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Working With It - Sizing

Loop Length

Loop Length Sizing the loop length requires a knowledge of the earth temperature and thermal conductivity at planned design depths.

  1. Define the buried heat exchanger location. This includes pipe depth and spacing for horizontal loops, or borehole depth, the number of U-bend loops and spacing of boreholes for vertical installations.

  2. Determine the underground soil temperatures. Annual air temperatures, moisture content, soil type and vegetative cover all have an effect. Several sources should be used, including local knowledge based on the loop contractor's experience and the measured data.

  3. Determine the maximum and minimum earth temperatures. This step uses information developed in step two, along with analytical equations.

  4. Determine the heat pump maximum and minimum entering fluid temperatures. Do this using the manufacturer's published data and estimates based on the site location

  5. Calculate the earth-to-loop temperature difference. This is the difference between the earth temperature minimum and the heat pump minimum entering fluid temperature for heating, and conversely the maximum temperature for the cooling cycle.

  6. Determine the pipe resistance to heat flow. This calculation uses thermal conductivity data for the pipe selected and the loop configuration.

  7. Determine the soil/field resistance to heat flow. This can be a time-consuming process, so using a software program is recommended.

  8. Calculate the heating and cooling run fractions. These are based on the building's heating and cooling loads, balance temperatures where no heating or cooling is required, heat pump capacity and hours in each temperature bin.

  9. Calculate the ground heat exchanger size based on:
    • The length of pipe for a horizontal system;
    • The feet of borehole for a vertical system; or
    • The defining parameter for other designs.

Design engineers typically examine several entering water temperatures to get a range of choices. Shorter lengths may be the lowest installation cost, but a longer length increases the heat pump capacity and efficiency, and lowers the operating cost. The final size selection will be a compromise.

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