and return headers connected to individual ground loops. The vertical bores are grouted with limestone slurry with a bentonite surface plug. The local ground temperature is 71°F (22°C). Figure 7 indicates that the ground loop leaving water temperature approached 95°F (35°C) in the library near the end of the school year. The water returning from the ground loop is about 6°F (3°C) cooler when the units are operating. This would indicate the two pumps are somewhat oversized. It should be noted the return air temperature is quite low (69°F [21°C]) and the unit cycles off, which indicates the system operates with more than sufficient capacity on a hot day. A 135,000 ft2 (12 500 m2) northwest Georgia middle school was built in 2004 with a 447 ton (1572 kW) geothermal heat pump system. The ground loop consists of 200 vertical 1 in. (25 mm) HDPE U-tubes, 260 ft (79 m) in length arranged in a 10 × 20 grid and backfilled with thermally enhanced grout. A total of 103 heat pumps are connected to a central loop served by two 20 hp (15 kW) pumps with variable speed drives. Ventilation air is provided by a 29,300 cfm (13 800 L/s) energy recovery unit (ERU) supplemented by 84 ton (300 kW) water-to-water heat pumps. The building received a low ENERGY STAR rating of 39, which may be a result of the elevated ground loop temperatures. Figure 8 indicates the ground loop LWT was 99°F 100 95 Temperature (°F) 90 85 80 75 70 65 3a 9a 3p 9p LWT EWT Return Air Hot (and Short) Loops May 27, 2011 (Installed 2001) Ground loop water and indoor air temperatures; ENERGY STAR rating = 100; outdoor temperature high = 100°F; central Texas elementary school (library), Lbore = 290 ft/ton, tgrn = 71°F. Figure 7 Hot day loop and air temperatures for “cold” central Texas school library. (37°C) during a day when the outdoor air high temperature was 97°F (36°C). The low differential loop temperature of less than 5°F (3°C) at full load indicates the pumps are delivering more than adequate flow rate. The owner prefers to operate both pumps continuously to prevent ground loop EWT above 105°F (41°C), a condition when heat pumps begin tripping out. www.info.hotims.com/41643-80 34 ASHRAE Journal September 2012