In Rockford, Ill., the owners of a new 12-business strip mall approved a plan for a network of water-to-air geothermal systems piped to a large main serving as a "thoroughfare." Each system would be able to pull and deposit British-thermal-unit-rich fluids as needed. With a total capacity of 90 tons, the project called for an extensive geothermal-exchange field.

Design/build contractor Area Mechanical started the project with a thorough conductivity test. Nobody was looking to waste money by drilling an oversized exchange field or, worse yet, come up short on capacity with an undersized field.

Sixty-four closed-loop vertical boreholes were drilled in an area now covered by asphalt, the strip mall's parking lot.

"The geo-exchange field takes up the entire parking lot," Ryan Kerry, part owner of Great Lakes Geothermal, the drilling subcontractor, said.

Great Lakes spent three-and-a-half months drilling the boreholes and trenching the lines to the building. Great Lakes also installed U-tubes and filled the boreholes with bentonite grout.

"Boreholes for the Rockford mall average 250 ft in depth," Kerry said. "We would normally go deeper, but we kept hitting gravel. Most likely, it's loose conglomerate from an old, now-dormant underground river bed."

According to Kerry, the soil makeup was well-suited for heat transfer, until the rigs hit the gravel veins.

Loop temperature averages in the low 40s during winter and the 60s to 70s throughout summer. The boreholes were set about 20 ft from each other—a greater-than-average distance—to help stabilize ground-loop temperatures year-round.

For each ton of heating and cooling capacity, there is about 180 ft of bentonite-grout-filled borehole.

"Illinois state code mandates that all geothermal boreholes are grouted," Kerry said.

To retain all of the energy-efficient heating and cooling, a tight building envelope was designed. The building was heavily insulated, and high-quality, low-E windows were specified. Awnings were installed over all of the windows and doors, and a flat white roof was installed to reflect the majority of the sun’s radiant energy during summer.

On the Inside
The first business to locate in the mall was a Subway restaurant. The 2,800-sq-ft eatery is heated and cooled with two ClimateMaster Tranquility geothermal systems suspended above the finished ceiling. One is a 3-ton TTH038; the other is a 4-ton TTH049. Both are hung from roof trusses.

"Initially, we considered having a primary pumping system with continuous flow through all the wells and all the units, but later discarded the idea, even though we knew there’d be greater overall system efficiency if we circulated fluids that way," Bill Sprague, project manager for Area Mechanical, said. "But the owner of the building insisted on having the ability to split up the utility bills. A primary pumping arrangement would have allowed for only one power meter."

"There's a total of eight circuits, each comprised of eight 2-in. geothermal loops," Jeff Hurst, geothermal product manager for Connor Co., Area Mechanical’s geothermal-equipment supplier, said. "These are connected, or fused together, in the geo-exchange field."

Area Mechanical heat-fused all of the geothermal lines. Reverse return piping was used to run each circuit into the building. The tubing starts at 11/4 in. at the first well and steps up as it joins the other wells. Two-in. tubing is used to run from the last well into the building. On the return side, the same tapering process is used, only in reverse order. This ensures an even draw from each of the eight wells connected to the circuit. The geothermal-exchange medium—1,800 gal. for the entire geothermal field and strip mall—is a 20-percent methanol mix.

"Once the circuits reach the building, they join a 4-in. main, or header, that all the units individually draw from," Hurst explained. "It's not a continuously running body of fluid. Fluids circulate only when there's a call for Btus."

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