CRD Geothermal System Design Principles 3 — Questions and Answers
Question 1: What is 'thermal short-circuiting' in a vertical borehole and how is it mitigated?
- Fluid bypassing the heat pump; fixed by adding a check valve
- Heat transfer between the supply and return pipes within the same bore; mitigated by using thermally enhanced grout and proper pipe spacing (Correct answer)
- Electrical short in the pump motor; fixed by grounding
- Ground water infiltration into the borehole; fixed by casing
Correct answer: Heat transfer between the supply and return pipes within the same bore; mitigated by using thermally enhanced grout and proper pipe spacing
Thermal short-circuiting occurs when heat from the warmer pipe leg transfers directly to the cooler leg inside the bore, reducing efficiency; proper grout and U-tube spacing reduce it.
Question 2: In the IGSHPA design method, what does the 'run fraction' (PLF) account for?
- Percentage of loop buried below frost line
- Fraction of time the heat pump operates at full load during the design month (Correct answer)
- Ratio of horizontal to vertical loop length
- Proportion of glycol to water in the heat transfer fluid
Correct answer: Fraction of time the heat pump operates at full load during the design month
The part-load factor (PLF or run fraction) reflects that the heat pump cycles on and off, allowing the ground some thermal recovery between runs.
Question 3: Which soil/rock type generally yields the highest ground thermal conductivity, allowing shorter bore lengths?
- Dry sand
- Saturated clay
- Granite or crystalline bedrock (Correct answer)
- Loose gravel
Correct answer: Granite or crystalline bedrock
Granite and other crystalline rocks have high mineral thermal conductivity (typically 2.5–3.5 W/m·K), reducing required bore length compared to sedimentary or unconsolidated soils.
Question 4: A Thermal Response Test (TRT) is performed on a pilot borehole. What is the primary output used by the designer?
- Borehole depth confirmation
- In-situ effective ground thermal conductivity and borehole thermal resistance (Correct answer)
- Groundwater flow velocity
- Soil pH and mineral content
Correct answer: In-situ effective ground thermal conductivity and borehole thermal resistance
A TRT measures the temperature rise of circulating fluid under a constant heat injection rate to back-calculate the ground's effective thermal conductivity and borehole resistance.
Question 5: For a residential open-loop (pump-and-reinjection) geothermal system, what is the minimum recommended well separation between the supply and injection wells?
- 10 ft (3 m)
- 50 ft (15 m)
- 100 ft (30 m) or more depending on aquifer properties (Correct answer)
- 500 ft (152 m)
Correct answer: 100 ft (30 m) or more depending on aquifer properties
At least 100 ft of separation is commonly required to prevent thermal short-circuiting where cooled/warmed return water re-enters the supply well before recovering.
Question 6: When calculating the total equivalent length for a vertical bore field, the designer adds a 'penalty temperature' (TP). What does TP represent?
- Temperature correction for altitude above sea level
- Long-term ground temperature rise (or drop) due to cumulative annual heat imbalance (Correct answer)
- Penalty for using a non-certified heat pump
- Temperature differential across the pump
Correct answer: Long-term ground temperature rise (or drop) due to cumulative annual heat imbalance
The penalty temperature accounts for the multi-year buildup of heat in the ground when annual heating and cooling loads are unbalanced, shifting mean ground temperature over the system's life.
Question 7: Which ASHRAE publication serves as the primary industry reference for ground-source heat pump system design in the United States?
- ASHRAE Handbook – HVAC Applications, Chapter on Geothermal Energy (Correct answer)
- ASHRAE Standard 90.1
- ASHRAE Guideline 14
- ASHRAE Standard 62.1
Correct answer: ASHRAE Handbook – HVAC Applications, Chapter on Geothermal Energy
The ASHRAE HVAC Applications Handbook chapter on geothermal energy is the principal US reference covering design methods, equipment selection, and loop sizing for GSHP systems.
CRD Geothermal System Design Principles 3
What is 'thermal short-circuiting' in a vertical borehole and how is it mitigated?