Open Loop systems, popularly called pump-and-dump, use plain domestic well water as the heat source for the geothermal heat pump (GHP) system. No buried closed-loop ground heat exchanger (GHEX) is actually used. Installation is often as simple as tee-ing directly into an available domestic water pipe in the basement and plumbing it to the GHP…then running a discharge pipe from there to some location on the property where the “used” geothermal water may be dumped directly into a drainage ditch, tile, or pond. Larger residential or commercial systems may be slightly more sophisticated, but the principle remains the same for all: Heat is extracted (or rejected) directly to (or from) the well water during GHP operation.
By eliminating GHEX material and installation costs, open loop systems generally have a significant first-cost advantage over closed loop systems. They also tend to operate at higher efficiencies than closed loops in The Adirondacks due to higher entering water temperatures during winter GHP operation and cooler temperatures during summer. Well capacity, recovery rate, temperature and quality–as well as water discharge opportunities on the site–are the general limiting factors. Some periodic back-flush cleaning of the internal GHP water coil may also be necessary to remove mineral deposits.
Due to relatively high water usage volumes during peak seasonal operation of an open loop system, local aquifer draw-down may be of concern in some areas; it may be of less concern in others where the aquifer may be more viable or where discharge water may be quickly returned to it. Local well drilling contractors and DOH officials are usually very helpful in determining this.
Smaller residential-sized systems generally fall within the currently specified water usage limits of 10,000 gal. per day and 1,000,000 gal. per year without requiring a permit. Any system exceeding these limits does require application to the DNR for a special water use permit. Also, discharging ground water directly into public surface water is not allowed; but it is permissible to discharge directly on top of the ground–as with lawn sprinkler systems–or to discharge into a private holding pond, subsurface drain tile, or leach bed (no deeper than 15 ft.).
A variation of the pump-and-dump sytem is one which pumps water directly from a lake or large pond then dumps it directly back during GHP heating and cooling operation. However, lake water quality and, more particularly, cold winter water temperatures impose some limitations on such applications in the northern climate.
Reinjection Wells: Although it will incur additional installation expense, it is possible to obtain a variance for a separate well to be drilled for the purpose of re-injecting water back into the same aquifer from which it was originally drawn. This may be the only option available on sites where other discharge opportunities do not exist.
Standing Column Wells: Coming into some use in the Adirondacks (albeit with mixed results) is the standing column reinjection well, which utilizes a coaxial heat exchange system inside a single domestic water well. Water is drawn from the bottom of the well through a “standing” thermally resistant pipe and is re-injected back into the annular space between the pipe and the surface of the borehole where heat exchange can occur as the water is recycled back to the bottom during GHP operation. This type of system is limited mainly to solid rock formations and requires precise engineering. It may be the only option in some circumstances where available space and surface discharge opportunities are completely limited and it is sometimes considered as a last resort.
*Note* - We do not typically install pump and dump geothermal systems. They tend to be more taxing on the system resulting in more failures. More general maintenance is required to guarantee the longevity of the system.