The following are some tips from several geothermal experts to help get a system off the ground — and into the ground — that is fully optimized.
One of the key considerations to keep in mind when designing and selecting a system is its life cycle costs, noted James E. Bose, Ph.D., P.E., executive director of the International Ground Source Heat Pump Association (IGSHPA). “Is there a reasonable payback?” he asked. “Other things, such as available space, etc., will certainly have to be considered.”
Expanding on this, Dean Grunseth, senior marketing manager, McQuay International (Minneapolis, MN), listed the following “key considerations” for a successful project:
Van C. Bourn, vice president, marketing and e-commerce, ClimateMaster Inc. (Oklahoma City, OK), commented that the system must be “designed to work in the intended conditions, i.e., low entering water temperatures if being installed in a northern climate.” He added, “Generally speaking, units that are only rated at ARI 320 conditions are only suitable for far southern U.S. installations (warm entering water temperatures).”
If geothermal is determined to be the right way to go, “One of the most important considerations is the calculation of the heating and cooling loads,” said Kent Kuffner, marketing manager, WaterFurnace International Inc. (Fort Wayne, IN).
“A thorough analysis of the loads and building usage is key to designing the system, selecting the units, and engineering the earth-loop system. The system designer, whether it’s the HVAC contractor, project engineer, or architect, must also consider space requirements for the earth loop. Other considerations include unit functionality, unit locations, pumping system design and location, and the internal piping system.”
“Horizontal loops are used where adequate land is available,” he said. “One or more trenches are dug using a backhoe or chain trencher. Pipes are inserted and the trenches are backfilled. A single pipe, or multiple pipes, can be placed in the trench. Multiple pipe trenches, while requiring more pipe, reduce the total required trench length.
“Vertical loops are used where space is more limited. Holes are bored using a drilling rig, pipe is inserted, and the holes are filled with a grout mixture. The pipes are connected horizontally a few feet below the surface.
“Pond loops can be installed if an adequately sized body of water is located close to the building. A series of coils are sunk to the bottom, connected by a header with supply and return pipes.
“Open loops are used where there is an abundant supply of quality well water.”
Horizontal loops will be “laid out in trenches three to six feet below the ground surface,” said Bourn. Vertical bore holes will be drilled 150 to 500 feet deep.
Bose noted, “The jobsite generally restricts the number of choices for installation. Space limitations may only allow vertical loops; rock close to the surface may not allow trenching for a horizontal installation. Drilling and trenching costs may dictate or lead a person to a conclusion.”
Bourn added, “Vertical loops typically cost the most but take the least amount of space. A vertical loop for a typical three-bedroom home can be installed in a 15- by 15-foot area, whereas for a horizontal loop, between 250 and 400 feet in length could be required.”
“The typical residential application is much less sophisticated than a large commercial system,” said Kuffner. “Residential homes less than 3,000 square feet typically require only one unit. Larger homes require more units. Depending on zoning needs, a zone control system can be added. Most manufacturers limit their residential product lines to six tons and less.”
A commercial system “can become much more sophisticated due to the need for mechanical rooms housing pumps, valves, and other geothermal loop components. Multiple units require more engineering of inside piping with consideration to pipe sizing and valving. In addition, a computerized control system may be utilized to control cycling, temperatures, and times of operations, and to monitor water and air temperatures, run times, and kW usage.” Unit sizes can range from 0.5 to 30 tons, he said.
Determining the load is especially important for a commercial application, stated Bose. “Commercial systems are dependent on building loads that must be determined by an energy analysis program that estimates monthly and/or annual usage of zone loads or building block loads. Residential ground heat exchanger loads can be estimated from outdoor temperatures. Residences generally have predictable internal thermal loads.”
“Residential systems may require supplemental duct heaters, where commercial loads typically do not,” commented Grunseth. “There are also differences in the loop length required per ton. Commercial loops are typically cooling dominant, where a residential loop may need to sustain a heating dominant loop.”
“Different soils will have an effect on ground heat exchanger lengths,” said Bose. “Heavy soils with high moisture content are better heat transfer mediums than a dry granular soil or sand,” Kuffner related.
“Running a thermal conductivity test prior to designing the loop can help the designer determine if the loop field will be larger than expected due to adverse heat transfer characteristics of the soil,” noted Grunseth.
The industry standard for piping, said Kuffner, is high-density polyethylene. Connections that will be buried in the ground are heat fused.
“Pipe size is selected to minimize friction loss, maintain good heat transfer between the circulating fluid and pipe wall, and minimize first cost” Bose remarked.
Pipe sizes generally range from 3/4 to 2 inches for residential or light commercial applications, Kuffner said. Larger commercial applications may use 6- to 12-inch pipe for main supply/return headers.
“Earth loops are generally designed for 3 GPM per ton of equipment to give the maximum benefit between flow, turbulence, and energy consumption,” said Bourn.
Should you use antifreeze in your circulating fluid? “Antifreeze is required when the loop field temperatures will drop below 40 degrees,” noted Grunseth.
Most residential geothermal units are sold with desuperheater capability, said Bourn. “Since the typical commercial installation either uses very little hot water or a very large amount, only a few commercial units are sold with the desuperheater option. For those projects requiring large amounts of hot water, water-to-water geothermal units are utilized.”
For more information, visit the following websites: Addison Products Co. — www.addison-hvac.com; ClimateMaster Inc. — www.climatemaster.com; International Ground Source Heat Pump Association — www.igshpa.okstate.edu; McQuay International — www.mcquay.com; WaterFurnace International Inc. — www.waterfurnace.com.
Publication date: 08/26/2002