Loop System Design Fundamentals
What the exam tests
This domain covers how a ground loop moves heat, how loop capacity is matched to the building load, and how loop type and length are chosen. Expect questions on heat transfer, thermal resistance, loop configurations, and the relationship between entering water temperature and heat pump performance.
Core rules
- A ground loop is sized for the more demanding of the heating and cooling loads, not the larger equipment tonnage alone.
- Loop length depends on soil or rock thermal conductivity, borehole thermal resistance, groundwater, and the minimum and maximum entering water temperatures the heat pump can tolerate.
- Vertical loops need less land; horizontal loops need more trench area; pond loops need adequate depth and volume.
- Balanced annual heat rejection and extraction preserve ground temperature; heavily cooling-dominated loads may need a larger loop or a hybrid design.
- Use a thermal conductivity test on commercial or uncertain sites instead of relying only on rules of thumb.
Worked example
A house has a 48,000 Btu/h heating load and a 36,000 Btu/h cooling load; the selected unit is 4 tons. In good soil, a vertical design at 180 ft per ton gives 4 × 180 = 720 ft. With 240-ft boreholes, that is 720 ÷ 240 = 3 boreholes. If the cooling load were larger, the design would follow the cooling block load, not the nominal tonnage.
Traps
Sizing only to the outdoor unit; ignoring the dominant load; applying one footage rule to rock and clay alike; forgetting that low entering water temperature reduces capacity and efficiency.