CRD Geothermal System Design Principles 4 — Questions and Answers
Question 1: What is the recommended minimum flow rate per ton of capacity for a residential closed-loop geothermal system using water as the heat transfer fluid?
- 0.5 GPM per ton
- 1.5–3.0 GPM per ton (Correct answer)
- 5.0 GPM per ton
- 10.0 GPM per ton
Correct answer: 1.5–3.0 GPM per ton
IGSHPA and equipment manufacturers recommend 1.5–3.0 GPM per ton to ensure adequate heat transfer and turbulent flow in the ground loop.
Question 2: A geothermal loop filled with 20% propylene glycol solution will have a freeze point of approximately:
- -50°F (-46°C)
- 15°F (-9°C) (Correct answer)
- 28°F (-2°C)
- 32°F (0°C)
Correct answer: 15°F (-9°C)
A 20% propylene glycol solution has a freeze protection point of approximately 15°F (-9°C), providing a safety margin below typical minimum EWT targets.
Question 3: In a two-stage or variable-speed compressor geothermal heat pump, what design advantage does variable capacity provide for residential systems?
- Eliminates the need for a ground loop entirely
- Better matches part-load demand, improving seasonal efficiency and comfort (Correct answer)
- Allows use of smaller diameter piping
- Removes the requirement for antifreeze
Correct answer: Better matches part-load demand, improving seasonal efficiency and comfort
Variable-capacity compressors modulate output to match the actual building load, reducing short-cycling and improving the system's seasonal COP and humidity control.
Question 4: Which loop configuration is most appropriate for a residential site with limited land area but access to deep drilling?
- Horizontal slinky loops
- Pond/lake loops
- Vertical closed-loop boreholes (Correct answer)
- Surface water open-loop
Correct answer: Vertical closed-loop boreholes
Vertical closed-loop boreholes require a small surface footprint because heat exchange length is achieved by drilling deep rather than spreading horizontally.
Question 5: What is the purpose of a 'header' in a closed-loop geothermal bore field?
- It pressurizes the refrigerant circuit
- It connects multiple parallel borehole loops to a common supply and return manifold (Correct answer)
- It filters sediment from the loop fluid
- It measures entering water temperature
Correct answer: It connects multiple parallel borehole loops to a common supply and return manifold
The header manifold connects individual borehole U-tubes in parallel, balancing flow distribution across the bore field and reducing total pressure drop.
Question 6: According to IGSHPA guidelines, what is the maximum allowable pressure drop recommended across a residential geothermal ground loop to minimize pump energy?
- 2 ft WC per 100 ft of pipe
- 4 ft WC per 100 ft of pipe (Correct answer)
- 10 ft WC per 100 ft of pipe
- 20 ft WC per 100 ft of pipe
Correct answer: 4 ft WC per 100 ft of pipe
IGSHPA recommends limiting friction loss to approximately 4 ft of water column per 100 ft of pipe to keep circulating pump power within an energy-efficient range.
Question 7: A designer is evaluating a pond loop for a residential system. What minimum pond volume (in acre-feet) per ton of system capacity is generally recommended?
- 0.25 acre-ft per ton
- 1.5 acre-ft per ton (Correct answer)
- 5 acre-ft per ton
- 10 acre-ft per ton
Correct answer: 1.5 acre-ft per ton
Approximately 1.5 acre-feet of pond volume per ton of heating/cooling capacity is the commonly cited minimum to provide adequate thermal mass and recovery.
CRD Geothermal System Design Principles 4
What is the recommended minimum flow rate per ton of capacity for a residential closed-loop geothermal system using water as the heat transfer fluid?