An open loop is simply pumping water from an aquifer through your geothermal unit and discharging that water back into the aquifer or ditch. The aquifer could be a well, lake, pond. It is strongly recommended that the water that gets pumped out of the aquifer gets put back in to keep your water supply strong. Common issues with open loops are that you can get sand, dirt, pebbles, and sludge in your water coax. It is strongly recommended that you have your water coax flushed from time to time to prevent serious problems down the road.
The illustration above depicts a typical open loop installation.