Residential Geothermal

Geothermal Heating and Cooling

What's so compelling about geothermal technology? Energy efficiency. In a technology defined by numbers, here's a fact that everyone will understand: With a geothermal system a reasonably tight 2000-sq.-ft. home can be heated and cooled for about $1 a day. Commensurate savings are common for larger homes. You'll pay several thousand more up front, but in many cases, the payback can come in as little as two or three years. And once the system is paid off, the annual return on investment can approach 20 percent. Add impressive durability and less-intrusive trenching methods and the scales begin to tip, at least in the new-home market.

Geothermal Basics

The heart of a typical geothermal system is a ground-source heat pump that cycles water through an underground piping loop. The water piped through this loop uses soil temperature to warm or cool the heat pump's refrigerant. Significantly, the heat pump is located indoors, like a furnace, which provides advantages we'll get to shortly.

While this equipment may sound exotic, its operation is fairly easy to understand when compared to that of conventional air-to-air heat pumps. A conventional heat pump is really just a central air conditioner that can reverse the flow of its refrigerant. The compressor is located outside the home, and, in the heating mode, it's able to extract some of the heat present in cold, outdoor air and deliver it indoors to a condensing coil. Unlike conventional furnaces, heat pumps don't have to create heat, they just harvest existing heat–and therein lie the savings.

Below about 10 degrees Fahrenheit, however, too little heat is present in the air and a backup heat source is needed to make up the difference, or, in many cases, take over entirely. Even within a heat pump's effective operating range, efficiency is directly tied to ambient temperature. The colder it gets, the less heat is available and the less efficient the system becomes.

In contrast, a ground-source heat pump, with its underground piping loop, is able to tap a warmer, more stable heat source. The soil below frost level–4 ft. to 6 ft. deep–stores the sun's energy at a more or less constant level, with temperatures keyed to latitude. Subsoil temperatures range from the low 40s in the North to the low 70s in the South.

For purposes of comparison, we'll use the 55 degrees F soil temperature common in much of the Midwest and Central Plains. This area of the country suffers some extreme temperatures, but also has a fair number of mild days, so it's a reasonable choice.

With a ground temperature of 55 degrees F, the system needs to boost the heat a mere 15 to 20 degrees to reach a comfortable indoor temperature. Compare this to the 40 to 60 degrees maximum differential that an air-to-air heat pump may handle, and even greater differentials expected of standard furnaces, and the logic comes into focus pretty quickly. The only influence outside air temperature has on the equation is in the home's ability to retain heat. Houses lose heat faster on colder days, so all systems work harder in cold weather. But while a ground-source heat pump may need to run more often on these days, it doesn't run less efficiently.

The geothermal principle works about as well for air conditioning. Instead of an outdoor compressor laboring against the heat of the day having to use hot air as its heat-shedding medium, a ground-source heat pump operates indoors, using ground temperature as its starting point. The result is a 20 percent to 40 percent savings over conventional heat pumps and air conditioners.

Of course, lower soil temperatures will reduce heating efficiencies and warmer soil will cut into air-conditioning savings. On average, however, ground-source heat pumps deliver three to four times the energy they consume.

Equipment

While the basic principles of geothermal heating haven't changed much in 20 years, the technology has, and that has made all the difference. By far, the biggest step forward has been in compressor technology. Until about 1990, all heat-pump and air-conditioner compressors had just one speed, and because every installation had to be sized to handle only a few extreme days, every system was, in a sense, oversized. As such, it was inefficient most days of the year.

This situation was remedied by two quite different improvements in compressor design, both 25 percent to 30 percent more efficient than previous technologies. The first is a 2-speed compressor that can idle along on mild days and rev up for extreme days. Because these compressors run more often, you'll also realize better humidity control during the air-conditioning season.

Almost simultaneously, scroll compressors came on the market. Scroll compressors are radically different in design–they use an orbiting coil instead of a piston–and boast a 30 percent improvement in efficiency. Because they have very few moving parts–and fewer still that make contact–they are built to last.

Another big difference between a standard heat pump and a ground-source pump is that a ground-source model is installed indoors. At first blush, this might seem unworkable, if only because of the noise. But these systems are quiet, almost as quiet as your refrigerator, which, after all, is really just another kind of heat pump. The noise we associate with an air-to-air unit comes from the large fan needed to pull air over the compressor coils. But ground-source compressors use water, not air, so they don't need fans. They have, instead, a quiet circulation pump and a compressor that runs at a lower pressure, both sealed in an insulated cabinet about the size of a washing machine.

This sealed environment also pays dividends in other ways. Compressor fins on outdoor units are more prone to casual damage–and the compressor fan draws in huge amounts of dirt and debris that reduce air flow and damage bearings. Further reductions in efficiency can be caused by corrosion or by an out-of-level condition that results from a settling of a compressor that's mounted on the ground. All these factors cause a compressor to work harder and run hotter, up to 450 degrees F on a really hot day, which is hot enough to do damage. Cold starts on cold days also take their toll.

But an indoor unit in its sealed chamber has none of these problems. As a result, the oldest models have been in place for 20 years and they seem to remain efficient with age. This may explain a frequently noted disparity between lab-certified efficiency ratings and field performance. In side-by-side tests comparing ground-source heat pumps to conventional heat pumps and air conditioners, the latter units gradually lose ground. In all likelihood, the difference is the environment they're installed in.

System Extras

Most compressor compartments also contain two add-ons–a resistance-heat grid and a desuperheater. Ground-source pumps in northern climates may need a little help on very cold days, and a small electric-resistance heater does the job. The added operating expense comes to about $30 to $40 a year. Though this may seem a net loss, it's really not. On-board resistance heat allows the pump and piping loop to be downsized slightly, which saves money.

A desuperheater is an auxiliary heat-recovery system that provides up to 60 percent of a home's domestic hot water. It's really just a second condenser located in the cabinet and connected to a standard electric water heater via a coaxial fitting. It delivers more heat in summer, but it helps in winter, too. The purchase price is a hefty $500, but again, the cost is misleading. Without a desuperheater, you'd need to install more underground piping to dissipate the extra heat. As you might expect, most units come with desuperheaters.

Control Units

Both the thermostats and the control panels for these systems are electronic. The thermostat is able to sense temperature changes to .1 degrees F and activate the system when it senses only a 1 degree temperature drop. Because the human body can sense only a 2 to 3 degree difference in temperature, the system is always one step ahead in comfort.

The microprocessor in the cabinet does double duty. It sequences the startup so that less stress is put on equipment, and it also has a built-in fault sensor that can identify the cause of a malfunction. The system faults appear on the thermostat so minor problems can be corrected immediately and more serious problems are diagnosed before the service technician arrives.

Loop Types

In nearly all cases the loop piping is made of flexible, high-density polyethylene that is warranted for 50 years and has a life expectancy of 200 years. Its flexibility and lack of "coil memory" also make it easier to install than the polybutylene used just a few years ago. In residential installations, it's usually 3/4 in. in diameter and is joined with heat-sealed (thermal-fusion) fittings.

When it comes to ground loops, there are two general system types–open loop and closed loop. Closed-loop systems are more common and can be trenched or bored underground horizontally or installed vertically like water wells. If you live next to a private lake, piping can even be laid underwater on the lake bed. You'd need at least 8 ft. of water over the pipe year-round but, if this option is available, it's far less costly than an underground loop.

The second option, an open-loop installation, is not as popular as it used to be. In this case, a dedicated well with a submersible pump serves as the source of water delivered to the heat pump. Once the water is cycled through the system, it's returned to the aquifer–typically through a second well drilled specifically for this purpose, or to a nearby stream or lake. While these systems are quite efficient, they tend to be more expensive. Water wells are costly and water quality can be a problem. You'd also have the added cost of running the submersible pump, typically $100 to $160 per year.

The most common installation is a horizontal loop. In this situation, an access pit is dug near the house, so the piping loop can be brought through the foundation wall and connected to the indoor compressor unit. From this pit, several piping loops are bored or trenched at least 5 ft. deep.

On average, a horizontal system requires 220 ft. of piping for every ton of compressor load (12,000 BTUs of heat). A newer 2000- to 2400-sq.-ft. home will require 3 tons of capacity and roughly 660 ft. of piping loop. Two pipes can be installed in each narrow trench or bore–one out and one return–so that's 330 ft. of trench. If a backhoe is used and a 3-ft.-wide trench is dug, six pipes can be laid in one trench, allowing a shorter trench. Prices vary, but expect to pay around $600 in trenching for every ton of capacity, or approximately $1800 for a 3-ton system.

Horizontal systems have always required lots of unencumbered space, but two recent developments have shrunk the lot-size requirements a little. First, new boring technology allows the operator to accurately steer a 5-in. boring machine under and around common obstructions. Starting from a header pit near the house, the machine can dive under outbuildings, trees and septic systems, and come up 100 ft. away. When finished, two pipes, fused with a "U" fitting on the far end, are pulled through most of the bore. The tail end of the bore hole is then backfilled or packed with a dense grouting material such as bentonite clay.

The other new twist has more to do with ingenuity than equipment. Instead of laying the pipe lengthwise in the bottom of a long trench, it is coiled in 2-ft.- to 3-ft.-dia. loops like a large Slinky toy. The coils are then laid down and covered with soil. This "Slinky" method greatly increases surface exposure and substantially reduces the amount of trenching needed. With these two innovations, a horizontal system can often be installed on a lot as small as 1/4 acre.

When a property won't accommodate even this much trenching or boring a vertical, closed-loop system is the next best option. In this case, a well driller typically drills several holes without casings 150 ft. to 200 ft. deep. The contractor then drops two pipes joined with a U fitting at the bottom into each hole and joins all pipes from all holes in a common pit 5 ft. to 6 ft. deep. Then the contractor runs a feed line and return line through the foundation wall and connects them to the compressor unit. Before filling the pit, each bore hole is grouted to meet state and local codes.

Vertical, closed-loop systems are actually more efficient, but more piping–typically 300 ft. per ton–is required. The drilling costs are also higher. Expect a vertical, closed loop to run $750 to $950 per ton of compressor capacity, or $2300 to $3000 for a 3-ton system.

Site

Every system will be slightly different if only because installers approach things differently. Every home is different, too, and needs to be considered individually. When estimating the heating and cooling load for your home, the contractor will need to factor in such things as insulation values, the number, placement and type of windows, weatherstripping, primary building materials and so on. Many will order an infrared heat-loss test from the utility company. Because design is so critical here, and because the ground loop is so permanent, it's important that the contractor get it right the first time.

Keep in mind that a leaky home will either need a larger system or fewer leaks. Because weatherizing a home is almost always cheaper than upsizing the system, you'll want to do the little things first, whether it's caulking joints or adding insulation. Every little bit helps.

Interestingly, soil type can also influence performance. Moist soils such as clay and loam are best. Dry, sandy soils, in contrast, contain millions of tiny air pockets which insulate against the heat-transfer process. In these cases, the contractor will need either to extend the piping loop–up to 30 percent–or to backfill the bottoms of the trenches with grout or a better soil.

Retrofits

As you might expect, most geothermal systems are installed in new construction and on good-sized lots. This is not to say that retrofits aren't a good idea, or that they're unworkable. In many cases they work well. Even modest city lots can often accommodate vertical loops. The problem is that most furnaces and air conditioners are replaced when they fail or when a real estate transaction requires it. Neither situation encourages a leisurely choice or an experimental mindset.

Of course, some existing homes will not accommodate these systems. A heat pump, like a furnace, needs ductwork, so you either need to have it in place or find a reasonable way to get it. And if your furnace is now in a closet, these larger, indoor heat pumps may not fit. Interestingly, the ClimateMaster Co. has introduced a geothermal system with an outdoor compressor designed specifically for tight-fit retrofits. Because it's in a sealed compartment, it should hold up well. In this case, only an evaporator coil is connected to the ductwork. The point is, if you're paying through the nose for an inefficient system, don't automatically assume that a geothermal system is out of the question.

Prices vary regionally, partly because some areas have experienced, well-equipped installers who compete with each other and others don't. In a mature market, you can often have a geothermal system for about $2000 more than a new air-to-air heat pump. In other areas, you could easily pay $4000 more.

In new construction, where a conventional furnace and air-conditioning package with ductwork would cost $5000 to $6000, a geothermal system would probably run $7000 to $8000. Still, the only way to know for sure is to ask several contractors. Get tight bids that include an estimate of payback.

Payback is hard to pin down nationally, but should be relatively easy to estimate on a house-to-house basis. If your contractor seems uncertain, call your utility company. There are now enough of these systems in place for you to have access to a close estimate.

The other significant factor in the payback question is the type of energy you're now using. Nationally, payback runs between two and six years. If you're currently using an electric-resistance-heat furnace, you're looking at a short turnaround. Oil-fired furnaces are next in line in terms of energy costs and electric heat pumps follow. The longest paybacks will come against natural gas, which is still relatively inexpensive.

Finally, although deregulation has really upset things lately, electric utility companies have typically been willing to underwrite some of the cost of geothermal installations with rebates or rate guaranties. The discounts can amount to hundreds of dollars, so be sure to ask.