Ground-Source Heat Pumps (Earth-Energy Systems)

A ground-source heat pump uses the earth or ground water or both as the sources of heat in the winter, and as the "sink" for heat removed from the home in the summer. For this reason, ground-source heat pump systems have come to be known as earth-energy systems (EESs). Heat is removed from the earth by using a liquid, such as ground water or an antifreeze solution; the liquid's temperature is raised by the heat pump; and the heat is transferred to indoor air. During summer months, the process is reversed: heat is taken from indoor air and transferred to the earth by the ground water or antifreeze solution. A direct-expansion (DX) earth-energy system uses refrigerant in the ground-heat exchanger, instead of an antifreeze solution.

Earth-energy systems can be used with forced-air and hydronic heating systems. They can also be designed and installed to provide heating only, heating with "passive" cooling, or heating with "active" cooling. Heating-only systems do not provide cooling. Passive-cooling systems provide cooling by pumping cool water or antifreeze through the system without using the heat pump to assist the process. Active cooling is provided as described below, in "The Cooling Cycle."

How Does an Earth-Energy System Work?

All EESs have two parts: a circuit of underground piping outside the house, and a heat pump unit inside the house. Unlike the air-source heat pump, where one heat exchanger (and frequently the compressor) is located outside, the entire ground-source heat pump unit is located inside the house.

The outdoor piping system can be either an open system or closed loop. An open system takes advantage of the heat retained in an underground body of water. The water is drawn up through a well directly to the heat exchanger, where its heat is extracted. The water is discharged either to an above-ground body of water, such as a stream or pond, or back to the same underground water body through a separate well.

Closed-loop systems collect heat from the ground by means of a continuous loop of piping buried underground. An antifreeze solution (or refrigerant in the case of a DX earth-energy system), which has been chilled by the heat pump's refrigeration system to several degrees colder than the outside soil, circulates through the piping and absorbs heat from the surrounding soil.

The Heating Cycle

In the heating cycle, the ground water, the antifreeze mixture or the refrigerant (which has circulated through the underground piping system and picked up heat from the soil) is brought back to the heat pump unit inside the house. In ground water or antifreeze mixture systems, it then passes through the refrigerant-filled primary heat exchanger. In DX systems, the refrigerant enters the compressor directly, with no intermediate heat exchanger.

The heat is transferred to the refrigerant, which boils to become a low-temperature vapour. In an open system, the ground water is then pumped back out and discharged into a pond or down a well. In a closed-loop system, the antifreeze mixture or refrigerant is pumped back out to the underground piping system to be heated again.

The reversing valve directs the refrigerant vapour to the compressor. The vapour is then compressed, which reduces its volume and causes it to heat up.

Finally, the reversing valve directs the now-hot gas to the condenser coil, where it gives up its heat to the air that is blowing across the coil and through the duct system to heat the home. Having given up its heat, the refrigerant passes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger, or to the ground in a DX system, to begin the cycle again.

Domestic Hot Water

In some EESs, a heat exchanger, sometimes called a "desuperheater," takes heat from the hot refrigerant after it leaves the compressor. Water from the home's water heater is pumped through a coil ahead of the condenser coil, in order that some of the heat that would have been dissipated at the condenser is used to heat water. Excess heat is always available in the summer cooling mode, and is also available in the heating mode during mild weather when the heat pump is above the balance point and not working to full capacity. Other EESs provide domestic hot water (DHW) on demand: the whole machine switches to providing DHW when it is required.

Water heating is easier with EESs because the compressor is located indoors. Because EESs have relatively constant heating capacity, they generally have many more hours of surplus heating capacity than required for space heating.

The Cooling Cycle

The cooling cycle is basically the reverse of the heating cycle. The direction of the refrigerant flow is changed by the reversing valve. The refrigerant picks up heat from the house air and transfers it directly, in DX systems, or to the ground water or antifreeze mixture. The heat is then pumped outside, into a water body or return well (in an open system) or into the underground piping (in a closed-loop system). Once again, some of this excess heat can be used to preheat domestic hot water.

Unlike air-source heat pumps, EESs do not require a defrost cycle. Temperatures underground are much more stable than air temperatures, and the heat pump unit itself is located inside; therefore, the problems with frost do not arise.

Parts of the System

Earth-energy systems have three main components: the heat pump unit itself, the liquid heat exchange medium (open system or closed loop), and the air delivery system (ductwork).

Ground-source heat pumps are designed in different ways. Self-contained units combine the blower, compressor, heat exchanger, and condenser coil in a single cabinet. Split systems allow the coil to be added to a forced-air furnace, and use the existing blower and furnace.

Energy Efficiency Considerations

As with air-source heat pumps, earth-energy systems are available with widely varying efficiency ratings. Earth-energy systems intended for ground-water or open-system applications have heating COP ratings ranging from 3.6 to 5.2, and cooling EER ratings between 16.2 and 31.1. Those intended for closed-loop applications have heating COP ratings between 3.1 and 4.9, while EER ratings range from 13.4 to 25.8.

The minimum efficiency in each range is regulated in the same jurisdictions as the air-source equipment. There has been a dramatic improvement in the efficiency of earth-energy systems. Today, the same new developments in compressors, motors and controls that are available to air-source heat pump manufacturers are resulting in higher levels of efficiency for earth-energy systems.

In the lower to middle efficiency range, earth-energy systems use single-speed rotary or reciprocating compressors, relatively standard refrigerant-to-air ratios, but oversized enhanced-surface refrigerant-to-water heat exchangers.

Mid-range units employ scroll compressors or advanced reciprocating compressors. Units in the high efficiency range tend to use two-speed compressors or variable-speed indoor fan motors or both, with more or less the same heat exchangers.

Earth-energy systems now can be qualified under Canada's ENERGY STAR® High Efficiency Initiative. In Canada, ENERGY STAR currently includes the following product specifications for earth-energy systems:

Table 1:

Key ENERGY STAR Criteria for Ground-Source Heat Pumps (2004)
Product Type Minimum EER Minimum COP Water Heating (WH)
Closed-loop 14.1 3.3 Yes
• with integrated WH 14.1 3.3 N/A
Open-loop 16.2 3.6 Yes
• with integrated WH 16.2 3.6 N/A
DX 15.0 3.5 Yes
• with integrated WH 15.0 3.5 N/A

To be allowed to display the ENERGY STAR symbol, products must meet or exceed technical specifications designed to ensure that they are among the most energy efficient in the marketplace. Minimum requirements vary from one category to another, but typically an ENERGY STAR model must be from 10 to 50 percent more efficient than a conventional model.

Sizing Considerations

Unlike the outside air, the temperature of the ground remains fairly constant. As a result, the output of an EES varies little throughout the winter. Since the EES's output is relatively constant, it can be designed to meet almost all the space heating requirement – with enough capacity left to provide water heating as an "extra."

As with air-source heat pump systems, it is generally not a good idea to size an EES to provide all of the heat required by a house. For maximum cost-effectiveness, an EES should be sized to meet 60 to 70 percent of the total maximum "demand load" (the total space heating and water heating requirement). The occasional peak heating load during severe weather conditions can be met by a supplementary heating system. A system sized in this way will in fact supply about 95 percent of the total energy used for space heating and water heating.

EESs with variable speed or capacity are available in two-speed compressor configurations. This type of system can meet all cooling loads and most heating loads on low speed, with high speed required only for high heating loads.

A variety of sizes of EESs are available to suit the Canadian climate. Units range in size from 7 kW to 35 kW (24 000 to 120 000 Btu/h), and include domestic hot water (DHW) options.

Design Considerations

Unlike air-source heat pumps, EESs require that a well or loop system be designed to collect and dissipate heat underground.

Open Systems

As noted, an open system uses ground water from a conventional well as a heat source. The ground water is pumped into the heat pump unit, where heat is extracted. Then, the "used" water is released in a stream, pond, ditch, drainage tile, river or lake. This process is often referred to as the "open discharge" method. (This may not be acceptable in your area. Check with local authorities.)

Another way to release the used water is through a rejection well, which is a second well that returns the water to the ground. A rejection well must have enough capacity to dispose of all the water passed through the heat pump, and should be installed by a qualified well driller. If you have an extra existing well, your heat pump contractor should have a well driller ensure that it is suitable for use as a rejection well. Regardless of the approach used, the system should be designed to prevent any environmental damage. The heat pump simply removes or adds heat to the water; no pollutants are added. The only change in the water returned to the environment is a slight increase or decrease in temperature.

The size of the heat pump unit and the manufacturer's specifications will determine the amount of water that is needed for an open system. The water requirement for a specific model of heat pump is usually expressed in litres per second (L/s) and is listed in the specifications for that unit. A heat pump of 10-kW (34 000-Btu/h) capacity will use 0.45 to 0.75 L/s while operating.

Your well and pump combination should be large enough to supply the water needed by the heat pump in addition to your domestic water requirements. You may need to enlarge your pressure tank or modify your plumbing to supply adequate water to the heat pump.

Poor water quality can cause serious problems in open systems. You should not use water from a spring, pond, river or lake as a source for your heat pump system unless it has been proven to be free of excessive particles and organic matter, and warm enough throughout the year (typically over 5°C) to avoid freeze-up of the heat exchanger. Particles and other matter can clog a heat pump system and make it inoperable in a short period of time. You should also have your water tested for acidity, hardness and iron content before installing a heat pump. Your contractor or equipment manufacturer can tell you what level of water quality is acceptable and under what circumstances special heat-exchanger materials may be required. Installation of an open system is often subject to local zoning laws or licensing requirements. Check with local authorities to determine if restrictions apply in your area.

Closed-Loop Systems

A closed-loop system draws heat from the ground itself, using a continuous loop of special buried plastic pipe. Copper tubing is used in the case of DX systems. The pipe is connected to the indoor heat pump to form a sealed underground loop through which an antifreeze solution or refrigerant is circulated. While an open system drains water from a well, a closed-loop system recirculates its heat transfer solution in pressurized pipe.

The pipe is placed in one of two types of arrangements: vertical or horizontal. A vertical closed-loop arrangement is an appropriate choice for most suburban homes, where lot space is restricted. Piping is inserted into bored holes that are 150 mm (6 in.) in diameter, to a depth of 18 to 60 m (60 to 200 ft.), depending on soil conditions and the size of the system. Usually, about 80 to 110 m (270 to 350 ft.) of piping is needed for every ton (3.5 kW or 12 000 Btu/h) of heat pump capacity. U-shaped loops of pipe are inserted in the holes. DX systems can have smaller diameter holes, which can lower drilling costs.

The horizontal arrangement is more common in rural areas, where properties are larger. The pipe is placed in trenches normally 1.0 to 1.8 m (3 to 6 ft.) deep, depending on the number of pipes in a trench. Generally, 120 to 180 m (400 to 600 ft.) of pipe are required per ton of heat pump capacity. For example, a well-insulated, 185 m2 (2000 sq. ft.) home would probably need a three-ton system with 360 to 540 m (1200 to 1800 ft.) of pipe.

The most common horizontal heat exchanger design is two pipes placed side-by-side in the same trench. Another heat exchanger sometimes used where area is limited is a “spiral” – which describes its shape. Other horizontal loop designs use four or six pipes in each trench, if land area is limited.

Regardless of the arrangement you choose, all piping for antifreeze solution systems must be at least series 100 polyethylene or polybutylene with thermally fused joints (as opposed to barbed fittings, clamps or glued joints), to ensure leak-free connections for the life of the piping. Properly installed, these pipes will last anywhere from 25 to 75 years. They are unaffected by chemicals found in soil and have good heat-conducting properties. The antifreeze solution must be acceptable to local environmental officials. DX systems use refrigeration-grade copper tubing.

Neither vertical nor horizontal loops have an adverse impact on the landscape as long as the vertical boreholes and trenches are properly backfilled and tamped (packed down firmly).

Horizontal loop installations use trenches anywhere from 150 to 600 mm (6 to 24 in.) wide. This leaves bare areas that can be restored with grass seed or sod. Vertical loops require little space and result in minimal lawn damage.

It is important that horizontal and vertical loops be installed by a qualified contractor. Plastic piping must be thermally fused, and there must be good earth-to-pipe contact to ensure good heat transfer, such as that achieved by Tremie-grouting of boreholes. The latter is particularly important for vertical heat-exchanger systems. Improper installation may result in less than optimum heat pump performance.

Installation Considerations

As with air-source heat pump systems, EESs must be designed and installed by qualified contractors. Consult a local heat pump contractor to design, install and service your equipment to ensure efficient and reliable operation; also, be sure that all manufacturers' instructions are followed carefully. All installations should meet the requirements of CSA C448, an installation standard set by the Canadian Standards Association.

The total installed cost of earth-energy systems varies according to site-specific conditions, but can be up to twice the cost of a gas, electric or oil furnace with add-on air conditioning. The total installed costs of open or ground water EESs can be less; the extra cost is due to ground collectors, whether they are open or closed-loop. Ductwork must be installed in homes without an existing air distribution system. The difficulty of installing ductwork will vary, and should be assessed by a contractor.

Installation costs vary depending on the type of ground collector and the equipment specifications. To be economically attractive, the incremental costs of a typical installation should be recovered through energy cost savings within five years. Check with your electric utility to assess the benefits of investing in an earth-energy system. Sometimes a low-cost financing plan or incentive is offered for approved installations.

Major Benefits of Earth-Energy Systems

Efficiency

In Canada, where air temperatures can go below –30°C, and where winter ground temperatures are generally in the range of –2°C to 4°C, earth-energy systems have a coefficient of performance (COP) of between 2.5 and 3.8.

A ground water EES installation in southern Canada will have a heating seasonal performance factor (HSPF) of between 10.7 and 12.8, compared with an HSPF of 3.4 for electrical-resistance heating. Similarly, a closed-loop EES in southern Canada will have an HSPF of between 9.2 and 11.0, with the higher value achieved by the most efficient closed-loop heat pump available.

Energy Savings

Earth-energy systems will reduce your heating and cooling costs substantially. Energy-cost savings compared with electric furnaces are around 65 percent.

On average, an EES will yield savings that are about 40 percent more than would be provided by an air-source heat pump. This is due to the fact that underground temperatures are higher in winter than air temperatures. As a result, an EES can provide more heat over the course of the winter than an air-source heat pump.

Actual energy savings will vary depending on the local climate, the efficiency of the existing heating system, the costs of fuel and electricity, the size of the heat pump installed, and its coefficient of performance at CSA rating conditions. Later in this booklet, heating energy-cost comparisons will be made between earth-energy systems and electric heating systems, as well as air-source heat pumps.

Domestic Hot Water Heating

EESs also provide savings in domestic hot water costs. Some have a desuperheater that uses some of the heat collected to preheat hot water; newer designs can automatically switch over to heat hot water on demand. These features can reduce your water heating bill by 25 to 50 percent.

Maintenance

EESs require little maintenance on your part. Required maintenance should be carried out by a competent service contractor, who should inspect your unit once a year.

  • As with air-source heat pumps, filter and coil maintenance has a dramatic impact on system performance and service life. A dirty filter, coil or fan can reduce airflow through the system. This will reduce system performance and can lead to compressor damage if it continues for extended periods.
     
  • The fan should be cleaned to ensure that it provides the airflow required for proper operation. The fan speed should be checked at the same time. Incorrect pulley settings, a loose fan belt or incorrect motor speed can all contribute to poor performance.
     
  • Ductwork should be inspected and cleaned as required to ensure that airflow is not restricted by loose insulation, abnormal buildup of dust or other obstacles, which occasionally find their way through the grilles.
     
  • Be sure that vents and registers are not blocked by furniture, carpets or other items that would impede airflow.
     
  • In open systems, mineral deposits can build up inside the heat pump's heat exchanger. Regular inspection and, if necessary, cleaning by a qualified contractor with a mild acid solution is enough to remove the buildup. Over a period of years, a closed-loop system will require less maintenance because it is sealed and pressurized, eliminating possible buildup of minerals or iron deposits.

Service contracts are similar to those for oil and gas furnaces.

Operating Costs

The operating costs of an earth-energy system are usually considerably lower than those of other heating systems, because of the savings in fuel. Qualified heat pump installers should be able to give you information on how much
electricity a particular earth-energy system would use.

However, the relative savings will depend on whether you are currently using electricity, oil or natural gas, and on the relative costs of different energy sources in your area. By running a heat pump, you will use less gas or oil, but more electricity. If you live in an area where electricity is expensive, your operating costs may be higher. The payback on an investment in an earth-energy system may be anywhere up to a decade or more. Later in this booklet, operating cost estimates are provided for EESs.

Life Expectancy and Warranties

EESs have a life expectancy of about 20 to 25 years. This is higher than for air-source heat pumps because the compressor has less thermal and mechanical stress, and is protected from the environment.

Most ground-source heat pump units are covered by a one-year warranty on parts and labour, and some manufacturers offer extended warranty programs. However, warranties vary between manufacturers, so be sure to check the fine print.

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