Geothermal loop systems are either closed or open loop, meaning they either constantly recirculate water through sealed underground pipes during the operation of a heat pump or they simply return water back to its source, as in the case of a water well, aquifer or pond. Under these two categories there are four basic types of loop systems – horizontal, vertical or well/pond closed loop systems and open loop. There is also something called a “hybrid” type system as well.
Selection of one over another type of loop system depends on climate, soil conditions, land availability, access to a well or pond, local installation costs and regulations. Let’s examine each in turn.
Commonly employed for residential installations if sufficient land is available, a horizontal-closed loop involves digging trenches at least 4 ft. deep. Piping for the loop is laid down on the floor of the trench, with two pipes laid – for example, one at 6 ft. and the other at 4 ft. Two pipes can also be laid side-by-side at 5 ft. in a 2 ft. wide trench. Pipe length totals vary quite a bit for all closed loop systems depending on design variables, but horizontal-closed loop systems usually range between 400 to 600 ft. per ton.
A newer option and one that requires less trenching (hence digging) is the closed loop Slinky Coil installation. Slinky coils are flattened, overlapped plastic piping strung out in a trench in a circular coiled loop. A slinky loop concentrates the heat transfer surface and reduces the land use requirement significantly, but large amounts of piping are required.
Residential and commercial buildings (especially schools) often use vertical systems due to land restrictions. 4” or larger diameter holes are drilled about 20 ft. apart, somewhere between 100 to 400 ft. deep. Inserted into these holes are two pipes that are connected at the bottom with a U-bend that forms the loop. The vertical loops are the connected to a trenched horizontal manifold pipe that connects to the heat pump. Vertical-closed loop systems can cost more to drill. However, depending on soil and site conditions going vertical can be the best method.
Pond/Lake Closed Loop
Properties, residential or commercial, that have an adequate body water like a pond or front a lake can connect the geothermal loop from the home or building to the water source. A supply line pipe is run underground and into the water and coiled in circles deep enough under the water’s surface to be safe from freezing. This is a very economical method because minimum land excavation is required. Slinky-type coils of pipe are used and typically weighted down with cement blocks to hold the coils on the bottom.
An open-loop system extracts heat from water that comes from a well or surface body water and is circulated through the heat pump. Once the water circulates through the system it is returned to the ground through a recharge well, or through surface discharge depending on where it came from. The body of water used should be fresh water, and the open-loop system needs to conform to local codes and regulations. Open-loop systems have advantages: they have higher efficiencies than closed-loop systems due to the fact that water conducts heat better than earth does, there is minimal digging required and minimal use of piping. If water quality is poor, there are special heat exchangers and separators that can be used to ensure dependable performance.
Standing Well Column
A standing well column is a variation of an open-loop system using deep vertical wells where water is drawn from the base of the standing columns and then returned to the loop. A standing column well system can bleed a portion of return water to the aquifer during peak heating or cooling times rather than re-injecting all of it. This bleeding cools the column during heat rejection and heats it during heat extraction.
Geothermal heat pumps can be used with other HVAC technologies to produce a “hydbrid” system especially for big cooling needs. For example a ground-source heat pump can be teamed with outdoor air using a cooling tower or a dry cooler.