Ground-Source Heating & Cooling Gets Better - Continued
The refrigerant moves directly from the geo field to the unit's compressor with no stops or intermediate heat exchangers required, enabling super-efficient transfer of thermal energy. Only a small amount of electricity is needed to power the system's compressor. The rest of the heating or cooling energy comes from the earth's temperature just below the surface (see table, below).
Heating & Cooling Efficiency: How Direct-Exchange Geothermal Compares
This data comes from Audit, by Elite, a widely used professional energy-analysis software. The test case is a typically insulated 2,436-square-foot new home in Chicago, which has cold winters and hot, muggy summers. The home is inhabited by a family of three who are in the house 12 hours a day; the cooling load is 24,717 Btu/hr and the heating load is 43,668 Btu/hr. Also included in the calculation is the heating of all domestic hot water. The example assumes $.09 per kwh for electricity, $1.30 per therm for natural gas, $1.62 per gallon for propane, and $1.55 per gallon for fuel oil. Domestic hot-water design assumes a 60-gallon tank with 50°F entering water temperature and a tank temperature of 125°F.
Total heating cost
Total cooling cost
Water heating cost
Total operating cost
Average monthly cost
Closed-loop, water-source geothermal
Air source, 12 SEER HP
12 SEER AC w/80% nat-gas furnace
12 SEER AC w/80% fuel-oil furnace
12 SEER AC w/80% propane furnace
Load Calcs Critical
Most hvac contractors are familiar with Manual J load calculations, which are used to determine heating and cooling loads, though sadly too few perform one when sizing a system. It's a fairly complex process, but should always be done, especially in the case of geothermal systems. According to ECR, all EarthLinked installers are required to submit the Manual J load calcs with each order to ensure proper operation and maximum efficiency.
"Without a Manual J load calculation, the risk is that the system will be oversized or undersized, each presenting a different set of problems," says Tiller.
An undersized system struggles to maintain comfortable indoor temperatures and will run continuously to meet the demand for heating or cooling; this will most likely shorten the life of the system. An oversized system will run in bursts. The cooling or heating need will be met quickly, instead of gradually, setting a pattern of short cycling. It's uncomfortable for the homeowners and reduces overall system efficiency.
"Regardless of the type of geothermal system your customer is considering, be sure they get a Manual J load calculation as part of the deal," Tiller says. "In the case of new construction or for a building that will be substantially retrofitted, my advice is to wait until all changes have been made to the plan. By doing this, you won't run the risk of unanticipated, last-minute change orders adversely affecting the load calculation after the equipment has been sized and ordered."
Evaluating the Site
After the load calc, the next step in planning a customer's installation is site evaluation, Tiller says. Evaluating the site "not only affects the type of earth loop best suited to the project, it also plays a key role in determining the overall cost of the system," she says.
Depending on the site, the hvac contractor will choose a vertical, horizontal, or diagonal earth-loop installation. Jefferson says the horizontal earth-loop application is the least expensive to install, chiefly because excavating and backfilling are less expensive than drilling. But in areas where rock layers obstruct excavation, it may be necessary to drill holes for the diagonal and vertical applications. Drilling is also a viable option on small lots.
Five loops per ton
100-foot length per loop
Typical pit depth 4 to 6 feet
One loop per ton
250-foot length per loop
Typical trench depth 4 to 6 feet
One to four loops per ton
50- to 100-foot drilling depth
15- to 45-degree drilling angles
3-inch minimum borehole diameter
One to two loops per ton
75- to 100-foot drilling depth
3-inch minimum borehole diameter
Figure 3. Depending on the site, earth loops can be installed horizontally, in pits or trenches, or in vertical or diagonal boreholes.
Both ECR and American Geo recommend that soils be tested for high concentrations of acids, chlorides, hydrogen sulfide, sulfates, and ammonia, all of which should be avoided because of the potential for copper corrosion.
ECR recommends that soil samples be taken with a coring tube, specifically the LaMotte model 1016 (www.lamotte.com). Your local soils engineering firm should be familiar with these tests.
Also, locations with a pH higher than 11 or lower than 6 and coastal areas with brackish water marshes, saltwater intrusion, or acidic peat bogs should be avoided unless cathodic protection is provided. ECR offers a cathodic protection system; it emits a small, self-adjusting electric current that prohibits corrosion.
Once the earth loop is in, installation of a ground-source heat pump is very similar to an air-source installation. Geothermal condensing units (the enclosure with the compressor) don't require air circulation, as do typical air-source systems, so they may be installed in a basement or utility room. Ductwork and the setting of equipment are comparable as well.
Unlike standard air-source heat pumps, geothermal condensers do not require air circulation, so they can be installed in a basement or utility room, which lengthens service life.
Typically, a geothermal system will provide service for 25 to 30 years, which is twice the life expectancy of air-source heat pumps. This is because the stable heat source prevents thermal stresses to the compressor, and the enclosed unit is out of the weather.
John Vastyan is a freelance journalist in Manheim, Pa., who specializes in the plumbing and mechanical, radiant heat, and geothermal industries.