55 50 45 40 35 12a EWT LWT Maximum Approach = Tavg–Tg (°F) 55 50 45 40 35 30 25 20 15 5 10 15 Years of Operation 20 25 119 ft/ton 116 ft/ton 116 ft/ton 290 and 310 ft/ton kgrout = 0.38 15 ft Separation Troom <70°F Temperature (°F) 6a 12p 6p Jan. 21, 2011 (Installed 2007) 12a Winter ground loop water temperatures; ENERGY STAR rating = 97; outdoor temperatures: High 6°F, Low –6°F; central Illinois elementary school, Lbore = 174 ft/ton, tgrn = 55°F. Figure 12: Cold day loop temperatures for 37,400 ft2 (3450 m2) central Illinois elementary school. raise concern about the expected life of ground loops with imbalanced cooling loads compared to heating loads. Older GSHP systems appear to have lower approach temperatures. Results are not adjusted for many important factors such as vertical bore length, ground thermal properties, and vertical bore separation distance. Newer systems tend to have slightly shorter ground loops but this is offset somewhat since older systems tend to have smaller vertical bore separation distances and Maximum approach temperature vs. GSHP age; maximum average applied temperature = [(EWT+LWT)/2] – Tground. Figure 13: Maximum average ground loop-to-ground approach temperatures vs. GSHP age. lower conductivity grout and fill. Figure 13 does provide some factors that likely influence the loops with the largest approach factors. Three of the newer systems with high approach temperatures have vertical bore lengths less than 120 ft/ton (10.4 m/kWT). Two systems with long loops but large approach temperatures have low thermal conductivity grout (0.38 Btu/h·ft·°F www.info.hotims.com/41643-32 42 ASHRAE Journal September 2012