A look at the system's control panel. "We put $1 of electricity into the system to get $5.50 worth of heat out of it. Any time oil costs more than $1.34 a gallon, we're making money," Terrien says.

It will take roughly eight years of $4,760 savings for the Coburn House geothermal system to pay back the additional $40,000 it cost to install.