CRD Geothermal System Design Principles 2 — Questions and Answers
Question 1: Which parameter most directly determines the required length of a vertical bore field in a residential geothermal system?
- Soil moisture content
- Ground thermal conductivity (Correct answer)
- Borehole diameter
- Grout thermal resistance
Correct answer: Ground thermal conductivity
Ground thermal conductivity governs how efficiently heat transfers between the grout and surrounding soil, directly setting the required bore length per ton of capacity.
Question 2: In a closed-loop geothermal system, what is the primary purpose of the expansion tank?
- Increase system pressure to improve efficiency
- Absorb fluid volume changes due to temperature fluctuation (Correct answer)
- Remove air bubbles from the loop
- Maintain glycol concentration
Correct answer: Absorb fluid volume changes due to temperature fluctuation
The expansion tank accommodates the volumetric expansion and contraction of the loop fluid as its temperature changes, preventing dangerous pressure spikes.
Question 3: A designer uses a Ground Loop Heat Exchanger (GLHE) software tool and obtains a g-function. What does the g-function represent?
- Grout thermal conductivity vs. depth
- Dimensionless temperature response of the bore field over time (Correct answer)
- Glycol flow rate correction factor
- Ground surface heat flux profile
Correct answer: Dimensionless temperature response of the bore field over time
The g-function is a dimensionless thermal response factor that describes how the borehole field temperature changes over time under a unit heat pulse.
Question 4: What is the typical design entering water temperature (EWT) range for a residential heat pump operating in heating mode in a US northern climate?
- 32–40°F (0–4°C) (Correct answer)
- 25–35°F (-4 to 2°C)
- 45–55°F (7–13°C)
- 55–65°F (13–18°C)
Correct answer: 32–40°F (0–4°C)
Northern climate ground loops are typically designed so that minimum EWT stays at or above 32–40°F to avoid freezing and maintain heat pump efficiency.
Question 5: Which heat pump rating condition is used by AHRI Standard 870 to certify residential geothermal water-to-air units?
- EWT 50°F heating / 86°F cooling (Correct answer)
- EWT 32°F heating / 77°F cooling
- EWT 41°F heating / 59°F cooling
- EWT 60°F heating / 104°F cooling
Correct answer: EWT 50°F heating / 86°F cooling
AHRI 870 rates residential water-to-air heat pumps at 50°F EWT for heating and 86°F EWT for cooling as the standard certification conditions.
Question 6: When sizing a horizontal slinky loop, which factor primarily determines how many trenches are needed?
- Depth of frost line only
- Trench length available and soil thermal conductivity (Correct answer)
- Pipe wall thickness
- Antifreeze type used
Correct answer: Trench length available and soil thermal conductivity
The number of trenches is driven by the available land area (trench length) and the soil's ability to recover heat, both of which set the loop's heat exchange capacity.
Question 7: A CRD designer specifies a desuperheater on a residential geothermal heat pump. What is its primary benefit?
- It increases COP during peak winter demand
- It pre-heats domestic hot water using waste heat from the refrigerant (Correct answer)
- It reduces ground loop pumping energy
- It provides auxiliary electric backup heating
Correct answer: It pre-heats domestic hot water using waste heat from the refrigerant
A desuperheater captures superheat from the refrigerant discharge gas to pre-heat domestic hot water, reducing water heater energy consumption.
CRD Geothermal System Design Principles 2
Which parameter most directly determines the required length of a vertical bore field in a residential geothermal system?