Vertical Closed Loop

Vertical closed-loop ground heat exchangers represent the most common configuration for ground source heat pump installations where land area is limited or soil conditions favor deep drilling over horizontal trenching. These systems utilize vertical boreholes ranging from 100 to 500 feet deep, with heat transfer pipe installed in the borehole and the annular space filled with thermally conductive grout.

Borehole Construction and Drilling Methods

Vertical boreholes are typically drilled to depths of 150 to 400 feet, with diameters ranging from 4 to 6 inches. Three primary drilling methods are employed based on geological conditions and project requirements.

Rotary drilling utilizes a rotating drill bit with circulating fluid to remove cuttings. This method provides the fastest penetration rates in sedimentary formations and unconsolidated materials. The drilling fluid creates a filter cake on the borehole wall that stabilizes the hole during pipe installation.

Cable tool drilling employs a heavy cutting bit raised and dropped repeatedly to fracture rock. This percussion method works effectively in consolidated rock formations where rotary drilling would be inefficient. The cuttings are periodically removed using a bailer.

Down-hole hammer drilling combines rotary motion with pneumatic percussion. Compressed air powers a hammer mechanism behind the bit while simultaneously removing cuttings. This method excels in hard rock formations and produces clean, stable boreholes with minimal wall damage.

Borehole diameter directly affects thermal performance. Larger diameter holes (6 inches) provide more space for grout and reduce thermal resistance, but increase drilling costs. The standard 4.5 to 5-inch diameter provides an acceptable balance between thermal performance and installation expense.

Heat Exchanger Pipe Configurations

Two fundamental pipe configurations are used in vertical closed-loop systems, each with distinct thermal and hydraulic characteristics.

U-Tube Configuration

Single U-tube construction employs one continuous loop of high-density polyethylene (HDPE) pipe, typically 3/4-inch or 1-inch nominal diameter. The pipe is bent into a U-shape with a tight radius return bend at the bottom. This configuration provides the simplest installation with minimal joints and potential leak points.

Double U-tube systems install two separate U-tube loops in the same borehole, effectively doubling the heat transfer surface area. This configuration reduces the required borehole depth by approximately 30 to 40 percent compared to single U-tube for the same capacity. However, thermal interference between the two loops slightly reduces the benefit, and installation complexity increases.

U-tube spacing within the borehole significantly impacts thermal performance. Spacers are installed at 10 to 20-foot intervals to maintain separation between supply and return pipes, minimizing thermal short-circuiting. The pipes should be positioned against the borehole wall rather than clustered at the center to maximize effective borehole diameter.

Coaxial (Concentric) Configuration

Coaxial heat exchangers consist of one pipe installed concentrically within another, creating annular flow paths. Fluid typically flows down through the center pipe and returns through the outer annulus, or vice versa depending on design objectives.

The primary advantage of coaxial design is elimination of thermal short-circuiting between supply and return flows. The configuration provides inherently higher thermal efficiency than U-tube designs in the same borehole. Installation is simplified since the assembly is lowered as a single unit.

Disadvantages include higher material costs, increased pressure drop in the annular flow path, and potential for installation damage to the thin-walled outer pipe. Coaxial systems are most advantageous in deeper boreholes (over 400 feet) where the thermal performance improvement justifies the additional cost.

Grout Thermal Properties and Selection

Borehole grout serves three critical functions: provides structural support to the heat exchanger pipes, prevents cross-contamination between aquifers, and facilitates heat transfer between the pipe and surrounding formation.

Standard bentonite grout consists of bentonite clay mixed with water to form a pumpable slurry. While providing excellent sealing properties, bentonite has poor thermal conductivity (0.4 to 0.5 Btu/hr-ft-°F), creating significant thermal resistance between the pipe and formation.

Thermally enhanced grout incorporates silica sand, graphite, or other conductive additives to increase thermal conductivity to 0.9 to 1.3 Btu/hr-ft-°F. This represents a 60 to 100 percent improvement over standard bentonite. The enhanced conductivity reduces borehole thermal resistance by 20 to 30 percent, allowing shorter total borehole length for the same capacity.

Grout thermal conductivity must be verified through testing per ASTM C518 or ASTM D5334. Field-mixed grouts often exhibit lower conductivity than laboratory samples due to inconsistent mixing ratios and entrapped air. Pre-mixed thermally enhanced grouts provide more consistent performance but increase material costs by 50 to 150 percent.

The grout must maintain adequate viscosity during pumping while achieving complete fill of the annular space without voids or channels. Tremie pipe installation from the bottom of the borehole ensures complete displacement of water and drilling fluids.

Borehole Field Layout and Spacing

Multiple boreholes are arranged in patterns to meet the total system capacity while accounting for thermal interference between adjacent boreholes.

Minimum borehole spacing ranges from 15 to 25 feet center-to-center, with the specific value determined by ground thermal properties, operating hours, and system balance. Tighter spacing (15 feet) is acceptable for cooling-dominated applications or balanced heating/cooling loads. Heating-dominated systems require wider spacing (20 to 25 feet) to prevent progressive ground temperature degradation over seasonal cycles.

Common field layouts include rectangular grids, linear rows, or irregular patterns constrained by site boundaries. Rectangular grids with 20-foot spacing in both directions provide uniform thermal distribution and simplified piping headers. Linear arrangements work well for narrow sites but may require increased spacing in the row direction.

The number of boreholes and total depth are interdependent. A system requiring 30,000 feet of borehole can be configured as 100 boreholes at 300 feet each, or 150 boreholes at 200 feet each. Deeper boreholes reduce drilling mobilization costs and surface piping complexity, while shallower configurations may avoid difficult drilling conditions at depth.

Ground Thermal Properties

Accurate determination of subsurface thermal properties is essential for proper system sizing. Three properties govern heat transfer performance:

Thermal conductivity (k) represents the rate of heat flow through the formation, typically ranging from 1.2 Btu/hr-ft-°F for dry sediments to 2.5 Btu/hr-ft-°F for saturated rock. Higher conductivity allows more efficient heat transfer and shorter borehole requirements.

Thermal diffusivity (α) indicates how quickly temperature changes propagate through the formation. It is calculated as α = k/(ρc), where ρ is density and c is specific heat. High diffusivity formations respond quickly to thermal loads but also recover faster during off-cycles.

Undisturbed ground temperature establishes the baseline for heat transfer calculations. This temperature typically equals the annual average air temperature plus 2 to 4°F, occurring at depths below 30 feet where seasonal variations are eliminated.

Thermal Response Testing

Thermal response testing (TRT) provides direct measurement of effective ground thermal conductivity and borehole thermal resistance under field conditions. This in-situ test eliminates the uncertainty of estimating properties from geological data alone.

The test involves circulating heated fluid through a completed test borehole at constant power input for 48 to 72 hours while monitoring supply and return temperatures. The temperature response curve is analyzed using line-source theory to extract thermal conductivity and borehole resistance values.

TRT results typically show 15 to 30 percent variation from handbook values based on geological surveys. This difference can significantly impact the required borehole field size. Testing is economically justified for systems with more than 20 boreholes, where the cost of testing (5,000 to 10,000 dollars) is offset by optimized field sizing.

The test borehole should be constructed with the same diameter, depth, pipe configuration, and grout as the production field. Testing should occur after grout has fully cured (minimum 7 days) to obtain representative thermal properties.

System Sizing Guidelines and Design Calculations

Vertical closed-loop systems are sized based on peak heating and cooling loads, ground thermal properties, and operating characteristics. The fundamental sizing parameter is borehole length per ton of capacity.

Typical sizing ranges from 150 to 300 feet of borehole per ton of installed heat pump capacity. The specific value depends on multiple factors:

• Ground thermal conductivity: Higher conductivity reduces required length proportionally
• Grout thermal properties: Thermally enhanced grout reduces requirements by 15 to 25 percent
• Pipe configuration: Double U-tube reduces length by 30 to 40 percent versus single U-tube
• Operating hours: Systems with longer run times require more length to prevent temperature drift
• Heating/cooling balance: Heating-dominated systems in cold climates require longer loops

A preliminary estimate uses 200 feet per ton for moderate climates with average soil conditions. This should be verified through detailed design calculations using methods from ASHRAE Handbook or IGSHPA guidelines.

Design load analysis must account for both peak instantaneous loads and seasonal energy extraction or rejection. The ground loop must handle peak loads during the design day while maintaining acceptable entering fluid temperatures (25 to 35°F for heating, 85 to 95°F for cooling).

Long-term ground temperature drift is evaluated through annual energy balance calculations. In heating-dominated climates, the ground loop extracts more heat annually than it rejects, causing progressive temperature decline. Conversely, cooling-dominated applications face progressive temperature rise. Hybrid systems incorporating supplemental heat rejection (cooling towers) or heat addition (solar collectors) prevent long-term drift in severely unbalanced applications.

Fluid flow rates through the ground loop affect both heat transfer and pumping energy. Design flow rates typically range from 2.5 to 3.0 gallons per minute per ton of capacity. Lower flow rates reduce pumping costs but increase temperature differential and reduce heat transfer effectiveness. Higher flows improve heat transfer but increase pressure drop and pump power consumption.

The circulating fluid consists of water with antifreeze (propylene glycol or methanol) at concentrations of 15 to 25 percent by volume to prevent freezing. The antifreeze reduces the fluid’s thermal capacity and increases viscosity, both of which must be accounted for in hydraulic calculations.

Installation Quality and System Longevity

Properly installed vertical closed-loop systems provide 50-plus years of reliable operation with minimal maintenance. Critical installation quality factors include:

• Continuous pipe without underground joints or fusion welds
• Pressure testing to 100 psi for minimum 30 minutes before and after installation
• Complete grout fill verified through calculated grout volume versus actual pumped quantity
• Proper pipe weighting to prevent flotation during grout placement
• Header piping designed to balance flow among parallel borehole circuits

The installed system should undergo commissioning that includes flow measurement in each circuit, pressure drop verification, and heat transfer performance testing. Baseline performance data enables future troubleshooting and performance monitoring throughout the system life cycle.