An open loop system (aka ‘pump and dump’ loop) is simply pumping water from a well or pond through the heat exchanger, and then discharging it back into the ground.
The most common open loop system involves two wells; one that sucks the water up to the heat pump, and the other expels it back into the ground. No water is used up or harmed in any way during this process; it is just pumped through a system that adds or removes a few degrees of heat, then returned to its source.
These types of systems are known as open loop because the supply and discharge are not connected. An open loop system is the least expensive to install compared to the other loop types provided there is sufficient ground water available in the area. It is even more ideal if there is an existing well feeding the property, because that could be the supply well, and only one more needs to be drilled to be the discharge well.
Open loop systems also have greater performance than closed loop systems, because no heat is being constantly removed from one volume of earth, so incoming water temperatures are just the temperature of the aquifer, and remain constant throughout the year.
Scaling of the heat exchanger is another issue to take into account, because constantly running hard mineral water through the system can be an issue. There are specially built cupro nickel heat exchangers that prevent much of this, and should be used on any geothermal heat system that uses an open ground loop.