ACHRNEWS

Common Geothermal Configurations

February 6, 2006
Pictured is a drawing of a horizontal ground loop system.
OKLAHOMA CITY - Geothermal heating and cooling can be achieved with at least four different configurations. ClimateMaster, an Oklahoma City-based manufacturer of water-source heat pumps, has provided details on those configurations, as well as answers to commonly asked questions about the technology.

A vertical ground loop system may be installed in vertical boreholes 150- to 300-feet deep. Each hole contains two pipes with a U-bend at the bottom. After the pipe is inserted, the hole is backfilled and grouted. The number of loops required depends on ground conditions, air conditioning and heating loads, and the depth of each hole. This design is well suited for retrofit applications where landscaping is already complete.

A horizontal ground loop system is usually selected when adequate space is available and trenching can be easily accomplished. A series of pipes is laid out in trenches 4 to 6 feet below the ground surface, then backfilled. Often multiple pipes are used to maximize the heat transfer capacity of each trench. The amount of pipe and the size of the ground loop field are based on ground conditions, heating, and cooling requirements.

Pictured is a drawing of a surface water or lake loop system.
An application located near a lake or pond can benefit from a surface water system (lake loop). Many local building codes require a retention pond to control rain run-off from large roof and parking areas of new buildings. These retention ponds can become low-cost and efficient energy sources. Because of the excellent thermal properties of water, a loop may be submerged in a series of coils beneath the water surface. A surface water system does not use lake water directly, but is a closed loop system.

An open loop (well water) system may be utilized with water-source heat pumps. In this application, ground water is pumped through supply piping from the well to the building's heat pumps. Upon leaving the building, the water is pumped to a pond, stream, or back into the ground via a discharge well.

This design limits the amount of piping and excavation required. When ground water is used on commercial buildings, an additional heat exchanger is usually installed between the building water piping system to maintain a closed piping system inside the building.

FAQS

Doesn't geothermal cost more to install than traditional HVAC systems?

In many cases a geothermal system can be less expensive than a traditional HVAC system. The system requires much less space and labor than typical pipe systems. Mechanical room size can be reduced, allowing more occupant space for lease or rent.

Can the earth loop be located under a parking lot?

Yes. Vertical earth loops may be installed under a parking lot provided that a 95-percent compaction requirement for the horizontal supply-return trenching is specified for the trench immediately under the lot or walkway.

Since heat transfer depends greatly on moisture being present in the soil, this application should be limited to vertical (not horizontal) loops.

Although moisture does penetrate through the asphalt, vertical loops will generally get most of their moisture from groundwater, not surface penetration.

Do I still need a boiler for backup?

No. Cooling-dominated commercial buildings - even in northern climates - require loops that are designed to handle heavy heat-rejection loads. What little heating the building requires can easily be met by the earth loop.

Pictured is a drawing of a vertical ground loop system.
What is my risk of underground leaks?

The risk of earth loops leaking is low as compared with other piping systems. Geothermal systems use heat-fused, high-density polyethylene pipe in all underground piping, similar to piping systems used in the natural gas distribution industry. Most pipe is warranted for more than 25 years and installers typically offer warranties on installation as well.

Why are maintenance costs reduced for the geothermal system?

Maintenance is reduced primarily because of the elimination of the cooling tower and boiler. By eliminating this equipment, it is possible to eliminate chemical and other costs associated with the prevention of scaling and bacterial growth, eliminate year-round tower operation that requires additional expenses during the colder weather months, and eliminate boiler maintenance. Plus, water-source heat pumps have factory-sealed refrigerant circuits. Maintenance consists of filter changes.

Is it always cost effective to design the loop to handle 100 percent of the heat-rejection load?

No. For commercial installation, full-time loads can greatly escalate the size of the earth loop. In these situations, downsizing the earth loop to handle a more balanced rejection-extraction load and adding a small, closed-circuit tower can reduce overall costs. The tower, designed to only operate during hot summer months, will reduce installed costs and still maintain lower operating costs.

For more information, visit www.climatemaster.com.

Publication date: 02/06/2006