Definitions of terms used in the HVAC industry
These definitions should help you to understand the terminology used to describe parts used and operations done by those in the industry.
Absolute: As in absolute pressure. The pressure above 0# not including normal pressure at sea level of the atmosphere above.
Ambient air: Temperature of the air of the surrounding area.
Adjust burners: Older furnaces had air adjustments on their burners. They should be adjusted to just get the yellow off the flame.
Air handler: A part of a conditioned air system containing a circulating fan to move air. Could be a gas, oil or electric furnace
Alternating current: An electric current that changes its polarity from positive to negative usually at a rate of 60 cycles per second. Common source would be power supplied to homes and businesses by a utility.
Ambient: The temperature of the surrounding area
Amperage: A measurement of the volume of electricity passing through a circuit.
Bimetal: 2 metals (usually copper and steel) bonded together whose expansion rates with temperature changes are different. This causes the metal to bend or warp with temperature changes. Normally used in all types of thermostats.
BTUH: British Thermal Units is the term for determining heat volume.
It is defined as: the amount of heat required to raise the temperature of one pound of water 1 degree Fahrenheit.
Circulating fan: Fan assembly that moves air through the furnace throughout the structure.
CFCs: Abreviation for chloroflourocarbon. A class of refrigerants developed in the 1930s that had the advantage of non flammability and non toxicity. They were phased out of production in 1995
Chassis ground: An attachment of a circuit to the metal of an appliance.
Closed: The position of an electrical switch that allows electricity to pass through.
Combustion air: Air used to supply the burners with enough oxygen to allow complete combustion.
Compression ratio: As applied to refrigeration, it is the difference between evaporating pressure and condensing pressure expressed in absolute pounds. The lower the difference between evaporating and condensing pressure, the higher the efficiency of the refrigerant. Energy usage increases as compression ratio increases.
Compressor: A device used to increase gas pressure
Condensation: The change of state of a substance from a gas to a liquid releasing the latent heat of condensation.
Continuity: An electrical term indicating that the wire or component tested is broken or more accurately has an open circuit.
Dedicated breaker: A circuit breaker in the service panel that has only one appliance wired to it.
Diagnostic code: These are lights on the IFC that indicate by numbers of flashes the conditions that are present in the furnace.
Direct current: An electric current that keeps the same polarity all the time. Common source would be a battery.
Discharge: The high pressure outlet of a refrigeration compressor
Downflow: A furnace design that moves air from the top of the furnace to the bottom. Used in buildings with no basement.
Drop in replacement: A refrigerant that can be used to replace a refrigerant that is no longer available without changing oil or replacing other parts.
Ductwork: These are usually galvanized metal piping that distributes heated air throughout the structure.
Fixed orifice device: A metering device that uses a reduced diameter in the liquid line to reduce the high side pressure to low side pressure.
Flame rod: A small diameter stainless steel rod on a gas furnace used to sense flame rectification by being placed in the flame.
Flame safety control: A control, sometimes incorporated into an IFC, designed to insure that gas is not moved through the burners or flame is established under dangerous conditions. In the oil furnace it is usually called the cad cell relay. In this application, it will shut down the oil supply if light is not sensed within a preset time after the oil is sprayed
Forced air: An appliance that uses a fan to force the air throughout the structure.
Gas regulator: A device that lowers line gas pressure to the proper pressure for the appliance.
Hard lockout: The term used to describe the condition that will not allow restart. Usually unit will attempt restart in one hour.
HCFCs: Abbeviation for Hydrochlorofluorocarbon. A refrigerant class developed in the 1930s used mostly in air conditioners with less damage to the ozone layer than CFCs. Phase began in 2010.
Heat exchanger: A sheet metal barrier between the burned gasses and the air passed through the furnace.
Hermetic compressor: A sealed motor-compressor unit that is welded together and is not generally repaired.
Hidden heat: Heat absorbed or released from a substance when it changes state.
High side: This refers to the refrigeration cycle. Any point between the discharge of the compressor and the expansion valve. This includes discharge line, condenser and liquid line.
HSI: Hot surface ignitor. A device that when electricity is passed through, warms to a high temperature to ignite gas burners.
Hygroscopic: The ability of a lubricating oil to absorb moisture.
IFC: Integrated furnace control Electronic control used to control most operations for newer gas furnaces including flame safety control, circulating fan control, inducer control and safety sensors.
Indoor unit: The term used to describe the inside unit that moves air through the structure
Inducer: A fan assembly used to draw burnt gasses from a gas furnace and create negative pressure in the heat exchanger
Interim replacement: A refrigerant used to replace banned refrigerants on existing equipment thereby extending the life of older equipment
Gas line pressure: The supply pressure that the utility sets for the pressure from the meter to the appliance.
Latent heat: Heat that is absorbed by a change of state (as in water to steam) or released (as in steam to water) that cannot be read by a thermometer.
Line voltage: Electrical term for the voltage used to operate the larger loads of the furnace (such as circulating fan motor or inducer). For most gas furnaces 120v.
Limit switch: A heat actuated switch that usually opens on a rise in temperature. Usually used to turn off burners or other energy source in unsafe conditions.
Load: An electrical term for a device that consumes electricity and does work. (example: a light, a motor)
Lockout mode: A term to indicate that the furnace has seen an unsafe condition and will not continue the sequence. A blinking light sequence will indicate where the failure is. Rebooting the unit will clear the code, but if the condition is still there, it will reoccur.
Low side: This refers to the refrigeration cycle. Any point between the expansion valve and the suction inlet of the compressor including the evaporator and the suction line.
Make: An electrical term meaning to allow electrical power to pass through.
Miscible: As to the ability of a refrigerant to mix with refrigerant oil.
Normally open: The position of contacts of a switch when power is not applied. Open means no power can pass through.
Nozzle: A part of an oil burner that takes high pressure oil and sprays it into a fine mist to prepare it to burn.
OEM: A part designed for a specific appliance model as opposed to general replacement part
Ohmmeter: An electric meter used for measuring the resistance of a circuit
Open: The position of an electric switch that will not allow electricity to pass through or, when used in relation to motor windings, a broken wire in the windings.
Outdoor unit: An air conditioning condensing unit consisting of the compressor, condenser, fan and attendant controls.
Overfired: The condition of a furnace having too much gas inputted into the burners, resulting in damage to the heat exchanger. See setting the firing rate
Pilot dropout: The amount of time it takes for the pilot safety to shut off all gas supply to the burners when the pilot light is extinguished.
POE: Abbreviation for Polyol Ester oil. A modern lubricate used in refrigeration systems, usually using HFC refrigerants.
Primary: Electrical term for the winding of a transformer that is connected to the power source. (Household outlet etc).
Primary control: A term used for oil furnace flame safety controls such as the cad cell relay or the stack switch. They are used to shut off the burner if there is no flame detected.
Prepurge: Operation of inducer prior to flame initiation to clear any gasses from the heat exchanger.
Reciprocating: A refrigeration compressor that uses pistons moving up and down to increase pressur
Refrigerant: A substance used to absorb heat by evaporating in the cooled space and releasing heat outside the structure.
Recover: To remove, without any attempt to clean, refrigerant from a refrigeration system. The refrigerant is then sent to a reclaim company where it is either destroyed or cleaned to ARI 700 standards. It can then be reused.
Refrigerating effect: The amount of BTUs removed from a conditioned space or medium.
Regulator: A device that reduces pressure to a lower pressure.
Return air: Ductwork designed to bring air that is circulated through the structure back to the furnace to be heated.
RMS: (root means square) A measurement of a meter's accuracy. An RMS meter is more accurate throughout its range than a meter that is not RMS.
Rollout switch: A high temperature limit switch that must be manually reset placed near the burners to detect flames that have "rolled out" from the front of the furnace.
Secondary: Electrical term for the winding on a transformer that is used as a power source. In furnaces, it is usually a low voltage source for control power.
Secondary heat exchanger: This is a part on high efficiency furnaces. It is a finned coil mounted directly above the blower assembly designed to remove more heat from the exhaust gasses.
Sensible heat: Heat that can be read on a thermometer. As heat (rated in BTU) is added or taken away, the temperature changes. (See also latent or hidden heat)
Series: An electric term to indicate a switch that if opened will shut off the power to some control or load resulting in interruption of fuel delivery.
Short: An electrical term indicating that the circuit has bypassed the load and passed through to the other side of the circuit with little or no resistance causing excessive current and consequent opening of circuit protection (fuse or breaker)
Split system: An air conditioning system that has a separate evaporator to be installed with an existing air handler or furnace in a remote location from the outdoor unit.
Subcool: Sensible heat removed from a liquid after all gas is condensed. Only a liquid can be subcooled.
Suction: The low side pressure in a refrigeration system
Superheat: Sensible heat added to gaseous refrigerant after all liquid is evaporated. Only a gas can be superheated.
Terminal board: Connection screws used for connecting thermostat wires to the furnaces
Transformer: A small coil that transforms line voltage to control voltage. Usually 24 v a/c
Upflow: A furnace design that moves air from the bottom of the furnace to the top of the furnace. Used in buildings with a basement.
Vaporization: The changing of a liquid to a gas accompanied by absorption
Venting: Piping used by furnaces to remove burned gasses from the structure to the outside.
2 stage furnace: This furnace uses 2 regulators to control the amount of gas burned. The first stage usually runs at 1/2 the pressure of the second stage.
Absolute: As in absolute pressure. The pressure above 0# not including normal pressure at sea level of the atmosphere above.
Ambient air: Temperature of the air of the surrounding area.
Adjust burners: Older furnaces had air adjustments on their burners. They should be adjusted to just get the yellow off the flame.
Air handler: A part of a conditioned air system containing a circulating fan to move air. Could be a gas, oil or electric furnace
Alternating current: An electric current that changes its polarity from positive to negative usually at a rate of 60 cycles per second. Common source would be power supplied to homes and businesses by a utility.
Ambient: The temperature of the surrounding area
Amperage: A measurement of the volume of electricity passing through a circuit.
Bimetal: 2 metals (usually copper and steel) bonded together whose expansion rates with temperature changes are different. This causes the metal to bend or warp with temperature changes. Normally used in all types of thermostats.
BTUH: British Thermal Units is the term for determining heat volume.
It is defined as: the amount of heat required to raise the temperature of one pound of water 1 degree Fahrenheit.
Circulating fan: Fan assembly that moves air through the furnace throughout the structure.
CFCs: Abreviation for chloroflourocarbon. A class of refrigerants developed in the 1930s that had the advantage of non flammability and non toxicity. They were phased out of production in 1995
Chassis ground: An attachment of a circuit to the metal of an appliance.
Closed: The position of an electrical switch that allows electricity to pass through.
Combustion air: Air used to supply the burners with enough oxygen to allow complete combustion.
Compression ratio: As applied to refrigeration, it is the difference between evaporating pressure and condensing pressure expressed in absolute pounds. The lower the difference between evaporating and condensing pressure, the higher the efficiency of the refrigerant. Energy usage increases as compression ratio increases.
Compressor: A device used to increase gas pressure
Condensation: The change of state of a substance from a gas to a liquid releasing the latent heat of condensation.
Continuity: An electrical term indicating that the wire or component tested is broken or more accurately has an open circuit.
Dedicated breaker: A circuit breaker in the service panel that has only one appliance wired to it.
Diagnostic code: These are lights on the IFC that indicate by numbers of flashes the conditions that are present in the furnace.
Direct current: An electric current that keeps the same polarity all the time. Common source would be a battery.
Discharge: The high pressure outlet of a refrigeration compressor
Downflow: A furnace design that moves air from the top of the furnace to the bottom. Used in buildings with no basement.
Drop in replacement: A refrigerant that can be used to replace a refrigerant that is no longer available without changing oil or replacing other parts.
Ductwork: These are usually galvanized metal piping that distributes heated air throughout the structure.
Fixed orifice device: A metering device that uses a reduced diameter in the liquid line to reduce the high side pressure to low side pressure.
Flame rod: A small diameter stainless steel rod on a gas furnace used to sense flame rectification by being placed in the flame.
Flame safety control: A control, sometimes incorporated into an IFC, designed to insure that gas is not moved through the burners or flame is established under dangerous conditions. In the oil furnace it is usually called the cad cell relay. In this application, it will shut down the oil supply if light is not sensed within a preset time after the oil is sprayed
Forced air: An appliance that uses a fan to force the air throughout the structure.
Gas regulator: A device that lowers line gas pressure to the proper pressure for the appliance.
Hard lockout: The term used to describe the condition that will not allow restart. Usually unit will attempt restart in one hour.
HCFCs: Abbeviation for Hydrochlorofluorocarbon. A refrigerant class developed in the 1930s used mostly in air conditioners with less damage to the ozone layer than CFCs. Phase began in 2010.
Heat exchanger: A sheet metal barrier between the burned gasses and the air passed through the furnace.
Hermetic compressor: A sealed motor-compressor unit that is welded together and is not generally repaired.
Hidden heat: Heat absorbed or released from a substance when it changes state.
High side: This refers to the refrigeration cycle. Any point between the discharge of the compressor and the expansion valve. This includes discharge line, condenser and liquid line.
HSI: Hot surface ignitor. A device that when electricity is passed through, warms to a high temperature to ignite gas burners.
Hygroscopic: The ability of a lubricating oil to absorb moisture.
IFC: Integrated furnace control Electronic control used to control most operations for newer gas furnaces including flame safety control, circulating fan control, inducer control and safety sensors.
Indoor unit: The term used to describe the inside unit that moves air through the structure
Inducer: A fan assembly used to draw burnt gasses from a gas furnace and create negative pressure in the heat exchanger
Interim replacement: A refrigerant used to replace banned refrigerants on existing equipment thereby extending the life of older equipment
Gas line pressure: The supply pressure that the utility sets for the pressure from the meter to the appliance.
Latent heat: Heat that is absorbed by a change of state (as in water to steam) or released (as in steam to water) that cannot be read by a thermometer.
Line voltage: Electrical term for the voltage used to operate the larger loads of the furnace (such as circulating fan motor or inducer). For most gas furnaces 120v.
Limit switch: A heat actuated switch that usually opens on a rise in temperature. Usually used to turn off burners or other energy source in unsafe conditions.
Load: An electrical term for a device that consumes electricity and does work. (example: a light, a motor)
Lockout mode: A term to indicate that the furnace has seen an unsafe condition and will not continue the sequence. A blinking light sequence will indicate where the failure is. Rebooting the unit will clear the code, but if the condition is still there, it will reoccur.
Low side: This refers to the refrigeration cycle. Any point between the expansion valve and the suction inlet of the compressor including the evaporator and the suction line.
Make: An electrical term meaning to allow electrical power to pass through.
Miscible: As to the ability of a refrigerant to mix with refrigerant oil.
Normally open: The position of contacts of a switch when power is not applied. Open means no power can pass through.
Nozzle: A part of an oil burner that takes high pressure oil and sprays it into a fine mist to prepare it to burn.
OEM: A part designed for a specific appliance model as opposed to general replacement part
Ohmmeter: An electric meter used for measuring the resistance of a circuit
Open: The position of an electric switch that will not allow electricity to pass through or, when used in relation to motor windings, a broken wire in the windings.
Outdoor unit: An air conditioning condensing unit consisting of the compressor, condenser, fan and attendant controls.
Overfired: The condition of a furnace having too much gas inputted into the burners, resulting in damage to the heat exchanger. See setting the firing rate
Pilot dropout: The amount of time it takes for the pilot safety to shut off all gas supply to the burners when the pilot light is extinguished.
POE: Abbreviation for Polyol Ester oil. A modern lubricate used in refrigeration systems, usually using HFC refrigerants.
Primary: Electrical term for the winding of a transformer that is connected to the power source. (Household outlet etc).
Primary control: A term used for oil furnace flame safety controls such as the cad cell relay or the stack switch. They are used to shut off the burner if there is no flame detected.
Prepurge: Operation of inducer prior to flame initiation to clear any gasses from the heat exchanger.
Reciprocating: A refrigeration compressor that uses pistons moving up and down to increase pressur
Refrigerant: A substance used to absorb heat by evaporating in the cooled space and releasing heat outside the structure.
Recover: To remove, without any attempt to clean, refrigerant from a refrigeration system. The refrigerant is then sent to a reclaim company where it is either destroyed or cleaned to ARI 700 standards. It can then be reused.
Refrigerating effect: The amount of BTUs removed from a conditioned space or medium.
Regulator: A device that reduces pressure to a lower pressure.
Return air: Ductwork designed to bring air that is circulated through the structure back to the furnace to be heated.
RMS: (root means square) A measurement of a meter's accuracy. An RMS meter is more accurate throughout its range than a meter that is not RMS.
Rollout switch: A high temperature limit switch that must be manually reset placed near the burners to detect flames that have "rolled out" from the front of the furnace.
Secondary: Electrical term for the winding on a transformer that is used as a power source. In furnaces, it is usually a low voltage source for control power.
Secondary heat exchanger: This is a part on high efficiency furnaces. It is a finned coil mounted directly above the blower assembly designed to remove more heat from the exhaust gasses.
Sensible heat: Heat that can be read on a thermometer. As heat (rated in BTU) is added or taken away, the temperature changes. (See also latent or hidden heat)
Series: An electric term to indicate a switch that if opened will shut off the power to some control or load resulting in interruption of fuel delivery.
Short: An electrical term indicating that the circuit has bypassed the load and passed through to the other side of the circuit with little or no resistance causing excessive current and consequent opening of circuit protection (fuse or breaker)
Split system: An air conditioning system that has a separate evaporator to be installed with an existing air handler or furnace in a remote location from the outdoor unit.
Subcool: Sensible heat removed from a liquid after all gas is condensed. Only a liquid can be subcooled.
Suction: The low side pressure in a refrigeration system
Superheat: Sensible heat added to gaseous refrigerant after all liquid is evaporated. Only a gas can be superheated.
Terminal board: Connection screws used for connecting thermostat wires to the furnaces
Transformer: A small coil that transforms line voltage to control voltage. Usually 24 v a/c
Upflow: A furnace design that moves air from the bottom of the furnace to the top of the furnace. Used in buildings with a basement.
Vaporization: The changing of a liquid to a gas accompanied by absorption
Venting: Piping used by furnaces to remove burned gasses from the structure to the outside.
2 stage furnace: This furnace uses 2 regulators to control the amount of gas burned. The first stage usually runs at 1/2 the pressure of the second stage.