Drill and Fill: Installers thread pipe into a hole a few inches wide and over 100 feet deep. As wind and solar hog the alt-energy spotlight, this technology has remained underground.
“You’re not making heat, you’re moving heat,” Colorado geothermal installer Jim Lynch says. Installations like Lynch’s tap into the earth below the frost line–which always stays around 50 degrees Fahrenheit–to reduce a home’s heating and cooling loads. All HVAC systems require energy-intensive heat movement, a task responsible for over half of the average house’s total energy demand. Geothermal works more efficiently because the system’s mild starting point creates an efficient shortcut to the target temperature. Imagine a 100-degree Florida day or a 0-degree Michigan night: Spot the system 50 degrees, and it doesn’t work so hard to get the house comfortable.
Unlike wind and solar, geothermal’s power source never varies.
Bob Brown, vice president of engineering with equipment maker WaterFurnace, says, “The ground’s there all the time. It’s great for heating and it’s great for cooling. All I’ve got to do is bury a plastic pipe, put fluid in and, lo and behold, I’ve got a great system.”
How Geothermal Happens
* In the ground: A water-filled, closed loop of 1-inch high-density polyethylene (HDPE) pipe ferries heat between the earth and the house. Pipes descend 4- to 6-inch-diameter vertical wells–the number and depth depend on the house’s site and size–before ganging together in a header and bringing lukewarm water in through the basement walls. Drillers backfill each hole with bentonite grout (or new enhanced grouts, engineered with fly ash) to maximize thermal conductivity.
* In the house: Pumps cycle water through the pipe loop to the heart of the system: the geothermal unit, which acts as furnace and air conditioner. This machine uses refrigerant and the temperate water from the underground pipes to heat or cool air. The air is then circulated through standard ductwork. With a device called a desuperheater, the unit uses excess heat to warm up domestic hot water at no added cost. The results feel the same as those from any standard forced-air HVAC system.
Air in the ducts (1), refrigerant in the geothermal unit (2), and water in pipes (3) flow past each other like interlocking gears. Water brought from underground transfers heat to the refrigerant, or absorbs heat from it, depending on the season. Like an air conditioner, the unit compresses or expands the refrigerant to raise or lower its temperature. Finally, the refrigerant, now heated to 180 F or chilled to 40 F, fills condenser/evaporator coils. Air in the ducts blows across the coils to be cooled or warmed, then flows through the house.
* The bit: This mud-drilling bit grinds soft earth and funnels it back into hollow, 20-foot drill-shank sections. Corkscrew auger bits, in contrast, pound through solid rock. A new mud bit spinning at 1000 rpm, pushing downward with between 300 and 500 pounds of pressure, is good for five 150-foot holes.
* The pipe:
Water-filled HDPE pipes absorb heat through their walls. This sawed-off cross-section shows two pipes fused in a butt joint made by pressing the molten edges together at over 500 F. The joint, stronger than the walls of the pipe itself, resists rust, rot and leaks for a purported 200-year life span.
* The unit:
A combined furnace and air conditioner, the geothermal unit manages all-season climate control from the basement. Using the same principles as a refrigerator, which removes heat from food, this machine and the buried pipe remove heat from the earth or from the house. Wired to a 50-amp circuit, it works without venting, combustion or risk of carbon-monoxide poisoning.
Vertical coils (1) fuel a system by using less total HDPE pipe than horizontal coils (2), in which loops of pipe fill shallow trenches exposed to constant heat just below the frost line. In pond systems (3), a blanket of water insulates coils anchored on racks. Hard ground can inhibit deep digging, stopping Colorado installers like Jim Lynch from doing simple vertical work: “Texas, Nebraska–that’s some easy drilling down there,” Lynch says. His clients receive options 2 and 3. If an existing system gets a geothermal upgrade, it may operate as geothermal 90 percent of the time, while the old boiler or furnace fires up only on the coldest days of the year. The payback period on retrofits averages 12 to 15 years; on new installations, it can get as low as three to six.
A typical 2000-square-foot home in Commack, N.Y., was recently retrofitted with a geothermal system. Tax credits, the inefficiency of the existing system and a low-interest loan combined to create immediate savings. The monthly payment is now $24 lower than the old monthly HVAC expense.
Installation cost: $30,000 — $11,000 (tax credit) = $19,000
Annual costs: $3945 (old system) — $2076 (geo) = $1869 saved
Payback period: $19,000 / $1869 = 10.17 years
Monthly fuel costs for old system: $329
Monthly geothermal costs: $173 (power) + $132 (loan) = $305
1. It’s a geyser. Hot springs and other steamy subterranean liquids are not related to residential geothermal. Those are unusual local seismic circumstances. Home systems work everywhere.
2. The water table is in the way.
Installers drill straight through it. On Long Island, where the water table is just a few feet below the surface, saturated sand makes for some of the best drilling and most efficient heat transfer possible.
3. It generates electricity.
Industrial-scale geothermal power plants can generate electricity. Home systems don’t–but they do save electricity (or fuel) by replacing conventional home heating and cooling with more efficient equipment.