Study for the CGD by practicing design decisions, not by memorizing isolated facts. Geoexchange design couples three quantities that must stay distinct in your head: the building's peak heating and cooling loads, the annual energy extracted from and rejected to the ground, and the ground's thermal properties as actually measured on the site. Set up your study around worked borefield scenarios where you compute the annual load balance, interpret a thermal response test as a range rather than a single value, choose entering fluid temperature targets deliberately, and decide when a hybrid heat rejection source is warranted.
Separating peak loads from annual ground loads in borefield sizing
Borefield sizing depends on two different load quantities. Design peak heating and cooling loads set the hourly fluid temperatures the loop must hold, while the annual net ground load, extracted energy minus rejected energy, drives multi-year ground temperature change. Sizing from the wrong quantity produces a system that fails in the wrong way.
Keep three worksheets separate from the start of any design: a peak load calculation at design conditions, a monthly or annual energy profile of heat extracted and rejected, and a ground thermal model. The peak loads determine required heat pump capacity and the worst-hour entering fluid temperature. The annual balance determines whether the ground itself stays stable. A field can satisfy every peak comfortably and still drift year after year because the seasonal energy flows never cancel.
Worked scenario: a cold-climate school extracts roughly 300 MWh of heat per year and rejects 120 MWh, for a net annual extraction of 180 MWh. A designer sizes 40 bores at 100 m strictly against peak loads with generous safety margin and declares the design done. In a simplified check, if that imbalance is absorbed by roughly 144,000 cubic meters of ground with a volumetric heat capacity near 2.2 MJ per cubic meter per kelvin, the bulk ground temperature falls on the order of two kelvin per year. Within a few heating seasons, entering fluid temperatures sink below heat pump limits and capacity collapses. The better decision is to compute the net annual ground load first, then choose between a larger field, a supplemental heat source, or a load-side change, and document which failure mode the design protects against.
Reading a thermal response test as a range, not a single number
A thermal response test yields an effective ground thermal conductivity that blends formation conductivity, borehole and grout resistance, and test artifacts into one fitted value. Treat it as a design input with uncertainty: run every borefield calculation across a low-to-high conductivity band and note which end governs each season.
Interpret the test before using it. Check the undisturbed ground temperature used, whether applied power was steady, how long the test ran, and whether groundwater flow or layered geology could distort the fit. A short test mostly characterizes conditions near the borehole; advection from moving groundwater can make the fitted conductivity look better than conduction alone in winter or worse in still conditions. When comparing a test result against regional geology references, explain any large gap rather than averaging it away.
Worked scenario: two response tests on one campus return noticeably different effective conductivities. A designer averages the two values, plugs the midpoint into the sizing software once, and moves on. The stronger approach asks why they differ: different depth intervals, different moisture or groundwater conditions, or unstable test power. Then run the sizing across the bounding values and check what the bore count and the worst-hour entering fluid temperature do at each bound. If the high-conductivity case permits two fewer bores but the low case forces a different configuration, the conservative bound governs the decision. Why it matters: the single-number habit hides the site's actual uncertainty, and the documented range is what justifies the final field to a reviewer.
Choosing entering fluid temperature targets that hold at both extremes
The design entering fluid temperatures, a minimum for heating mode and a maximum for cooling mode, lock in heat pump capacity and efficiency, and they couple directly to ground temperature, loop flow, and field size. Set these targets from the load and ground analysis, then verify the equipment selection against them.
Trace the coupling explicitly: undisturbed ground temperature, plus or minus the seasonal and multi-year temperature change, sets the fluid temperature range available to the loop; the loop temperature difference and flow set how much of that range each bore uses; and the resulting entering fluid temperature determines heat pump output and coefficient of performance at the design hour. Change one target and the others move. A modest relaxation of the heating-mode minimum can shrink a borefield materially, while an aggressive target can quietly force extra bores that add cost without adding delivered comfort.
Compare two deliberate choices on the same building. Suppose the site's undisturbed ground sits near ten degrees Celsius and loads are moderate. A designer who imports a cold-climate minimum target from a different region sizes a field for conditions that will not occur there, paying for depth the loads never require. The better decision states the target with reasoning: the coldest entering fluid temperature the selected heat pumps can accept with required capacity, checked against the projected multi-year ground temperature from the annual balance. Documenting that chain, ground temperature to loop range to equipment limit, is the habit that separates a defensible design from a copied template.
Deciding when a cooling-dominant building needs a hybrid heat rejection source
When annual rejected energy substantially exceeds annual extracted energy, the ground warms year over year and the cooling-mode design fails over time. Compare three responses, an enlarged borefield, a supplemental heat rejection device such as a dry cooler or cooling tower, and load-side measures, on cost, ground stability, and control complexity.
Worked scenario: a cooling-dominant medical clinic rejects roughly 420 MWh of heat annually but extracts only 140 MWh, a net rejection of 280 MWh. The designer's first instinct is to enlarge the borefield until the imbalance is absorbed, an approach that spends heavily and still leaves the ground warming monotonically, pushing cooling-mode entering fluid temperatures upward each year until equipment capacity or limits are reached. The better decision sizes the core borefield for balanced operation and adds a hybrid heat rejection device sized to shed most of the imbalance, for illustration, enough to cut the net annual rejection to well under a third of its original value, leaving drift slow enough to stay within limits over the design life.
The control sequence then becomes part of the design, not an afterthought. Specify when the loop bypasses the ground and rejects through the hybrid device, usually whenever rejecting through the ground would push the loop above its cooling-mode maximum, and confirm the sequence in both shoulder seasons and peak weeks. Why it matters: a hybrid changes the worst-hour analysis, the annual balance, and the commissioning checklist at once, and a design that ignores the sequencing produces a device that runs at the wrong times and saves nothing. Compare the hybrid against the pure-enlargement option on first cost, ground temperature drift, and operability before committing.
Matching loop configuration to site constraints and load character
Configuration choice is a design decision with its own criteria: available land area, geology and drilling conditions, the load balance from your annual analysis, and water availability or permitting for surface systems. Compare the main options against the specific project before defaulting to vertical bores.
Vertical closed-loop borefields need the least land area and reach stable temperatures at depth, but they inherit drilling costs, grout thermal performance, and borehole resistance from the response test. Horizontal systems trade trench length and land area for lower installation cost and shallower, more seasonally variable ground temperatures, which shifts the entering fluid temperature analysis. Pond and surface water loops can be economical where a suitable water body exists, but thermal stratification, seasonal temperature swings, and permitting change the analysis again rather than simply extending it.
Use the load character you computed earlier to narrow the field. A strongly heating-dominant building on a small urban lot pushes toward vertical bores with careful attention to the extraction imbalance; a balanced-load campus with open land can justify horizontal circuits; a cooling-dominant site adjacent to a permitted water body may combine surface rejection with a smaller ground field. Then check the configuration against the conductivity range from the response test, because an option that only works at the optimistic conductivity bound is not actually an option. Documenting why rejected configurations were rejected is as valuable as the one you choose.
| Configuration | Suits best when | Key design constraint | Fails when |
|---|---|---|---|
| Vertical closed-loop borefield | Land is limited; loads are large; stable temperatures needed | Borehole and grout resistance; annual load imbalance drives multi-year drift | Sized on peaks alone or one averaged conductivity value |
| Horizontal closed-loop | Open land exists; budget favors trenching over drilling | Land area; shallower seasonal temperature swings | Land is too small or loads demand stable deep temperatures |
| Surface water loop | Suitable permitted water body is adjacent | Stratification and seasonal water temperatures; permitting | Water body is small, shallow, or thermally limited in summer |
| Hybrid with heat rejection device | Annual rejection far exceeds extraction | Control sequencing between ground and device | Device and ground are never coordinated in the controls |
A pencil-and-paper borefield check with a self-review rubric
You can rehearse the central decision without software. Take a monthly load profile, compute the net annual ground load, estimate the multi-year ground temperature drift, and check the seasonal swing separately. Score your work against the rubric below before checking the arithmetic.
Exercise setup. Build or take a simplified twelve-month profile with heating extraction and cooling rejection in energy units. Add the extracted energy, add the rejected energy, and take the difference as the net annual ground load, noting its sign: negative means net extraction and cooling ground, positive means net rejection and warming ground. Then, as a coarse illustration only, estimate the bulk temperature drift by dividing the cumulative net energy over the design life by the ground volume the field plausibly influences and the ground's volumetric heat capacity. Repeat with the bore count doubled and observe how the drift per year scales.
Expected observations. First, an imbalanced profile produces a monotonic drift, not an oscillation that returns each year; the seasonal swing rides on top of it but never cancels it. Second, doubling the bore count roughly halves the drift per unit of ground, which is why imbalance can be bought out but at a steep cost. Third, the sign of the net load tells you which season's design limit erodes first. Self-check rubric: one point for a correctly signed net annual load, one for separating seasonal swing from multi-year drift, one for stating which design limit erodes first, one for naming two sensitivity checks you would run, such as conductivity bounds and heat pump limits. Four points means the reasoning chain is intact; below three, redo the profile before moving on.
A preparation sequence organized around design decisions, not recall
Sequence your preparation the way a design actually proceeds: load analysis first, ground interpretation second, target-setting and configuration third, hybrid and documentation last. Finish each stage by producing a short written justification, because writing the rationale is itself exam-relevant practice.
A realistic, adaptable sequence: first, restate the core concepts in your own words, including peak versus annual loads, thermal response testing, borehole resistance, entering fluid temperature coupling, and configuration options, using a mind map to show how each parameter flows into the next. Second, drill load analysis and response-test interpretation with the pencil-and-paper exercise above and with range-based conductivity checks. Third, run full scenario practices, two or three per week, where you commit to a configuration, targets, and hybrid decision, then write the two-paragraph justification a reviewer would expect. Fourth, review documentation and professional-standards expectations: assumptions stated, sources of ground properties identified, control sequences specified.
For administrative matters, rely on the Association of Energy Engineers directly: AEE certifications generally combine education and experience eligibility, an approved training program, and an application followed by an exam that may be taken live or remotely per current AEE terms; confirm the CGD-specific requirements, schedule, and fees on the AEE certifications page rather than from secondary sources. Readiness checks before you sit the exam: you can compute a signed net annual ground load from a monthly profile without notes; you can explain why a response test result is a range; you can trace ground temperature to entering fluid temperature to equipment limits; and you can justify a hybrid decision in writing in under ten minutes. If any check fails, return to the matching stage rather than re-reading everything.
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.