Consulting-Specifying Engineer - November 2008 - (Page 38)

Drilling of well field for Manheim Township High School in Lancaster, Pa. Source: Consolidated Engineers 3. Estimate the building’s monthly loads. This process is typically completed using a dynamic (hour-by-hour) simulation software package for large commercial buildings; however, the bin calculation method can be used for smaller buildings. Consult the ASHRAE Handbook of Fundamentals for more information on these calculation methods. Monthly loads take into account the heating and cooling loads; the type and size of equipment selected; and the thermal characteristics of building, climate, and soil. Matching heating and cooling loads will ensure that the system is used to its full potential and also enable more complete charging and discharging of the earth around the loopfield. Installations with unbalanced annual loads have, over time, degraded performance of GSHP systems. 4. Select equipment. Once the cooling and heating loads are known, it is time to select equipment. When selecting water-to-air heat pumps, the rating conditions for the heat pumps should use 77 F entering liquid temperature (ELT) and 80.6 F entering air temperature (EAT) in cooling mode, and 32 F ELT and 68 F EAT in heating mode. With ELT approaching 32 F, consider a glycol/water mix to prevent the possibility of freezing the liquid. Evaluate whether the equipment size should be based on the heating load or the cooling load. In climates where heating operation predominates and cooling operation is minimal or non-existent, optimum heat pump size may be 65% to 75% of the design heating load. It may be advantageous to specify two-speed or variable speed systems where heating requirements are much greater than cooling. This would accommodate sizing to or oversizing on the heating side, while allowing desirable operation for cooling. Or, as discussed, hybrid systems may be chosen. Provide careful matching of heat pump and ground exchanger capacity to building load. Water flow through each unit should be designed to simplify water balancing; keep unit water pressure drops as close to each other as possible. Ensure that the geoexchange pump and piping accessories are all bronze or stainless steel. The high-density polyethylene piping used for geoexchange systems is not an oxygen barrier material, and the fluid will always have a certain amount of dissolved oxygen that will corrode any unprotected ferrous material in the piping system. 5. Select the type of indoor distribution system. An air system is typical for commercial and institutional applications. A waterto-air system is often considered for larger multizone commercial buildings. Radiant heating and cooling systems are also an option, though much less common. 6. Size the indoor distribution system. This means calculating air and water flow and selecting diffusers, registers, and grilles and Common parameters: Quick reference for evaluating vertical ground loops • Bore holes are typically 150 to 450 ft in depth. • Typical systems require between 150 to 200 ft of bore per ton of peak block load. • Ground loop heat exchanger costs can vary from $1,200 to $2,000 per ton installed, depending on the drilling conditions and the size of the system. • At 20 ft bore spacing, a shallow field of 150 ft bores requires approximately 1 acre per 100 tons of peak block load. For example, for a 50,000-sq-ft office building with a ground loop sized for the peak heating load of 750,000 Btuh (62.5 tons) and 400-ft boreholes: • Number of boreholes required = (62.5 tons x 200 ft of bore/ ton)/400 ft per bore = 31.25 bore holes • Cost of boreholes = 62.5 tons x $1,600/ton = $100,000 • Acres required for boreholes = 62.5 tons x (1 acre/100 tons) x (150 ft/bore/400 ft/bore) = 0.2325 acres (roughly 100 ft x 100 ft square) 38 Consulting-Specifying Engineer • NOVEMBER 2008

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Consulting-Specifying Engineer - November 2008 Contents Viewpoint News M/E Roundtable Comissioning Lighting Controls Prevent Plugholing: Smoke Control Done Right Commercial Geothermal Heating and Cooling System Design Codes &�Standards Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - November 2008

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