GeoExchange Systems Design
What size of heat pump is required for a given building?A heat loss - heat load assessment will determine how much heat the building requires. The installed heat pump and supplementary backup unit must meet this load, subject to the conditions in the CSA standard. The quantity of loop must be consistent with the size of the heat pump. On average, a 4-ton unit will be sufficient for a 1,700-ft² home.
Is the geoexchange system designed to provide 100 percent of building heat and water heating? A heat loss calculation will determine how much heat is required for both applications, but most systems are sized to meet less than the full design load and rely on an electric backup heater to provide supplemental heat on very cold mornings during the winter. In all cases, full cooling load can be met by a system.
Is there a national or local standard for the design of a geoexchange system?
Proper design is covered under national CSA standards C445 (residential) and C447 (commercial). As of March 2002, both applications will be covered under CSA C448. We are not aware of any provincial regulations, while some municipalities or other regulatory agencies may have guidelines or rules pertaining to the professional installation of systems.
Proper design is covered under national CSA standards C445 (residential) and C447 (commercial). As of March 2002, both applications will be covered under CSA C448. We are not aware of any provincial regulations, while some municipalities or other regulatory agencies may have guidelines or rules pertaining to the professional installation of systems.
What is an open loop?One pipe is used to take water directly from a lake or river, or an underground well or aquifer. The water is circulated through the heat pump to extract the heat, and a second pipe is used to return the water to the same source. Standards dictate that the amount of water extracted cannot impact negatively on the ecosystem, and consideration must be given to discharge areas to avoid impacts (e.g.: if slightly warmer water is discharged into a lake during summer, the discharge should not be located near sensitive aquatic life).
What is a lake loop or river loop? The pipe is anchored to the bottom of a water body of sufficient depth, and extracts heat directly from the water. The pipe must be weighted to avoid shifting, and must be protected at the point of entry into the lake to avoid being severed by ice movement in the spring. The position is determined to avoid damage from boat anchors. The amount of heat extracted from the water is so low and diffuse that there is no ecological damage to marine life or aquatic systems, even in the immediate vicinity of the loop.
A drill is used to bore a cavity, into which a section of looped pipe is inserted and sealed according to provincial regulation. This configuration is the most efficient, since it extracts both solar heat from near-surface soil, as well as magmatic heat from deeper areas. This option is used when surface property is limited.
A trench of at least 4 feet depth is dug. At the bottom, the high-density polyethylene pipe is placed and the trench is carefully back-filled to avoid air pockets or damage from sharp rocks. There can be from one to six pipes placed in each section of trench, subject to appropriate spacing to allow for thermal separation. The pipe does not have to be perfectly level.
Most consumers will receive excellent performance and savings with a geoexchange unit that is designed to provide 70-75% of the total heat loss for the building. When ambient temperatures are above the design balance point, the geoexchange unit will provide all heating required; only when outdoor temperatures drop below that point does the geoexchange unit require supplemental heat from an electric resistance-heating element inside the unit. Most outdoor winter temperatures are above the balance point, and the geoexchange system can provide 95% of the load if the unit is sized to 70%. Some dealers prefer to install a system that meets 100% of heat loss; this eliminates the need for supplemental heating but increases the cost of installation (more pipe is required).
A closed loop is a continuous pipe that circulates a fluid (often with an antifreeze mixture) to extract heat from the surrounding soil or water. An open loop is, essentially, two pipes: one to suck in water that is moved through the heat pump to transfer heat, the other to take the cooled water and discharge it to the appropriate location, usually downstream from the first pipe.
A geoexchange system can heat either air or water. If air, ductwork is required to circulate the warmed air throughout the building. Due to the low temperature rise of a geoexchange system, the volume of circulated air is higher than a combustion furnace and, therefore, the ductwork must have a higher capacity in cubic feet per minute.
Both loops extract heat from the surrounding earth (soil or water) for transfer into the heat pump. Temperature changes in water temperature can be more dramatic than changes in soil temperature (e.g.: if your water loop were located near a glacial stream) and water levels are more prone to change (e.g.: drought conditions).
What are the options if there is insufficient room for a horizontal loop? The loop can be installed in a vertical borehole(s). An open-loop system may be possible if there is sufficient water. The use of coiled loop pipe or numerous pipes in the same trench may be a feasible option.
There are three basic sections: energy supply comes from the ground loop (or vertical or lake loop); the conversion of heat occurs in the heat pump and compressor; and the warmed energy is delivered through the circulation system (ductwork or in-floor radiant heating).
Can a geoexchange system heat a building without supplemental heat? A system can be designed to provide 100% of heating needs on the coldest days in northern Canada, but the cost to install such a system would be excessive. A system designed to provide 70% of a calculated heat load is the optimal sizing if there is a supplemental heating element, but the history of earth energy systems includes past errors in heat loss calculating and, consequently, some systems have been installed under-sized. The new CSA standard for installation (C448) has been revised to overcome this deficiency.
How long should the buried ground loop be? The length of pipe depends on two factors: the amount of heat required in the building (the more heat required, the larger the heat pump and the longer the loop must be) and the thermal transfer characteristics of the soil (dry light soil with entrained air can require three times the amount of loop to provide the same heat as wet, packed soil). Design software will calculate the amount of pipe for a specific site. An industry rule of thumb is that an average home requires a four-ton unit, which requires 400-500 feet of horizontal loop or 300-350 feet of vertical borehole for each ton (up to six pipes can be installed in a horizontal trench if spaced properly).
Are there advantages of an open-loop system over a closed-loop system? Open-loop units usually are less expensive to install (no need for trenching or drilling) and are more efficient (the heat in the supply water is transferred without the need for antifreeze). Closed-loop units offer more security of supply (less chance of constriction of the water supply) and have no environmental impact. Often, closed loops operate at a lower cost than open systems due to lower pumping power required.
What is the link between size of heat pump and length of loop? The higher the heat load (demand for heat), the larger the heat pumps. The larger the heat pump, the longer the loop required.
How much power does a geoexchange system consume? The system requires electricity to operate the loop pump, the compressor and the circulating fan (in forced air distribution). It also uses electricity in the backup mode to deliver sufficient heat on very cold days, and requires power throughout the year to operate the water-heating component. The amount of power used will depend on the size of the unit, operating times and sizing considerations, but an average residential unit would require 11,000 kWh per year, which represents an annual cost of $935 (based on 2001 retail prices).
Can any pond be used as the heat source? The body of water must be large enough to contain sufficient latent heat to meet the demand of the building. The larger and deeper the better; stagnant or glacier-fed bodies are not feasible.
Can a geoexchange system be used for radiant floors or hydronic heat? The heat pump can produce water that is hot enough for use in these applications. In most cases, it is not warm enough to be used in old-style under-the-window radiator systems.
Where can the ground loop be located? The CSA standard does not allow pipe to be placed near septic tanks or hydro lines, and must be a minimum distance from neighbouring property lines.
How does heat circulate if the building has no ductwork? Without ducts, the only option is to circulate heated water through a radiant in-floor configuration, or small fan coil units.
How much groundwater does an open-loop system require to operate? Depending on the heat load of the building, most units require 8 to 12 gallons per minute of water; more if the unit is heating potable / service water at the same time.
If the entering water temperature drops, what is the impact on the geoexchange system? Lower water temperature (either direct from a lake/well or cooler fluid returning in a closed loop) reduces the available heat that can be transferred to the building; i.e.: the building gets colder. If the entering water temperature drops below a set temperature, the geoexchange system will shut down.
What should be done with discharge water from an open-loop system? The CSA standard and provincial / local regulations require that the water be returned to source after passing through the heat pump. The system removes only a small amount of heat, so the water discharged is not classified as sewage. Care must be exercised to ensure that open-loop systems protect the environment to the greatest degree possible and comply with all laws, standards and regulations.
Will my existing ductwork function with this system? Your contractor can determine the ductwork requirements for your system and identify if modifications are needed.
Will baseboards work to circulate heat? Baseboards usually can only conduct electricity. A geoexchange system requires either ductwork to circulate warm air or radiant in-floor coils to circulate warm water.
Does a geoexchange system require additional insulation in the building? The lower the demand for heat, the smaller the heat pump and the lower the installed cost. Insulation, along with any energy management or conservation technique, is always the best first step in any heating system.
What will cause a geoexchange system to fail? The main reason for system failure is under-sizing in design, which means that the building requires more heat than the geoexchange system can reasonably provide, and the resulting strain causes a component to fail.
Why is there supplemental heating? To reduce installation costs and provide better performance, an electric resistance heating element is included in unit to boost the heat during extremely cold temperatures. A fossil fuel furnace can also be integrated.
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