Shallow geothermal systems based on ground source heat pumps constitute an energy-efficient solution for space heating and cooling; however, their application in Brazil remains limited, particularly with respect to horizontal ground heat exchangers installed in residual soils. This paper presents a case study on the analytical design of a horizontal Slinky-type ground heat exchanger intended for the thermal conditioning of a 25 m2 experimental test room located in Maringá, Paraná, southern Brazil, operating approximately 40 h per week. The subsurface profile is composed of residual clayey soil derived from basaltic bedrock, whose geotechnical and thermal properties were determined through laboratory and in situ investigations. The design was carried out using an analytical methodology specifically developed for horizontal geothermal systems, which accounts for seasonal ground temperature variations, soil thermal conductivity and diffusivity, and peak building thermal loads. The results reveal a pronounced asymmetry between operating modes, with a required pipe length of approximately 306 m for summer cooling and 126 m for winter heating. A compact horizontal Slinky configuration was adopted to satisfy site constraints while maximizing heat exchange per unit area. Sensitivity analysis indicates that soil thermal conductivity is the dominant parameter governing exchanger length, followed by thermal diffusivity and heat pump performance. The findings highlight the importance of site-specific thermal characterization and the use of appropriate design methodologies for horizontal geothermal systems in subtropical and residual soil environments.