An open loop is an earth loop that uses the water from a well to heat and cool your home. The water is pumped from the well through the geothermal heat pump's water-to-refrigerant heat exchanger coil and then returned to the earth. In the cooling season it rejects heat from your home into the water, and in the heating season it absorbs heat from the water into your home.
Open loops were common 30 years ago, but since then, the closed earth loop has taken over the lead. This is partly due to the major improvements in closed loop technology, and partly because of environmental concerns in some areas. The EPA has been considering outlawing "pump and dump" open loop systems, but using open loops that return the water back into the earth via an injection well will most likely continue to be acceptable practice. You can still use an open loop with your geothermal heat pump, if it is allowed in your county and state.
For a well to be used as an open loop, it must meet three criteria: well capacity, water chemistry, and pumping power costs.
Many areas of the country do not have enough water in the earth to satisfy the water flow rates of a geothermal heat pump. The amount of water required for the operation of a geothermal heat pump on an open loop is 1.5 gallons per minute, per ton of capacity. For example, if you need a 3-ton geothermal heat pump, your water requirements would be 4.5 gallons per minute. The temperature of the water may increase the flow requirements. During heating mode, if the water temperature is lower than 41 degrees F, the flow must be increased until the leaving water temperature stays above the freeze protection settings. In warmer climates, during cooling mode, the water flow rates may need to be increased so the geothermal heat pump's efficiency will be acceptable.
The annual amount of water used by a 3-ton geothermal heat pump is about one million gallons a year. This is a very large amount of water, but since geothermal heat pumps don't change the water quality, only water temperature, all of the water used by a geothermal heat pump can be safely returned back to the earth without contaminating the ground/aquifers or wasting any water.
If you are going to install the system on an existing well that already supplies water for domestic use, then the well will have to have sufficient capacity to meet the needs of both the geothermal heat pump and the domestic water usage.
There are many factors that will determine if your water's chemistry is satisfactory. Bad water chemistry will either scale the heat exchanger excessively, which is a nuisance, or dissolve the water-to-refrigerant heat exchanger out of your geothermal heat pump, which is a disaster. The mineral profile, pH, and temperature of the water must all be within the correct ranges for the well water to be compatible with a geothermal heat pump's water coil heat exchanger.
Water that has a pH level too high or too low will dissolve the heat exchanger, and can't be used. Also, the warmer the water, the faster an acid or alkaline will dissolve the heat exchanger, and the smaller the range of acceptable pH's. In the south, ground water temperatures are higher, and more time is spent in cooling mode (when the geothermal heat pump is adding heat to the water), so southern open loops must have more neutral pH than northern open loops.
Some types and amounts of dissolved minerals will scale the heat exchanger over time. Scaled heat exchangers can be cleaned, but it is a nuisance and an added cost. The frequency that cleaning is needed will determine whether or not this type of well is usable. Most of the time it would be better to use a closed earth loop and forget about the inconvenience of cleaning the water coil, no matter how many years it would last between cleanings.
The costs of the well pump will also effect whether or not a well should be used for an open loop. The main costs of pumping well water are how far it must be lifted up against gravity, and the type of motor used in the pump. Most electric motors that are used on submersible water well pumps are permanent split capacitor (PSC). Until a few years ago, that was the highest efficiency motor available for residential use, but now there are constant pressure/ECM well pumps available. The ECM (electronically commutated motor) has a very high efficiency and will cut the cost of pumping water by 60 percent, compared to a PSC motor. The ECM motor also makes the pump variable speed, which means your water pressure will be constant. The downside to the constant pressure/ECM pump is the initial cost. Depending on the model, the pump may cost $1200.00 to $1800.00.