Six steps for designing ground heat exchangers

Designing ground-coupled heat pump (GCHP) HVAC systems requires careful consideration of system configuration and the ability to clearly navigate the design paths within the owner’s budget. A GCHP system can be either an economical system or a source of continual heartburn for the engineer and owner. Understanding GCHP design considerations and operational requirements is necessary to design an economical and efficient project.

By Gayle Davis, PE, CGD; Stanley Consultants, Austin, Texas September 7, 2017

Learning objectives:

  • Understand the basic concepts and equipment of a ground-source heat pump system.
  • Review key elements of ground-coupled heat pump (GCHP) design.
  • Understand maintenance and operations requirements for GCHP and incorporate them in the design.

The term ground-source heat pump (GSHP) refers to the family of heat pump systems that use the ground, groundwater, or surface water as a heat sink and source to condition buildings. This article focuses on ground-coupled heat pump (GCHP) systems, which fall under the GSHP family. A GCHP system is characterized as a reversible vapor-compression-cycle heat pump connected to a closed-loop ground heat exchanger. Ground-coupled systems typically use water-to-air heat pumps or water-to-water heat pumps. Conventional hydronic pumps are used to circulate a water or water-antifreeze mixture between the building piping system and the ground heat exchanger.

GCHPs are typically connected to either a vertical ground heat exchanger (GHEX) or a horizontal GHEX. Vertical GHEX systems are constructed of two small-bore (0.75- to 1.5-in. diameter) high-density polyethylene (HDPE) pipes inserted in a 4- to 6-in.-diameter vertical borehole filled with thermally enhanced grout. Borehole depths usually range from 100 to 400 ft deep, depending on soil conditions and drilling equipment. Vertical GHEX systems achieve high system performance by taking advantage of the relatively constant temperature and soil thermal properties of the deep earth. Ground temperatures are nearly equal to the average annual air temperature at depths of 12 ft or more, providing a constant-temperature heat source and sink.

Horizontal GHEX systems have either a shallow trench, typically 4 to 8 ft deep, or are horizontally bored. Trenched systems can contain several different piping arrangements, such as single-pipe, multiple-pipe (2-, 4-, or 6-pipe arrangements), or spiral-pipe. Horizontally bored systems are drilled at depths greater than 12 ft and are grouted similar to vertical GHEXs. Due to their shallow depths, horizontal GHEX systems are subjected to seasonal ground-temperature swings and varying soil thermal properties, which result in lower system performance. Table 1 provides a summary of the advantages and disadvantage to consider when selecting between a vertical or horizontal GHEX.

The decision between a vertical or horizontal GHEX system is a multifaceted process. There are many factors to consider:

  • Land availability.
  • Required system capacity.
  • Excavation costs.
  • Drilling costs.
  • Labor costs.

Vertically bored systems typically require less land area than a horizontal system. Available land area at the project site will allow the designer to make an initial determination between the two ground heat-exchanger systems. For most projects, the decision between vertical or horizontal GHEXs is based on economics. Based on the combination and evaluation of the above parameters, borefield economics typically show that horizontal systems are cost-effective for systems with capacity requirements of less than 15 tons. Horizontal systems larger than 15 tons are often costly due to excavation and backfill costs. Vertical systems are typically used for systems larger than 15 tons.

Designing GCHP systems can take several iterations and design paths based on owner input, ground thermal conductivity, codes, budgetary constraints, and which energy-rating program (U.S. Green Building Council LEED or Energy Star) used. Commercially available ground heat exchanger design software programs, such as GLHEPRO, GSHPCalc, and LoopLink PRO, have been developed to aid with the design of the ground heat exchanger and to allow designers to evaluate alternative designs. These programs are specifically programmed to analyze the effects of borehole spacing, unbalanced design loads, and soil properties to determine the required ground heat exchanger loop lengths. Use these six design strategies when designing ground heat exchangers.

1. Determine ground properties

Soil composition, undisturbed soil temperatures, thermal conductivity, and allowable drilling depths are completely out of the designer’s control. These properties are estimated using one or more of the three available methods. The first method is to estimate the information based on “local information” and “best guess” of the soil composition. For obvious reasons, this is the least accurate method and is not often used. The second available method is to obtain soil-formation information from local drilling logs, discussions with local drilling contractors, or geological reports; the engineer then estimates soil properties from soil thermal property tables using a weighted-average process. This method provides a more reasonable estimation of soil thermal properties and is typically used for horizontal GHEXs.

The best and most accurate method to estimate soil thermal properties is to perform a formation thermal conductivity (FTC) test. Vertical GHEXs require a more accurate estimation of the site ground thermal properties due to installation costs and the system effects of oversizing or undersizing the GHEX. International Ground Source Heat Pump Association 2016 Standards and ASHRAE 2015 Handbook—HVAC Applications provide recommended FTC test procedures and specifications.

In short, a test borehole is drilled to an estimated design depth and a drill log is used to record the soil and rock types encountered as the drill progresses. A vertical U-bend loop is installed and grouted in place. The test loop is connected to equipment designed to introduce a controlled amount of heat to the loop, and then temperatures are recorded over a 36- to 48-hour period. The data gathered is used to determine the undisturbed soil temperature, average thermal conductivity, and average thermal diffusivity. The test loop is capped and later incorporated into the GHEX. The soil information is input into a ground heat exchanger design software package to determine the required loop lengths.

A bonus feature to conducting a FTC test is the production of drill log reports. The FTC report and drill logs are often incorporated within the construction documents to provide contractors with important information about the efforts that will be required to install the GHEX formation. This information may alleviate bid uncertainties and reduce the vertical GHEX bid price.

2. Grout thermal conductivity effect

Grout conductivity is the major controlling factor when determining the required vertical borehole length. Soil and the U-bend pipe thermal conductivity are predefined parameters and beyond the designer’s control. However, the designer has direct control over the specification of the grout thermal conductivity. Grout is the pathway or roadblock to heat transfer between the pipe and the ground. Therefore, it makes sense to increase the thermal conductivity (reduce the thermal resistance) as much as economically reasonable.

As the grout thermal conductivity is increased, the overall thermal resistance of the ground heat exchanger is reduced. As a result, the GHEX can deliver or remove the same amount of heat with shorter boreholes or fewer boreholes. Nevertheless, the trade-off between increased grout conductivity and reduced borehole design lengths follow the law of diminishing returns. Eventually, the design borehole lengths or number of boreholes will be dictated by the soil thermal conductivity, and increasing the grout thermal conductivity will not result in a decrease in the borehole lengths or the number of boreholes.

GHEX designers will need to perform an economic analysis to determine which grout thermal conductivity will result in the optimal performance for the best value, given the local conditions. Typically, grout thermal-conductivity analyses start with conductivities of 0.88 to 1.00 Btu/hrxftx°F. Many thermally enhanced grout manufacturers have developed grout-volume calculators to assist designers with their economic calculations.

3. Outdoor air strategies

Often, the GCHP system is designed for the building’s total peak HVAC load. Careful consideration should be given to this approach if available land is limited or budget is a concern. Outdoor-air (OA) peak loads can account for a significant portion of a building’s conditioning loads. OA loads are calculated at peak occupancy, a load that the building rarely experiences. These high incidental loads can result in an oversized and inefficient heat pump system.

Oversizing the heat pumps could cause short cycling, which in turn can cause early equipment failure, humidity-control issues, and temperature-control problems. Also, installing a ground heat exchanger sized for the total peak load may be a higher project cost than treating the OA with a separate system. Generally, high-capacity requirements and low run time will result in less-than-desirable payback for GCHP systems.

Systems with high OA conditioning loads are usually designed with dedicated outdoor-air systems (DOAS), energy-recovery ventilation (ERV) systems, or heat-recovery ventilation (HRV) systems, dependent on building size and OA requirements. Typically, these OA-treatment systems are completely decoupled from the ground heat exchanger, which has its own heating and cooling systems, so the OA loads do not affect the ground heat exchanger sizing and performance. However, when the OA load is relatively small and constant, the fresh-air load may be added to the ground-source heat pump system. Several heat pump manufacturers offer heat pumps with integrated ERV or HRV options.

An alternative option to installing a separate heating and cooling system for the DOAS would be to install a water-to-water heat pump (WWHP) to serve the DOAS. This approach will require analysis to determine if it will provide an acceptable performance and economic benefit when compared with air-cooled chillers and hot-water boilers that serve the DOAS. The WWHP alternative should still incorporate an energy-recovery process to precondition the OA to reduce the load on the GCHP system.

4. Minimize pump energy

The system pumps are often one of the highest energy-using components in GCHP systems. Reducing or minimizing system pump energy begins early in the design stage by minimizing the system flow rate and head loss.

First, let’s consider a method for reducing the system flow rate. GCHP pump flow rates are typically sized for 3 gpm/ton of installed capacity. Sizing flow rates for installed capacity results in higher flow than necessary and increases system costs. Consider designing for a system flow rate between 2.5 to 3.0 gpm/ton of peak block load as an alternative. Sizing the system flow rate for the peak block load provides sufficient flow to meet the heating and cooling load and reduces system costs. Reduce the hydronic system head loss by sizing the piping system for a friction loss rate not to exceed 4.0 ft of head per 100 ft of piping. In addition, select heat pumps with hydronic coil losses of less than 15 ft of head and select control and balance valves with losses of less than 5 ft of head.

GCHP systems may require a water-antifreeze solution when designed for northern climates due to the reduced loop temperatures. Design calculations will show if the heat-transfer fluid’s minimum temperature will fall below freezing. If an antifreeze solution is required for the system, use the lowest concentration necessary. Antifreeze increases the fluid density of the pumped solution. This, in turn, increases the friction loss and requires a larger pump to circulate the fluid. In addition, antifreeze reduces the fluid’s capacity to transfer heat, which affects the GHEX and heat pump sizing and typically requires the systems to be enlarged to accommodate the loss in capacity.

Pumps should be selected to operate as near as possible to their maximum efficiency point. Pump controls also can be used to reduce the system pump’s energy consumption. Smaller GCHP systems typically use a simple on/off pump control. However, larger systems use variable-flow controls with variable frequency drives (VFDs) and control valves to reduce the flow to meet the system demands. The designer should note that ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings requires a VFD on hydronic systems with pump power exceeding 10 hp and requires modulating control valves and variable flow.

5. Purging air and dirt

Dirt, sand, pipe shavings, air, and other construction debris must be flushed from the GHEX and interior piping. These contaminants must be removed before system start-up and full operation of the system occurs, as they may damage valves, pumps, and coils—and potentially block fluid flow in the GHEX or interior pipe system. As a preventive measure, designers should specify or state with drawing notes that all pipe ends should be temporarily sealed with caps immediately after installation and immediately after individual loop-pressure testing to minimize system contamination. Remember, a blocked GHEX loop means capacity loss.

Specify the use of a purging/flushing pump skid to perform the initial system flush at a minimum flow rate of between 2 and 3 fpm. This method will remove a majority of the debris and large air pockets from the system. Remaining air should then be “bled” from the system through air vents (manual and/or automatic). The designer should provide sufficient valves at the system’s manifold connections, where the supply and return headers and the GHEX and interior pipe connect. Proper system valve design and arrangement will allow for separate flushing of the GHEX and interior piping systems before the entire system is flushed.

The above methods and procedures will remove debris and free air from the systems during the construction or start-up and commissioning phase (if applicable), but will not help during operation and after maintenance or major repairs. The designer should consider including a high-efficiency, reduced-velocity coalescing air and dirt separator in the hydronic system. Coalescing air and dirt separators have internal media to reduce the fluid velocity and break surface tension to allow entrained air and suspended particles to be removed. Removing air and dirt from the system can reduce maintenance costs, reduce operating costs, improve system efficiency, increase heat transfer, and most important, extend the life of the system.

6. Improving maintainability

  1. Finally, as with any design, the system needs to be maintainable. Long-term system performance is quite often directly associated with maintenance. If the system is overly complex or hard to maintain, maintenance personnel may ignore maintenance items or disable control devices. Consider coordinating and incorporating the following items as necessary to increase system maintainability:
  • Locate the heat pump equipment for “easy” maintenance, such as at ground level or on mezzanine levels. Typically, if the equipment is accessible it will be maintained.
  • When possible, select heat pump units that use standard air filter sizes. Standard-size filters are off-the-shelf items and typically cost less than special-order filter sizes.
  • Incorporate pressure/temperature ports (aka P/T ports or Pete’s ports) at each heat pump connection and on each supply and return on the GHEX manifold. The P/T ports should be located as close to the heat pumps as possible to minimize the effects of heat gain or loss for temperature readings and to minimize pressure loss when taking pressure readings. Minimizing these losses or gains will allow for increased accuracy when troubleshooting or conducting a system-performance check.
  • GCHP systems are simple systems that typically do not require complex direct digital control systems to operate efficiently. Quite often, a programmable thermostat will provide sufficient control. They are inexpensive, readily available, and most have 7-day programming features to allow for setbacks. Most building owners and maintenance personnel will know how to operate a programmable thermostat.

Terminology and resources

The two predominant authorities on ground-source heat pump design are the International Ground Source Heat Pump Association (IGSHPA) and ASHRAE. Each of these organizations has developed design guide material to assist designers and engineers in understanding and designing ground-source heat pump systems. ASHRAE recently produced a revised edition of its design guide, Geothermal Heating and Cooling: Design of Ground-Source Heat Pump Systems, which incorporates new research, design, and installation practices since the initial publication in 1997. IGSHPA has published many manuals covering topics including design, installation, and soil and rock classifications.

Gayle Davis is a project manager and senior mechanical engineer with Stanley Consultants. He has experience in project management, design, and commissioning of mechanical systems for the built environment and central plants. He is a member of ASHRAE and IGSHPA, and is a certified geoexchange designer.