Signhild Gehlin is a technical expert at the Swedish Centre for Shallow Geothermal Energy, Lund, Sweden. Jeffrey D Spitler is a professor in the School of Mechanical and Aerospace Engineering, Oklahoma State University, Stillwater, Oklahoma. Göran Hellström is an Adjunct Associate Professor in the Department of Mathematical Physics at Lund University of Technology, Sweden. Deep Boreholes for Ground Source Heat Pump Systems – Scandinavian Experience and Future Prospects Signhild E A Gehlin, PhD Jeffrey Spitler, PhD, PE Göran Hellström, PhD Member ASHRAE Fellow ASHRAE ABSTRACT Ground source heat pump (GSHP) systems are commonly used in Sweden for both residential and commercial buildings. However, there are several key differences compared with GSHP systems utilized in the USA. Scandinavian systems are often heating-only, and instead of using grouted boreholes, groundwater-filled boreholes are often used. These boreholes are cased from the ground surface to the usually shallow bedrock. A single or double U-tube is commonly suspended in the borehole. These boreholes are often deeper than those commonly used in the USA. The average borehole depth has increased over time, and the average borehole depth for ground heat exchangers installed in 2013 in Sweden was 171 m (561 ft.) Boreholes as deep as 250-300 m (820-984 ft) are not uncommon and there is interest among installers of using even deeper boreholes. Incentives for deeper boreholes include limited area for drilling, pre-existing boreholes on neighboring properties, and deeper-than-usual layers of soil and unconsolidated rock. This paper reviews current Scandinavian practice for borehole design and discusses installations with boreholes 300 m (984 ft) deep or deeper. Aspects of the design include using larger pipe sizes or double U-tubes to keep pressure losses acceptable, larger borehole diameters to accommodate the larger pipe sizes, increased short-circuiting due to the long lengths, and design temperatures for heating-dominant systems due to the geothermal gradient. INTRODUCTION Approximately one fifth of the two million single-family houses in Sweden are today heated with a ground source heat pump (GSHP) (Gehlin et al. 2015). The typical domestic GSHP is a 5-10 kW (17,060-34,120 Btu/hr) capacity heat pump connected to a 100-200 m (328-656 ft) deep vertical groundwater-filled borehole in hard rock. Ground heat exchangers used in Scandinavia are commonly closed-loop systems, fitted with a single U-tube. The uppermost 6 m (20 ft.) or more of the borehole is cased with a steel casing and sealed to the bedrock to protect the groundwater from surface pollution (SGU 2008). According to the Swedish Geological Survey Well Database, the average borehole depth for GSHP systems has increased from 100 m (328 ft.) in 1995 to 171 m (561 ft.) in 2013 (Gehlin et al. 2015) (see Fig. 1). For single-family houses the increasing borehole depth is a result of more efficient heat pumps, as well as the trend to utilize GSHP with higher installed capacities to avoid the use of auxiliary heating by electric resistance heaters. The trend towards deeper boreholes is however more pronounced for commercial GSHP systems, where there