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Proper design conditions must be created for 100-percent-outside-air units. It is worth noting that ASHRAE's recommended cooling condition of 1 percent commonly is exceeded throughout the eastern United States, and engineers must consider similar types of abnormal design situations.

A weather system with dry-bulb temperatures in the mid-90s and wet-bulb temperatures in the 80s recently passed through the Midwest. The front affected Missouri, Kentucky, Illinois, Indiana, and southern Ohio for more than three days. Such conditions can cause a DOAS to act as a humidity injector for a building. Dehumidification controls become a necessity and the situation typically is not corrected immediately after installation.

HYBRID SOLUTIONS

While a pure GSHP system can be a goal, hybrid solutions also should be considered, especially for less-than-ideal weather conditions. For a heating project, a hybrid solution might be used because ground temperatures do not always provide water warm enough to satisfy extreme heating requirements. In such cases, a design incorporating a water heater in the loop after it enters the building can provide a heating boost, resulting in sufficient heat without lowering system efficiency throughout most of the year. Additionally, this approach can limit the cost of drilling more or deeper wells for ground-heat transfer. Because GSHP system designs require economic analysis, a designer should model a number of schemes for loop sizes, depths, pumping efficiencies, equipment sizes, run times, etc.

For cooling projects, cooling boosters commonly are incorporated into designs of hybrid systems. Open cooling towers are undesirable because of the need for makeup water, chemical treatment, and associated extra maintenance. However, it is popular to use closed-circuit evaporative coolers, which can provide booster cooling to a water system while a closed-piping arrangement is maintained. Such supplemental-cooler concepts are similar to the previously mentioned heating model. Supplemental cooling equipment may be needed only for extremes, but designers can choose to have the equipment operate during certain conditions because it can provide higher efficiencies within GSHP units.

With current software, engineers can increase a building's efficiency by incorporating online interactive control of systems that previously ran relatively wild without regard for operating costs. Hybrid systems should be considered in almost all preliminary design analyses. They can reduce first and operating costs while giving a building greater capacity and flexibility.

CONCLUSION

GSHPs are a relatively new technology that can be beneficial to building owners with respect to initial and operating costs and indoor space conditions. The challenge is for engineers to commit themselves to learning the technology, resulting in proper application.

Well-engineered buildings cannot be created from a recipe. However, keys to a successful GSHP project include:

• Knowing geological conditions.
• Evaluating bore-field size, depth, and spacing.
• Requiring an experienced driller.
• Making and reviewing test wells before final work is done.
• Preparing a cost analysis of the bore field.
• Knowing each zone's heating and cooling loads.
• Fully analyzing all possible pumping solutions.
• Evaluating control options.
• Remembering that GSHPs are unitary systems.
• Controlling humidity.
• Ensuring systems are balanced.

RESOURCES

ASHRAE. (1995). Commercial ground-source heat pump systems. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Sachs, H. (2002). Geology and drilling methods for ground-source heat pumps installations: An introduction for engineers. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Kavanaugh, S., Rafferty, K., & Geshwiler, M. (1997). Ground-source heat pumps: Design of geothermal systems for commercial and institutional buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Fisher, D., Rees, S., Padhmanabhan, S., & Murugappan, A. (2006). Implementation and validation of ground-source heat pump system models in an integrated building and system simulation environment. HVAC&R Research Journal, 12, 693-710.

ASHRAE. (2007). HVAC applications handbook. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASHRAE. (2008). ASHRAE handbook — HVAC systems and equipment. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

ASHRAE. (2009). ASHRAE handbook — fundamentals. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.

Mumma, S.A. (2009). 30% surplus OA: Does it use more energy. ASHRAE Journal, 6, 24-36.

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Anthony B. McGuire founded McGuire Engineers in 1986 after working in engineering firms in Chicago, Indianapolis, Boston, and New York. As chairman of the company, he is involved in the consultation, design, and construction of mechanical and electrical systems around the world. A registered engineer in 21 states, he has received numerous honors from professional engineering associations. He serves as an advisor to the Chicago Building Department and the Chicago Building Owners and Managers Association.

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