For The GSHP System Designer
Do
- Recognize that proper design of a ground heat exchanger (GHEX) piping system is critical, including considerations for long-term energy build-up or draw-down in the loopfield caused by unbalanced system ground loads. If this concept is unfamiliar, attend an appropriate industry training event or enlist the help of a professional GSHP system designer.
- Perform peak heating and cooling load and energy usage calculations for the HVAC system. Accuracy is critical as system design and performance will be directly related to these calculations.
- Consider providing domestic hot water (DHW) with the GSHP system. Determine the quantity of water to be used, the hot water generation (HWG) loads, and then select the appropriate HWG system type. Perform economic analysis calculations for the selected HWG system to determine feasibility along with HWG system payback.
- Check the suitability of the local soil and geology for the type of GHEX being considered.
- Consider running a formation thermal conductivity (FTC, in-situ) test if the project size is greater than 25 tons or if the local geological conditions are relatively unknown.
- Design a vertically- or horizontally-bored GHEX piping system using a high thermal conductivity, low permeability grouting material. The grout thermal conductivity value should be selected based on an honest and comprehensive comparative economic analysis.
- Check the site for access points for the GHEX installation equipment. A site visit is mandatory.
- Correctly size the equipment. Do not add a large factor of safety.
- Optimize the design of the interior piping and GHEX piping systems along with the selection of the associated circulating pumps. Advertised operating cost savings provided by a GSHP system can be quickly negated by a poor piping/pumping system design.
- Select the antifreeze type and concentration based on the minimum expected circulating fluid temperatures (if necessary). Freeze protection to 10°F below the average circulating fluid temperature under design conditions is required if fluid temperatures fall below 40°F at any point in the system at any time.
- For retrofit applications, ensure that a GSHP system retrofit is possible with the existing
distribution system.
- Water-air GSHP units typically provide 400 cubic feet per minute (cfm) airflow rates per ton of nominal capacity. Ductwork should be properly sized to handle the required amount of airflow without excessive restriction.
- Water-water GSHP units can efficiently provide hot water at temperatures up to
about 120°F (100°F hot water supply temperatures are optimum, the lower the
better). Beyond that, equipment capacities and efficiencies suffer greatly.
For a retrofit application with baseboard heat served by an existing boiler (150-180°F hot water supply temperatures), a GSHP system will not provide water temperatures that are high enough for the existing distribution system to be effective. Additional measures must be taken to properly heat the home. - Create a thorough, detailed set of specifications so that all parties understand what is expected through all phases of the GSHP system design and installation process.
- Refer to state and local codes to ensure that all aspects of the GSHP system design are in compliance including antifreeze type, interior piping material selection, grout material selection and placement method and ground connection method.
- ASK FOR HELP if GHEX design is unfamiliar territory. Paying for consulting fees up front is much less expensive than having to fix a poorly designed system after the fact.
Don’t
- Guess or use rules-of-thumb for heat loss calculations, GHEX design, or pipe sizing and associated pump selection.
- Assume there will be sufficient space for a GHEX without calculating the actual amount space required.
- Assume that even though there is sufficient space for GHEX installation, a system can be installed without considering buried utility locations, property setback and easement restrictions, and ensuring that heavy equipment access is not an issue.
- Allow the center to center spacing between vertical- or horizontal-bores to be less than 15 ft.
- Leave it to the drilling contractor to provide a loopfield design for the system. A qualified engineer should perform the analysis necessary to provide such a design.
Contact our engineering department to discuss possible options for your GSHP system and for geothermal design services.