CRD Geothermal System Design Principles 5 — Questions and Answers
Question 1: What does EER measure in the context of a geothermal heat pump operating in cooling mode?
- Energy extracted from the ground per hour
- Ratio of cooling output (BTU/hr) to electrical input (watts) at a single rating condition (Correct answer)
- Seasonal average efficiency over the entire cooling season
- Ground loop heat rejection rate
Correct answer: Ratio of cooling output (BTU/hr) to electrical input (watts) at a single rating condition
EER (Energy Efficiency Ratio) is the instantaneous cooling capacity in BTU/hr divided by electrical power input in watts at a specific set of rating conditions.
Question 2: What is 'borehole thermal resistance' (Rb) and why does it matter in loop design?
- Resistance to ground water entering the borehole; affects water quality
- Thermal resistance between the circulating fluid and the borehole wall; higher Rb requires longer bores (Correct answer)
- Electrical resistance of the circulation pump motor
- Rock compressive strength affecting drilling cost
Correct answer: Thermal resistance between the circulating fluid and the borehole wall; higher Rb requires longer bores
Borehole thermal resistance (Rb) quantifies how difficult it is for heat to move between the fluid in the U-tube and the surrounding ground; a higher Rb means less efficient heat exchange and greater required bore length.
Question 3: When a geothermal system is designed with a 'hybrid' configuration pairing a ground loop with a cooling tower, what problem does this solve?
- Insufficient heating capacity in extreme cold
- Cooling-dominated building loads that would thermally saturate the ground loop (Correct answer)
- Excessive domestic hot water demand
- High drilling costs in rocky soil
Correct answer: Cooling-dominated building loads that would thermally saturate the ground loop
A cooling tower (or fluid cooler) rejects surplus heat to the atmosphere in cooling-dominated climates, preventing multi-year ground temperature rise that would degrade heat pump efficiency.
Question 4: IGSHPA's block-load design method for residential systems requires knowing the design-month peak block load and the annual equivalent full-load hours. What are the annual hours used to calculate?
- Required compressor refrigerant charge
- The annual ground heat exchange needed to determine long-term temperature penalty (Correct answer)
- Number of service visits per year
- Minimum loop burial depth
Correct answer: The annual ground heat exchange needed to determine long-term temperature penalty
Annual equivalent full-load hours convert the seasonal energy into an annual ground heat exchange quantity, which is used to compute the multi-year temperature penalty from load imbalance.
Question 5: What pipe material is most widely used for residential closed-loop ground heat exchangers in the US, and why?
- Copper, for high thermal conductivity
- High-density polyethylene (HDPE), for flexibility, chemical resistance, and heat-fused leak-free joints (Correct answer)
- PVC, for low cost and rigid installation
- Stainless steel, for longevity in corrosive soils
Correct answer: High-density polyethylene (HDPE), for flexibility, chemical resistance, and heat-fused leak-free joints
HDPE is the industry standard because it withstands ground movement, resists corrosion and biological attack, and can be thermally fused to create joints with no mechanical fittings underground.
Question 6: A residential geothermal system shows a progressively declining COP over several heating seasons. Which design oversight is the most likely cause?
- Oversized circulator pump
- Thermally unbalanced annual loads causing cumulative ground temperature depression (Correct answer)
- Use of propylene rather than ethylene glycol
- Excessive duct static pressure
Correct answer: Thermally unbalanced annual loads causing cumulative ground temperature depression
When annual heating extraction greatly exceeds cooling rejection, the mean ground temperature drops each year, reducing EWT and heat pump COP over time.
Question 7: Which regulatory or certification body administers the Certified Residential Geothermal Designer (CRD) credential in the United States?
- ASHRAE
- IGSHPA (International Ground Source Heat Pump Association) (Correct answer)
- ACCA
- NATE
Correct answer: IGSHPA (International Ground Source Heat Pump Association)
IGSHPA administers the CRD credential, which validates competency in ground-source heat pump system design for residential applications.
CRD Geothermal System Design Principles 5
What does EER measure in the context of a geothermal heat pump operating in cooling mode?