{"id":60,"date":"2022-05-09T19:02:49","date_gmt":"2022-05-09T23:02:49","guid":{"rendered":"https:\/\/opentextbc.ca\/plumbing3g\/chapter\/closed-loop-systems\/"},"modified":"2022-09-07T11:55:34","modified_gmt":"2022-09-07T15:55:34","slug":"closed-loop-systems","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/plumbing3g\/chapter\/closed-loop-systems\/","title":{"raw":"Learning Task 3","rendered":"Learning Task 3"},"content":{"raw":"A closed-loop geothermal system is a good choice when open-loop sources are either not available or are not allowable for various reasons noted above. Closed-loop geothermal systems either extract low-temperature heat from the earth (in heating mode) or reject it (in cooling mode) by circulating 100% water or a water-based antifreeze solution through a closed assembly of piping loops that are buried in the ground. These loops can be installed either vertically or horizontally.\n\nThe choice of use of a horizontal earth loop installation is based on available area, soil conditions and availability of excavating equipment. Piping is buried 4 to 8 feet below the ground\u2019s surface and can be installed with one, two or four pipes in a narrow trench or as a series of coils known as \u201cslinkies\u201d that are installed in a larger excavated area.\n\nFigures 1 through 4 below show pipes installed in trenches created either by chain trenchers or backhoe-type excavators. Ground conditions typically determine which of the two are used; if the ground has soils with rocks that are no larger than fist-sized, the chain trenchers can be used. A \u00a0\u201cDitch Witch\u00ae\u201d is a well-known example of this type of tool. Trenches can be narrow, as shown in Figures 1 and 2, and therefore use less area for the system piping, with up to two pipes per trench. The narrow trench and its spoils will occupy less ground area before backfilling.\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"300\"]\"\" Figure 1 Single pipe in trench[\/caption]\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"300\"]\"\" Figure 2 Two pipes vertically in trench[\/caption]\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"400\"]\"\" Figure 3 Two pipes horizontally in trench[\/caption]\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"400\"]\"\" Figure 4 Four pipes in trench[\/caption]\n\nIf larger excavators are needed due to soil conditions or chain trencher availability, the trenches will be wider and will therefore have room available to install two or four pipes within them, such as in Figures 21 and 22. Using four-pipe trenches can reduce the area needed for the piping \u201cfield\u201d which may be advantageous.\n\nFigure 5 below shows pipe fashioned into coils, known as \u201cslinkies\u201d and laid within wide excavated areas. These layouts work well in soils that can be excavated and back-filled using a front-end loader.\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"500\"]\"\" Figure 5 \u201cSlinkies\u201d laid into large-area excavation[\/caption]\n\nNote that the coils are attached to two manifolds seen near the bottom of the picture. This means that the coils are piped in parallel, rather than in series, with each other and reduces the power that the circulator(s) must produce in order to move the water solution through the field. Figure 6 below shows the coils partially backfilled.\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"400\"]\"\" Figure 6 Partially backfilled \u201cslinkies\u201d[\/caption]\n\nNote that in Figures 5 and 6, there is a straight run of pipe beside each coil. This is the return line from the end of the coil. There will be a supply and a return manifold for each field.\n\nIf there is insufficient area in which to install horizontal earth loops, such as in an urban neighbourhood, vertical earth loops are a viable option. Equipment that is similar or identical to that which is used by well drillers create 6-inch bore holes that can be many hundreds of feet deep. Figure 7 below shows an example of a vertical field of bore holes.\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"400\"]\"\" Figure 7 Field of boreholes with vertical loops[\/caption]\n\nThe boreholes are normally 6 inches in diameter, with 125 to 150 feet of borehole required per ton of heat pump evaporator capacity. Once a borehole is established, a U-tube of HDPE or PeX tubing is inserted down into the full depth of each borehole. Because of the tight space in the borehole, a special pre-formed return bend is fusion welded to the ends of the vertical tubes and pressure tested before being installed. After insertion, the space between the tubing and borehole is filled with a grout mixture composed of sand and an expansive clay called Bentonite. The addition of grout increases the thermal conductivity between the earth and the tubing. As well, it acts to fill the void to prevent any ground surface contaminants from making their way down into the water aquifer.\n\nThe supply and return tube from each borehole are then connected together into supply and return manifolds or \u201cheaders\u201d, in much the same fashion as in a hydronic heating system. A reverse-return piping configuration, as shown in Figure 8 below, ensures equal flow through each borehole.\n\n[caption id=\"attachment_59\" align=\"aligncenter\" width=\"400\"]\"\" Figure 8 Reverse-return manifold piping[\/caption]\n

Piping for Closed Earth Loops<\/h1>\nThe most commonly used material for earth loop piping is HDPE (high density polyethylene) specifically designated as PE3608 based on the ASTM F-412 standard. This is a thermoplastic material that can be heat-fusion welded, making it very adaptable to all aspects of connecting piping for direct-earth burial. Although a DR (diameter-ratio) of 11 or lower is preferable for buried tubing, this thicker material isn\u2019t as favourable for heat transfer as is that with a DR of higher than 11 (remember that the lower the DR number, the thicker the pipe wall). When done correctly, heat-fused joints are stronger than the base pipe itself and can last for many decades. For information regarding the processes involved in heat-fusion welding, consult the ITA modules concerned with Level 1 of the BC Pipe Trades Training programs.\n\nCross-linked polyethylene tubing (\u201cPeX\u201d) can also be used for buried earth loops, although this practice isn\u2019t as common as is the use of non-cross-linked PE3608 tubing. PeX tubing cannot<\/em> be joined by heat-fusion methods, so any buried joints must be made using mechanical means that are approved by the manufacturer. If PeX tubing is to be used, the system is carefully pre-planned so that any buried tubing is continuous and free of joints or connections, and the manifolds are accessible either inside or outside the building.\n

Exterior versus Interior Manifolds<\/h2>\nA manifold is simply a series of tee connections into a main pipe, commonly referred to as a \u201cheader\u201d. Manifolds for earth loops are most commonly formed by heat-fusing tees into a header that will increase in size according to the amount of flow through each segment, with a header at the supply and return ends of the loops. These headers are buried underground along with the loops and are not accessible for flow adjustment, temperature monitoring or for the possible addition of more loops unless they are located within a buried vault that is freeze-protected, or an aboveground structure with the same considerations. For equal flow to be an option, the loops must be connected to the mains in a reverse-return configuration, identical to that found in hydronic heating systems, and the loops must also be of the same length. If monitoring and adjustment of flow rates and temperatures is desired, or if more loops are to be added, manifolds are best installed within the building.\n\nIf the piping manifolds are outside the building, there will only be two penetrations of the foundation wall necessary; one for the supply header and one for the return header. This makes sealing of the pipe penetrations simpler. Figure 9 below illustrates a common method of sealing pipe penetrations through foundation walls.\n\n\"\"\n\nThe mechanical seal shown above expands both outward and inward when compressed by tightening the through-bolts. The holes in the concrete foundation must be of a certain size in relation to the pipe and can either be pre-formed at the time of the pour or drilled through the wall using a \u201ccore drill\u201d machine. In either case, the interior surfaces of the hole must be smooth.\n\nThe area around the pipe penetrations should be backfilled with clean granular fill and compacted adequately so as to deter soil settlement that may put stress on the pipe at the penetration.\n\nThe two obvious disadvantages of installing manifolds inside buildings are that there are many more pipe penetrations in the structure, and that the individual loop lengths must be longer. The advantages of interior manifolds are:\n