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Home Energy Magazine Online November/December 1996

Do You Dig Ground Source Heat Pumps?

by Bill Rock Smith and Jennifer Arco

Bill Rock Smith is an independent consultant specializing in high performance homes and HVAC systems. Jennifer Arco is the Northeast territory manager at WaterFurnace, a heat pump manufacturer.

Ground source heat pumps are being aggressively marketed throughout the United States. Electric utilities love them, and manufacturers make some spectacular claims. But what is hype and what is fact?

Workers begin installation of the underground coil for a ground source heat pump. The coil will be laid out in a horizontal configuration. This Alaska residence used to be heated with wood from the forest visible in the background.

Ground source heat pumps have received a lot of attention and publicity over the last few years. This is the result of marketing efforts by the Geothermal Heat Pump Consortium, the International Ground Source Heat Pump Association, the Environmental Protection Agency, manufacturers, and numerous electric utilities. These "geothermal" systems are touted as highly efficient, cost-effective, and environmentally friendly. The question is, do they live up to the marketing hype? Looking at the equipment in a specific location-in this case the Northeast-it is possible to figure out which of the broad claims are accurate for local conditions.

Ground source heat pumps move heat from underground into the house for space heating and domestic hot water. They cool the house by moving heat from the house into the ground and the hot-water tank. Underground temperatures remain almost constant year round, regardless of air temperature variations. The area of constant temperature is the heat source for heating and the heat sink for cooling. The capacity and efficiency of a heat pump depends on the temperature difference between the house air and the heat source or sink. When a heat sink is cold, or a heat source is warm, the heat pump works more efficiently. Thus, the earth is a better source or sink than the outdoor air used by air source heat pumps.

Figure 1. Heat pump cycles. To change from heating to cooling, the refrigerant flow reverses in the air coil and coaxial heat exchanger.

Open and Closed Loops

Ground source heat pumps are generally categorized by the type of ground connection (loop) they use. The two main types are open loop and closed loop. The type of loop used affects the installed costs, maintenance costs, and system efficiency. In both cases, water or a water-antifreeze solution is circulated through the ground loop to the heat pump. The heat pump uses a refrigeration process to exchange heat to or from the liquid, cooling or heating the home (see Figure 1).

An open-loop system uses groundwater from a well and circulates it through the heat pump. The water is then discharged to a second well, or to a stream, river, lake, pond, ditch, or drainage tile. An open loop requires three elements: adequate water quantity, adequate water quality, and discharge capability. A water quality test is strongly recommended, as is a pump down-a 24-hour test that checks the recovery rate of the groundwater. It is difficult to guarantee the quantity and quality of a water source over the 20-year-plus life expectancy of the system. Also, due to the use of groundwater, many areas restrict open-loop systems.

Closed loops use the ground as the heat source or sink. The piping is sealed, and a solution of water and antifreeze is circulated through the underground pipes. The ground loop is installed vertically or horizontally, or is placed at the bottom of a pond (see Figures 2 and 3). The size of the loop is based on the load of the home and the heat transfer capability of the soil or pond. Today, polyethylene plastic is generally used for ground looping; some older systems used polybutylene. (Polybutylene pipe has fallen into disrepute for its tendency to leak. The pipe is no longer produced in the United States, and former manufacturers recently settled a class action lawsuit brought by dissatisfied homeowners.) To minimize the potential for leaks, installers use continuous lengths of pipe up to 500 ft long. They heat-fuse and pressure-test all joints before and after installation. Once the loop is in the ground, they pack the bores with bentonite clay. Closed loop systems are often warrantied for 50 years, and have a life expectancy of over 55 years.

Figure 2. Typical horizontal ground loop.

Figure 3. Typical vertical ground loop. Vertical bores cost more, but take up less space than horizontal trenches.

Open loops are generally less expensive to install and are slightly more efficient than closed loops, since the loop water remains warmer throughout the winter. (Groundwater is generally about 40^oF, while closed loop water can drop below the freezing point.) However, open loops typically require more maintenance since the heat exchanger must be cleaned periodically to remove mineral deposits. Also, the higher electrical requirement of a well pump can lower the unit's overall efficiency. This pump needs to be carefully sized to prevent short-cycling and premature failure.

Generally, maintenance problems go down as prices go up. The cheapest heat pumps are reinjection wells, which draw water from a household drinking well, reinjecting it in the same place. These systems can cost as little as half of what closed loops cost. Other open loops cost about a third less than closed loops. However, unlike any open-loop system, closed loops are almost maintenance-free, thanks to their known, constant quantity and quality of water solution. And their small circulators, located in the building, have lower power requirements and replacement costs than well pumps.

Water Heating

There are two ways that ground source heat pumps can provide hot water. On-demand systems have a thermostat in the hot-water tank; when the tank needs heat, it activates the heat pump. Most systems, however, have a desuperheater. These systems use a heat exchanger in the heat pump to add excess heat to the hot water. The excess can come from house cooling-the system heats domestic hot water with heat removed from the house-or from house heating when the house does not need the heat pump's full capacity. During peak space heating, the water is heated by electric resistance. The desuperheater can satisfy 25%-50% of the hot-water needs of a home.

Workers dig a bore for a ground-source heat pump's vertical ground loop.

Electricity's Bad Name

In the Northeast, for a system to be considered energy-efficient, it has to be fossil fueled. The high cost of electricity in the area (10¢-14¢/kWh) makes electric resistance a bad choice for space and water heating. Most homes that use electric space heating use either baseboard resistance or air source heat pumps; both cost two to three times as much to operate as fossil fuel systems. As a result, in most of New England it is difficult to sell a home that has any sort of electric heating system.

Hydronic baseboard or radiant floor heat distribution systems are generally considered the most comfortable. People complain that warm-air distribution is uneven, noisy, and uncomfortable, and that it is responsible for drying out the house. Among forced-air heating systems, air source heat pumps are even more problematic, due to their relatively cool supply air and high heating bills. The bad reputations of forced warm air and heat pumps make it difficult to convince customers that ground source heat pumps can be efficient and comfortable.

The operating costs for heating, cooling, and water heating with a ground source heat pump can compete with the costs of fossil fuel systems in the Northeast. Still, many people confuse ground source heat pumps with electric-resistance heat, partly because the systems use electric resistance for backup heating. Sizing a heat pump to minimize the use of this inefficient back-up increases the installed costs. Ground source heat pumps are already the most expensive space conditioners to install, due primarily to the high cost of ground looping-$1,000- $1,500 per ton of capacity in the Northeast.

Ground source heat pumps are starting to gain acceptance, however, even in the Northeast. This is due largely to rebates of $200 to $1,500 per ton offered by several major electrical utilities. Several utilities offer rebates only for systems installed in homes that meet specified performance levels. Heat pump systems are very sensitive to proper sizing. Ground looping gets more expensive as the system gets larger, so it is much more economical to install ground source systems in homes with low loads. Also, 90^oF-95^oF supply air is adequate in a home with well-insulated and sealed ducts, but is too cool for a low-performance home.

Digging Beneath the Hype

Ground source heat pumps, sometimes called geothermal systems, are being aggressively marketed nationwide. However, broad claims made by the ground source industry that hold true for most of the country may not be as valid in some regions. In fact, they can be misleading. The following are some of the broad claims of the industry, and how they really apply to the Northeast.

"Save up to 60% on utility bills." This is a general claim made by some manufacturers of geothermal systems. It is often accompanied by a graph comparing ground source to air source and fossil fuel. Electrical rates used for the graph are lower than typical New England rates. If one doesn't study the numbers, it looks like geothermal outperforms all systems. At the lower end of local electrical rates (10¢/kWh), geothermal is competitive with fossil fuel in the heating mode and is slightly better than conventional air conditioning in the cooling mode; its desuperheater is less efficient than fossil fuel for water heating. Using typical New England utility rates, annual costs for heating, cooling, and water heating (using a desuperheater) can often be higher for a geothermal system than for the popular combination of fossil fuel heating and electric-compressor cooling.

"Geothermal systems are 300%- 400% efficient." This means that they deliver three to four units of energy to the home for every unit they consume. Fossil fuel is typically only 60% to 90% efficient. In the Northeast, the cost of electric energy is three to four times greater than the cost of fossil fuel energy, so being 300% to 400% efficient makes the geothermal system economically competitive with fossil fuel. However, one must be sure that the efficiency rating includes the efficiency of the hot water system. Otherwise, the electric resistance heat for the hot water could make the system as a whole less efficient than a conventional system.

"A desuperheater can provide up to 50% of the home's hot-water needs." The key words here are "up to." Desuperheaters heat water only while the system is in operation, and the cooling mode produces the most hot water. A 50% contribution to the water heating load is optimistic, considering the low cooling loads in the Northeast.

"The utility is offering rebates." Some utilities are offering rebates as high as $1,500 per ton plus another $2,800 for building the home to prescribed standards. These rebates can offset some or most of the cost difference between the installation of geothermal and that of high-end fossil fuel systems combined with compressor-based cooling. It is important to compare the cost of the geothermal system (after rebates) with the system the customer would otherwise have used. If the customer would not have installed an air conditioner, the installation cost of the heat pump, even after the rebates, would be several thousand dollars higher.

Cut-away view of a ground source heat pump. This part of the heat pump is placed inside the house. Among other benefits, this protects the unit from theft-a problem that is increasingly affecting the outdoor units of air source heat pumps.

"Energy Crafted or NY-STAR homes with geothermal systems save 30%-50% over code homes." It is the higher insulation levels and airtightness of the house that produces the savings. A fossil-fuel home built to the same standards would produce the same savings.

So Why Bother?

Even in the Northeast, where installing and operating a ground-source heat pump is probably more expensive than anywhere else in the United States, there are non-economic reasons to consider these systems.

No combustion. Fossil fuel space- and water-heating systems produce carbon monoxide and other dangerous gases, often require the storage of toxic fuels, and have chimneys that must be maintained. The combustion gases can also contribute to air pollution. With an electric system, these problems are all transferred to the power plant, so the home is safer.

Three systems in one. Ground source systems are integrated, providing heating, cooling, and some water heating.

High-quality equipment. The components in units produced by the leading geothermal manufacturers are usually top-of-the-line. The quality of the cabinet and the general construction are superior to most fossil fuel equipment.

No outdoor unit. Ground source systems eliminate the noise and aesthetic problems that come with standard air conditioning systems. The condenser is contained in the geothermal indoor unit, and the heat exchanger is underground.

Energy-efficient blowers. Geothermal systems usually have variable-speed blowers that use less electrical energy as their speed is reduced. Since ground source systems operate at low blower speeds much of the time, they have longer run times than single-speed systems. This reduces system noise and improves comfort by reducing the force of the air blowing on occupants.

Better dehumidification in cooling mode. During the cooling season, the heat pump can operate at a lower blower speed, increasing dehumidification if that is desirable.

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