KNOW-HOW: GEOTHERMAL SYSTEMS UPDATE
Energy from the ground: it’s catching on with old-house owners on smaller lots.
Perhaps your treasured hot-water or steam system suffered a catastrophic freeze, or the forced-air HVAC system is reaching the end of its useful life. Or maybe heating and cooling bills in that rambling old pile you call home are cutting into retirement savings. If any of these circumstances sound familiar, your home may be a candidate for a geothermal HVAC system—even if the house is on a compact lot in a historic streetcar suburb.
Geothermal systems make use of the free, renewable energy stored in the earth. A geothermal heat pump uses the constant temperature underground as the exchange medium for heating and cooling, instead of outside air. Just a few feet below the surface, ground temperatures are significantly warmer than the air above during winter, and much cooler in summer, usually averaging around 50° F year round. For this reason, a geothermal heat pump uses less energy to warm indoor air when it’s cold, and to cool the air when it’s hot outside—all without directly burning any fossil fuel.
While the upfront costs of geothermal are about twice that of conventional systems, the energy savings are so significant that most homeowners will recoup their costs in five to 10 years— sooner, factoring in the state and federal tax incentives now available. (Several of the larger geothermal companies also offer financing on favorable terms.) The whisper-quiet systems make a lot of sense for older houses with generous floor plans and minimal insulation, too.
The key component of any geothermal system is the ground loop system. Composed of high-density polyethylene pipes that contain a water–ethanol mix to prevent freezing, the loop system is buried below the freeze line. During cold weather, the temperature-sensitive fluid circulates and picks up heat from the steady temperatures underground and delivers it to the house. The heat pump concentrates the thermal energy and transfers it to conventional ductwork or an under-floor radiant system, which circulates the heat. In warm months, the process is reversed and the cooler fluid exhausts heat out of the house. As a bonus, the unit can assist with hot-water heating.
There are several types of ground loop systems, and all require quite a bit of earth moving and lawn repair. The most common and cost-effective is the horizontal loop system. Piping is laid in trenches from 100' to 400' in length. Installing the trenches requires enough land, of course—at least .25 of an acre, and often up to .75 acre. In a variation called the “Slinky,” looping the pipe in overlapping circles allows the use of more pipe in a shorter trench, cutting down on installation costs and permitting horizontal installation on smaller sites.
Vertical loop systems are used in locations where there isn’t enough land to lay a horizontal system, like small lots in historic neighborhoods. In a vertical system, holes are bored with a drilling rig, and a pair of pipes with special U-bend fittings inserted into the holes. A typical home requires three to five bores with about a 15' separation between the holes.
If you’re lucky enough to have a pond a half-acre or larger on your property, your home may be a candidate for a pond loop
system. A series of coiled, closed loops are sunk to the bottom of
the water body. The loop is connected to the house via more pipe laid in a horizontal trench. Water-to-water heat transfer is much more efficient than water-to-air heat transfer, so this type of geothermal system is highly economical and efficient.
Whichever loop type is required, bear in mind that the life of a typical geothermal heat pump is 15 to 20 years; loop systems can last 20 years or more. Forced-air systems will require a water-to-air heat pump, while hydronic radiant heating systems require water-to-water heat pumps.
Although geothermal heat pumps can be integrated easily with existing systems such as forced-air or radiant floor heating, you’ll need space in a utility room or basement for all necessary components. Where there is no ductwork in place, a delivery system will have to be designed and installed, preferably one that disturbs as few original walls and ceilings as possible. For a two-storey house, for example, it may make sense to install a split system, placing ductwork and an air handler for the first floor in the basement, with a similar set-up in the attic to serve the upper floor.
Designing and installing a geothermal HVAC system is a complex process. It goes without saying that this is not a do-it-yourself project. Look for a contractor/engineering firm with significant experience in geothermal installations, preferably one certified by the International Ground Source Heat Pump Association (igshpa.org).
As part of a vertical-loop geothermal installation, a drilling rig prepares to bore deep holes in the front yard of a Queen Anne house in a historic Syracuse neighborhood.
LEFT Expect to do a little landscaping after the system is in, as shown in this historic geothermal install in the Hudson Valley. ABOVE Curling the polyethylene pipes in overlapping loops gives this variation on the horizontal loop method the nickname “Slinky.”