Geothermal System Design Principles Flashcards

Which parameter most directly determines the required length of a vertical bore field in a residential geothermal system?

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.

In a closed-loop geothermal system, what is the primary purpose of the expansion tank?

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.

A designer uses a Ground Loop Heat Exchanger (GLHE) software tool and obtains a g-function. What does the g-function represent?

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.

What is the typical design entering water temperature (EWT) range for a residential heat pump operating in heating mode in a US northern climate?

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.

Which heat pump rating condition is used by AHRI Standard 870 to certify residential geothermal water-to-air units?

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.

When sizing a horizontal slinky loop, which factor primarily determines how many trenches are needed?

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.

A CRD designer specifies a desuperheater on a residential geothermal heat pump. What is its primary benefit?

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.