CRD Ground Loop & Heat Pump Configuration 5 — Questions and Answers
Question 1: During a thermal response test (TRT), what two key ground parameters are determined?
- Soil pH and moisture content
- Ground thermal conductivity and borehole thermal resistance (Correct answer)
- Ground temperature and bore depth
- Soil density and specific heat capacity separately
Correct answer: Ground thermal conductivity and borehole thermal resistance
A TRT injects or extracts heat at a known rate and measures temperature response over time to calculate ground thermal conductivity and borehole thermal resistance.
Question 2: A header manifold in a parallel ground loop field is sized primarily to:
- Increase total fluid velocity to prevent freezing
- Ensure equal flow distribution to all parallel bore circuits (Correct answer)
- Reduce system operating pressure below 30 psi
- Allow individual bore circuits to be isolated for servicing
Correct answer: Ensure equal flow distribution to all parallel bore circuits
Proper manifold sizing and reverse-return or balanced-tee design ensures equal flow through each bore, preventing short-circuiting of cooler or warmer fluid.
Question 3: What is 'undisturbed ground temperature' (UGT) and why is it critical in loop design?
- The temperature at the frost line used to size antifreeze concentration
- The native soil temperature before system installation, representing the baseline for heat exchange calculations (Correct answer)
- The temperature difference between bore top and bottom used to set pump head
- The maximum allowable temperature rise during summer cooling mode
Correct answer: The native soil temperature before system installation, representing the baseline for heat exchange calculations
UGT is the soil temperature before any system influence; it sets the source/sink baseline that determines heat pump COP and required bore length.
Question 4: Which of the following best describes the 'EWT' that appears on a heat pump performance data table?
- Evaporator water temperature at the refrigerant side
- Entering water temperature to the heat pump from the ground loop (Correct answer)
- Equivalent water temperature used for air-to-water comparison
- Exit water temperature leaving the domestic hot water tank
Correct answer: Entering water temperature to the heat pump from the ground loop
EWT (Entering Water Temperature) is the ground loop fluid temperature entering the heat pump's water-to-refrigerant heat exchanger, directly affecting rated capacity and efficiency.
Question 5: What happens to heat pump heating COP as entering water temperature (EWT) decreases during cold winters?
- COP increases because the compressor runs cooler
- COP decreases because the temperature lift the compressor must overcome increases (Correct answer)
- COP stays constant because refrigerant charge compensates
- COP increases due to higher refrigerant density at low temperatures
Correct answer: COP decreases because the temperature lift the compressor must overcome increases
Lower EWT means the refrigerant must evaporate at a lower pressure, increasing compressor compression ratio and work input, thereby reducing COP.
Question 6: In a hybrid geothermal system, a supplemental cooling tower is added primarily to:
- Pre-cool outdoor air before it enters the heat pump
- Reject excess heat during cooling-dominated operation to reduce ground temperature rise (Correct answer)
- Provide backup heating when the heat pump is in defrost mode
- Supply condenser water to a conventional chiller as a backup
Correct answer: Reject excess heat during cooling-dominated operation to reduce ground temperature rise
A cooling tower dissipates excess heat rejection in cooling-dominated climates, preventing long-term ground warming that would degrade heat pump COP over years.
Question 7: Which design tool calculation method does ASHRAE recommend for sizing vertical bore fields in commercial and residential applications?
- Manual J heating load calculation
- The g-function method using dimensionless borehole response functions (Correct answer)
- The degree-day method based on local climate data
- The bin method using hourly weather data only
Correct answer: The g-function method using dimensionless borehole response functions
ASHRAE and IGSHPA recommend the g-function (Eskilson) method, which uses borehole response functions to account for thermal interference and long-term ground temperature changes.
CRD Ground Loop & Heat Pump Configuration 5
During a thermal response test (TRT), what two key ground parameters are determined?