By BUILDER Magazine Staff An Indiana company has patented a method that lets any geothermal heat pump use the municipal water supply as its working fluid. If successful, it could make geothermal cost competitive with gas.

"The water utility is a huge geothermal system already in place," says Jim Hardin, president of the Indianapolis-based Hardin Technologies, which owns the patent. Hardin has partnered with heat pump maker WaterFurnace International in Fort Wayne, Ind., to promote the technology, which would require return water lines from neighborhoods to utility substations. Hardin believes that the system would use 40 percent less energy than a standard gas furnace "by the time you pay for water, you'll probably save 20 percent on your heating and cooling bill."

Hardin Technologies vice president Lily Cramer says that most utilities have been receptive, as long as they can ensure the purity of their drinking water. The system has been successfully tested on a school in Comanche, Okla., for two years, and the company is seeking approval from state utilities.