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Geothermal Loops Horizontal Closed Loops... Horizontal loops are the most common configuration of closed loop systems in North America. A trench is dug on the property and High Density, Fusible, Polyethylene pipe is laid and appropriately spaced in the bottom of the trench, then buried in a continuous or parallel loop (depending on size of unit). The national Canadian installation standard (CSA C445) states that the loop must be located at least 600 mm (2 feet) below ground, but industry guidelines are at least twice that depth. The most common depth is to bury a loop at least 300mm (1 foot) below the frost level. It is possible to layer more than two pipes in each trench, thereby reducing the cost of digging. If a double layer of pipe is used in a single trench, then the trench must be deep enough to allow for thermal separation. It is important to backfill the trench properly, to avoid air pockets that can reduce the transfer of heat, and to ensure that the pipe is not damaged by large sharp rocks.
A closed pond, river or "lake loop" system is positioned on the floor of a body of water instead of being buried in the ground, as with a standard horizontal loop. The pipe must be weighted properly to remain on the bottom of the lake and to avoid shifting caused by spring ice movement. It is common to attach the loop pipe to a non-polluting plastic mesh, such as winter snow fencing, then floated out to the area of choice. This configuration will create a loop grid as one circuit. The circuits are then connected together to create one loop system, appropriately sized to the installed system. When the loop is filled, it will sink to the bottom of the lake, pond or riverbed. Weights are commonly attached to the top of loop grid to hold them in position. Over a short period of time the lakebed will cover the loop, creating a protective barrier and aquaculture. Care must be taken to avoid harming the pre-existing aquaculture. You should consider the positioning of the loop to avoid areas that boats commonly anchor. An anchor can cause a loop to be moved and or ruptured.
This is the most expensive type of closed loop but is a very efficient configuration, due to the fact that the under-earth level of heat increases and generally stabilises with depth. It is also more than likely that a drilled hole will pierce through an aquifer running water across the loop on a regular basis, which helps to increase efficiency. This option is viable when surface property is limited or has difficult terrain. Care must be taken to ensure that the vertical bore holes are drilled according to provincial/state/regional regulations.
A common question is, "Can I install my loop close to my septic system to take advantage of the heat that is going down my drain"? The answer is, it is not wise to place your loop close to your septic bed. Although a Geothermal System can easily take the heat away from the septic bed, a septic bed requires heat to help with microbial action to break down the sewage, which weeps from the system. If you take that heat away, the microbial action can stop and you may harm your septic bed. Local building codes will apply with this issue. There are methods to take advantage of grey water heat but this application should be discussed with your local building officials to ensure a proper system. A grey water re-capture system would require two separate sewage systems within your home, one for sewage and one for grey water.
Loose dry soil traps air and is less effective for the heat transfer required in GHSP technology than moist packed soil. Each manufacturer provides specifications on the relative merits of soil type; low-conductive soil may require as much as 50% more loops than a quality high-conductive soil. The rule of thumb here is, "the wetter the better".
The pipe that is most commonly used for GHSP installations is a high density, polyethylene pipe. There would normally be two grades: "series 125" for residential installations, and "series 160" for commercial installations. The pipe is heat fused at the time of installation to eliminate any underground mechanical joints. When a pipe is properly heat fused, the point of fusing is stronger than the pipe. Most loop pipe manufacturers offer a 50-year warranty. GSHP pipe comes in three common diameters: 0.75", 1" and 1.25". Two coiled loops (commonly called the "Svec Spiral" and the "Slinky") require less trenching than conventional straight pipe. As a result, the lower trenching costs and the savings in property disruption offsets the higher cost of coiled pipe. The ground overall mass required with straight verses the slinky pipe should be approximately the same. Care must be used when back filling a slinky type loop to ensure that pipes are spaced properly. In some cases a slinky loop requires sand back filling around the loop pipe itself. Although straight and slinky pipes are commonly used, the installing dealer will generally install their preferred pipe size and type.
GSHP technology relies on stable underground (or underwater) temperature to function efficiently. In most cases, the deeper the loop is buried, the more efficient the system. Normally a loop pipe will be buried approximately 1 foot or 30 cm below the frost level. A vertical bore hole is the most efficient configuration, but this type of drilling can be very expensive.
The longer the amount of piping used in a GSHP outdoor loop, the more heat that can be extracted from the ground (or water) for transfer to the house. Installing less loop than specified by the manufacturer will result in lower indoor temperature, and more strain on the system as it operates longer to compensate for the demand. However, excessive piping can also create a different set of problems, as well as additional cost. Each manufacturer provides specifications for the amount of pipe required. As a broad rule of thumb, a GSHP system requires 400 to 500 feet of horizontal loop, or 300 to 350 feet of vertical loop to provide heat for each ton of unit size.
The greater the distance between buried loops, the higher the efficiency. Industry guidelines suggest that there should be 3 meters (10 feet) between sections of buried loop, in order to allow the pipe to collect heat from the surrounding earth without thermal interference from the neighbouring loop. This spacing can be reduced under certain conditions. It is common to bury one set of loops above another set with a deeper trench. This would be covered under application designs. A rule of thumb here would be, "more ground mass is always better than less".
Closed-loop GSHP units can circulate any approved "anti-freeze" fluid inside the pipe, depending on the performance characteristics desired. Each manufacturer must specify which fluids are acceptable to any particular unit, with the most common being denatured ethanol or methanol (the latter is not approved for use in Ontario, Canada because of the high flash point).
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