demand were present in fewer than 10% of systems. These findings help designers and contractors refine sizing ratios and grid-connection design, and they provide baseline data for researchers and guideline developers.