The Glossary of Geothermal Heating and Cooling provides definitions of many terms used within the Geothermal heat pump industry. The terms in this glossary may be used by industry professionals, for education materials, and by the general public.
A
Active Borehole Length
The length of the U-bend in the borehole below the header trench (usually 4 to 6 feet less than the total borehole length from the surface).
Advanced Geothermal System (AGS)
See Closed-Loop Geothermal System entry.
Ambient Air
The surrounding air (usually outdoor air or the air in a specific location).
Ambient Ground Temperature
The natural temperature of the earth in a specific location. This temperature will typically be quite stable at a depth of 30' (9 m) and is usually close to the annual average air temperature of the area and is influenced primarily by the average air temperature and secondarily by the thermal energy absorbed from the sun. While the ambient ground temperature is constant, as heat is extracted or rejected via the ground heat exchanger (GHX), the temperature will vary. Typically, a GHX is designed to operate at a minimum fluid temperature of about 32°F (0°C) and a maximum fluid temperature of 90°F (32°C)
Annual Ground Load
Defined to be the difference between the annual amount of heat rejected to the GHEX in the cooling mode and the annual amount of heat extracted from the GHEX in the heating mode.
Antifreeze
A variety of Antifreeze solutions are used in geothermal ground loops. The most common types are: Propylene glycol, Methanol, and Ethanol
As-Built Drawing
A detailed drawing that shows everything included on the site plan in addition to the exact location, dimensions, and other pertinent details for a given GHEX installation after the installation is complete.
Auxiliary Heat
A supplemental source of heating to provide additional heat to assist a geothermal heating and cooling system. Auxiliary heat can also be used as back-up emergency heat if heat pump goes into fault.
Average Efficiency
Seasonal average versus peak Energy Efficiency Ratio (EER) or coefficient of performance (COP).
B
Balance Point Temperature
The outdoor air temperatures where internal heat gains from people, appliances, etc. offset the envelope heat loss to the atmosphere. It is at the balance point temperatures where no indoor heating or cooling will be required to maintain the temperature of the home at the thermostat set point.
Bend
A fitting either molded separately or formed from pipe for the purpose of accommodating a directional change.
Bentonite Grout
A common grout mix used to protect aquifers from ground or cross contamination and ensure good contact between loop and surrounding soil.
Best Practice
Commonly accepted practices among industry professionals.
Bin
In the bin method, a temperature increment, usually 5 F, into which the range of temperatures for an area are divided. Bins are used to produce a frequency distribution of hourly, monthly, or annual outdoor temperature occurrences for a specified location.
Bleed
Discharge of water from a standing column well to maintain desired temperatures
Block Load
Defined to be the sum of the zone loads. A block load calculation is necessary for a building with multiple zones served by a centralized heating/cooling system.
Blowers
Fans used to force air across the heat exchanger. With a ground source heat pump, the only blower used is to force air through the central heating system.
Brine
A mixture of water and antifreeze
Buffer Tank
A storage tank for geo conditioned water
Building Management System
A computerized or digital control system that controls many or all of the systems in a building, including the HVAC system, lighting system, building access controls, etc.
C
Centralized Pumping System
Flow centers located centrally in a ground source system that produce flow to all heat pump units in that system.
Circulating Pump(s)
The pump(s) that circulates the fluid in the closed-loop system during normal operation.
Closed-Loop Ground Source Heat Pump
The heat exchange loop in a Ground Source Heat Pump (GSHP) system that consists of the ground heat exchanger, the circulating pump, and the water-source heat pump in which the heat transfer fluid is not exposed to the atmosphere.
Closed-Loop Geothermal System
A subsurface circuit of wellbores containing a fluid heated by a geothermal hot rock resource, without direct contact of the fluid with the resource. Unlike the closed loop in a Ground Source Heat Pump, which is used for small-scale residential heating and cooling, Closed-Loop Geothermal Systems are used for utility-scale energy production (typically >1 megawatt). A Closed-Loop Geothermal System is sometimes referred to as an Advanced Geothermal System (AGS).
Coaxial Heat Exchanger
A tube in tube heat exchanger where water (or brine) is separated from refrigerant.
Coefficient of Performance
A measure of heat pump efficiency. It is calculated by dividing the heating or cooling output of a heat pump (in BTUs/hr) and dividing it by the energy input (converted to BTUs/hr).
Coil
A heat exchanger used to transfer energy from one source to another. In ground source heat pumps, water-to-refrigerant and refrigerant-to-air coils are used.
Combination GSHP Unit
A GSHP that has the ability to heat or cool air at full capacity or to heat or cool water at full capacity, but not both at the same time.
Condenser
A heat exchanger in which hot, pressurized (gaseous) refrigerant is condensed by transferring heat to cooler surrounding air, water, or earth.
Compressor
The central component of a heat pump system. The compressor increases the pressure of a refrigerant fluid, and simultaneously reduces its volume, while causing the fluid to move through the system.
D
Deep Earth Temperatures
Relatively constant temperature at a given depth which vary when heat is extracted or rejected by ground loops.
Degree Day
A measure of the severity and duration of an outdoor temperature deviation above or below a fixed temperature (65 F), used in estimating the heating or cooling requirement and fuel consumption of a building for either summer or winter conditions.
Delta P
Difference in pressure between two test points
Delta T
Temperature difference between two test sites such as supply and return air or entering and leaving water in ground loop.
Demand (DMD)
The electrical input required to operate a GSHP unit for space conditioning.
Design Loop Temperatures Temperatures that the ground loop is designed to stay above during the heating mode and below during cooling mode.
Design Outdoor Temperature
Outdoor air temperatures that coincides with the peak seasonal heating or cooling load.
Design Loads
The peak heating or cooling load used to select the equipment for a system (such as a heat pump) and to design the air distribution system (supply air diffusers, return air grilles, and the duct system). Design loads are based on standard or accepted conditions for a given locality (a design day).
Design Temperature, Summer
A specific temperature used in calculating the cooling load of a building. The summer design temperature is typically the outdoor air temperature that is exceeded 0.4% or 1.0% of the time.
Design Temperature, Winter
A specific temperature used in calculating the heating load of a building. The winter design temperature is typically the outdoor temperature that is exceeded 99.0% or 99.6% of the time.
Desuperheater
A device for recovering superheat from the compressor discharge gas of a heat pump or central air conditioner for use in heating or preheating water. Also known as a heat recovery water heater.
Dimension Ratio (DR)
A specific ratio of the average specified outside diameter to the minimum specified wall thickness (OD/t) for outside-diameter controlled plastic pipe.
Direct Expansion (DX) Earth-Coupled Heat Pump
A heat pump system in which the refrigerant is circulated in pipes buried underground.
Distributed Pumping System
A system made up of smaller, individual pumping stations (one flow center for each heat pump) each controlled individually by the operation of the specific heat pump unit that they serve.
Dual Circuit GSHP Unit
A GSHP which utilizes two compressors (generally of different capacities) connected to two refrigeration circuits to allow multiple modes of operation. This unit may use one compressor in heating or cooling of ducted air only, one compressor to heat water only, one compressor heating or cooling air while the other is heating water, or both compressors either heating or cooling air.
E
Efficiency
A measure of the useful output of a system divided by the input required to drive the system.
Energy Efficiency Ratio (EER)
A measure of cooling efficiency for heat pump equipment, expressed as the cooling energy removed from the space (Btu) divided by the electric energy consumed to provide that cooling (W).
Energy Loads
Used in predicting the energy necessary to operate the system for some prescribed time such as a month, year, or season. The calculation methodology may be the same as for the design load; however, the actual operating and weather data are used instead of design conditions.
Energy Model
A detailed heat loss and heat gain calculation for a building. The calculation takes into account heat transfer to and from the space inside the building to the outside air. It takes into account the construction of the building walls and roof, including insulation values, mass of the structure, orientation of the different components to the sun, color of the material. An 8,760 hourly model is used to calculate the heat loss/gain based on the historical average temperature data (usually 20 years) for the building location. The modeler then overlays building occupancy schedules, lighting schedules, ventilation, etc. to account for internal heat gains, heat losses and gains from the mechanical system, etc. in detail, on a room by room basis. The software than calculates the actual heating and cooling loads required for every hour of the year. The detailed energy analysis allows us to calculate the energy consumption per day, month or year. This can be translated into energy cost based on local utility rates. It also allows us to calculate energy transfer to and from the GHX and size the system accurately.
Emergency Heat
A backup heat source that is activated if heat pump shuts off. The most common form of emergency heat is electric resistance.
Entering Water Temperature (EWT)
The temperature of the water entering the heat pump from the ground loop heat exchanger. Systems are designed so that the entering water temperature does not fall below the Minimum Entering Water Temperature during heating or rise above the Maximum Entering Water Temperature during cooling. Entering water temperature has a significant effect on heat pump operating efficiency.
Equipment Loads
Loads served by the heating/cooling system that are not included in peak heating/ cooling block load calculations. These loads include duct and hydronic piping losses/gains as well as ventilation loads.
Evaporator
A heat exchanger in which cold, low-pressure (liquid) refrigerant is vaporized to absorb heat from the warmer surrounding air, earth, or water.
Expansion Valve
A device that reduces the pressure of liquid refrigerant entering the evaporator and meters and regulates the flow of refrigerant so that it can properly absorb heat.
F
Fan Coil
A water or refrigerant coil through which air is circulated for conditioning
Finish Tank
Water heater downstream of a buffer tank
Flow Center