Wright Mechanical Services

There are four basic types of ground loop systems. Three of these ­­ horizontal, vertical, and pond/lake ­­ are closed­ loop systems. The fourth type of system is the open­ loop option. Which one of these is best depends on the climate, soil conditions, available land, and local installation costs at the site. All of these approaches can be used for residential and commercial building applications.

Most closed­ loop geothermal heat pumps circulate an antifreeze solution through a closed loop ­­usually made of plastic tubing ­­ that is buried in the ground or submerged in water. A heat exchanger transfers heat between the refrigerant in the heat pump and the antifreeze solution in the closed loop. The loop can be in a horizontal, vertical, or pond/lake configuration. One variant of this approach, called direct exchange, does not use a heat exchanger and instead pumps the refrigerant through copper tubing that is buried in the ground in a horizontal or vertical configuration.

Direct exchange systems require a larger compressor and work best in moist soils (sometimes requiring additional irrigation to keep the soil moist), but you should avoid installing in soils corrosive to the copper tubing. Because these systems circulate refrigerant through the ground, local environmental regulations may prohibit their use in some locations.

Hybrid systems using several different geothermal resources, or a combination of a geothermal resource with outdoor air (i.e., a cooling tower), are another technology option. Hybrid approaches are particularly effective where cooling needs are significantly larger than heating needs. Where local geology permits, the “standing column well” is another option. In this variation of an open­loop system, one or more deep vertical wells is drilled. Water is drawn from the bottom of a standing column and returned to the top. During periods of peak heating and cooling, the system can bleed a portion of the return water rather than re­injecting it all, causing water inflow to the column from the surrounding aquifer. The bleed cycle cools the column during heat rejection, heats it during heat extraction, and reduces the required bore depth.

The heating efficiency of ground­source and water­source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in Btu per Btu of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit. Look for the ENERGY STAR® label, which indicates that the unit meets ENERGY STAR criteria. Manufacturers of high­efficiency geothermal heat pumps (GHPs) voluntarily use the EPA ENERGY STAR label on qualifying equipment and related product literature. Many GHPs carry the U.S. Department of Energy (DOE) and Environmental Protection Agency (EPA)ENERGY STAR label.

Although the purchase and installation cost of a residential GHP system is often higher than that of other heating and cooling systems, properly sized and installed GHPs deliver more energy per unit consumed than conventional systems. For further savings, GHPs equipped with a device called a “desuperheater” can heat household water. In the summer cooling period, the heat that is taken from the house is used to heat the water for free. In the winter, water heating costs are reduced by about half. Depending on factors such as climate, soil conditions, the system features you choose, you may recoup your initial investment in two to ten years through lower utility bills.

And ­­ when included in a mortgage ­­ your investment in a GHP will produce a positive cash flow from the beginning. For example, if the extra $3,500 cost of the GHP will add $30 per month to each mortgage payment, the energy cost savings will easily exceed that added mortgage amount over the course of each year. On a retrofit, the GHP’s high efficiency typically means much lower utility bills, allowing the investment to be recouped in two to ten years. It may also be possible to include the purchase of a GHP system in an “energy­efficient mortgage” that would cover this and other energy­saving improvements to the home. Banks and mortgage companies can provide more information on these loans.

There may be special provisions or incentives available from federal, state, and local governments; power providers; and banks or mortgage companies that offer mortgage loans for energy­saving home improvements. Be sure the system you’re interested in qualifies for available incentives before you make your final purchase.

Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open­loop system, or as a repository for coils of piping in a closed­loop system. Ground water can also be used as a source for open­loop systems, provided the water quality is suitable and all ground water discharge regulations are met. Before you purchase an open­loop system, be sure your system supplier/installer has fully investigated your site’s hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed­loop systems generally pose little to no environmental hazard.