Mechanical Air Service - HVAC Terms

A-coil: a heat exchanger consisting of two diagonal coils that are joined together in the shape of the letter A.

Access fitting: a valve that provides a port or creates an opening to a sealed system.

Acid condition: the presence of corrosive substances in a sealed refrigeration system.

ACR tubing: copper tubing that is specially sized and sealed for use in air conditioning and refrigeration systems.

AFUE: Annual Fuel Utilization Efficiency. A measure of a gas furnace's efficiency in converting fuel to energy ­ the higher the rating, the more efficient the unit. For example: A rating of 90 means that approximately 90 percent of the fuel is used to provide warmth to your home, while the remaining 10 percent escapes as exhaust.

Air conditioner: a device that modifies the temperature, humidity, cleanliness, or general quality of air.

Air conditioning: the science of controlling the temperature, humidity, cleanliness or general quality of air. Abbreviated: ac.

Air diffuser: an air supply outlet that delivers air in a specific pattern.

Air handler: air moving and/or mixing unit; may include blower, filters, dampers, etc.

Ambient temperature: the temperature of the immediately surrounding air or area.

Amperage: the rate of electrical current flow in a circuit.

Anticipator: a thermostat component that compensates for the control's thermal lag.

Bearing: a surface or roller designed to minimize friction between moving parts.

Belt drive: a means of transferring rotating force from a prime mover (such as a motor) to a pump or compressor.

Blower: an air-moving device; a fan.

Boiler: a sealed chamber in which water is converted to steam or is heated for circulation in a hydronic heating system.

Boot: a fitting that connects round to rectangular sheet metal ducts.

BTU (British thermal unit): the amount of heat that will raise or lower the temperature of one pound of water one degree Fahrenheit.

Bulb sensing: a fluid-filled bulb that responds to a temperature remote from its control.

Burner: a device that utilizes fuel to support combustion.

BUTH: BTU per hour. A measurement used to size HVACR equipment.

Bypass: a piping detour around a component; to circumvent any part of a circuit.

Calibrate: to adjust a control or device in order to maintain its accuracy.

Cap: a fitting that seals off the end of a tube or pipe.

Capacity: The output or producing ability of a piece of cooling or heating equipment. Cooling and heating capacities are referred to on BTUs.

Capillary tube: a small diameter tubing used in pressure sensing devices or as a metering device in refrigeration systems. Also called a cap tube.

Carbon monoxide: CO; a poisonous gas produced by incomplete combustion.

Celsius (centigrade): the metric temperature scale in which water freezes at 0 deg. And boils at 100 deg.

Central system: an air conditioning system controlled from or having all its main components located at a single point.

CFC chlorofluorocarbon: An ozone-depleting refrigerant that is being phased out.

CFM: Cubic Feet Per Minute. A standard measurement of airflow. A typical system requires 400 CFM per ton of air conditioning.

Check valve: a control fitting that permits the flow of fluid in one direction only.

Chiller: 1. a cooler that refrigerates a secondary refrigerant, such as water or brine, for circulation elsewhere. 2. Any machine that lowers the temperature of a fluid.

Chimney or flue: a vent for the products of combustion.

Circuit: 1. a path for electrical flow. 2. A complete, closed loop for fluid circulation.

Circuit breaker: a resettable switch that opens an electrical circuit in case of an excessive current flow.

Circulator: a pump used to push water through a hydronic system.

Coil: 1. a section of more than one length of pipe or tubing used to transfer heat. 2. a wire winding or loop.

Combustion: burning; the rapid oxidation that results from combining fuel, heat, and air.

Compressor: The heart of an air conditioning or heat pump system. It is part of the outdoor unit and pumps refrigerant in order to meet the cooling requirements of the system.

Condensate/condensation: water vapor that liquefies due to the lowering of its temperature to the saturation point.

Condensate pump: a water pump that disposes of or recirculates collected condensate.

Condenser: a heat exchanger in which compressed refrigerant vapor is cooled until it becomes a liquid.

Condenser fan: the fan that circulates air over and air cooled condenser. (usually located outside)

Condensing unit: the components that intake low-temperature/pressure refrigerant and convert it to a liquid suitable for the absorption of more heat; the compressor, condenser, receiver and their controls.

Contactor: a switch that can repeatedly cycle, making and breaking an electrical circuit. When sufficient current flows through a coil built into the contactor, the resulting magnetic field causes the contacts to be pulled in.

Coupling: a straight fitting for two lengths of pipe or tubing.

Damper: Found in ductwork, this movable plate opens and closes to control airflow. Dampers can be used to balance airflow in a duct system. They are also used in zoning to regulate airflow to certain rooms.

Damper motor: a motor which, through mechanical linkage, opens or closes a damper automatically.

Data plate: an equipment identification label; usually lists model and serial numbers and various unit ratings.

Degree day: the difference between indoor design temperature and each day's average outdoor temperature. Abbreviated: DD.

Dehumidifier: an air conditioner that removes moisture from air by cooling it.

Demand meter: an instrument that measures electrical power consumption, usually in kilowatt-hours.

Dew point: the temperature at which water vapor begins to condense out of humid air.

Diffuser: a grille over an air supply duct with vanes to distribute the discharging air in a specific pattern or direction.

Disconnect: a switch box that cuts off electrical power to a machine being serviced.

Door switch: a circuit control that is made and broken by the opening and closing of the equipment's door or lid.

Downflow furnace: a furnace that intakes air at its top and discharges it near its bottom.

Draft diverter: a fitting on the furnace flue that draws in room air to dilute the exhaust gases. It also minimizes the effect of downdrafts and extreme updrafts in the flue.

Drier: an accessory that removes moisture from a refrigeration system.

Drip pan: a pan used to collect evaporator condensate.

Duct: a pipe or conduit through which air is delivered.

Duct heater: an electric-resistance or hot water heater mounted inside an air-delivery duct to provide supplementary heat to the delivered air.

Ductwork: Pipes or channels that carry air throughout your home. In a home comfort system, ductwork is critical to performance ­ in fact, it's as critical as the equipment.

Economizer: a system of controls and dampers on an air conditioning system that mixes varying volumes of outdoor air with air in the conditioned space.
This saves money by using outdoor air for cooling whenever it is cold enough.

EER (energy efficiency ratio): the number of BTU produced per watt of electrical power consumed by an air conditioner.

Efficiency: the amount of usable energy produced by a machine, divided by the amount of energy supplied to it.

Elbow: any fitting that produces a bend in the duct or piping run it connects.

Electric heat: a heating system in which the energy source is electricity and the heat is produced by resistance elements.

Electronic leak detector: a test instrument that detects the presence of halide refrigerants in small air samples.

Electrostatic filter or precipitator: an air cleaning device that electrically charges foreign particles in the air and then collects them on positively charges plates.

Evacuate: to remove, through the use of a vacuum pump, all moisture and noncondensables from a system.

Evaporator coil or Indoor coil: The other half of your air conditioning system located inside your home in the indoor unit. This is where the refrigerant evaporates as it absorbs heat from the air that passes over the coil.

Fahrenheit: the temperature scale on which water freezes at 32 deg. And boils at 212 deg.

Fan coil: a heating and/or cooling unit consisting of a finned-tube coil and fan in a package.

Filter: a device that removes impurities from a fluid through a mechanical straining process.

FLA (full load amperage): the current draw of a motor under full load, the current flow in a circuit when the load is at its rated peak.

Flexible duct: a duct that can be gradually bent to go around obstacles.

Flue: a vent that carries the products of combustion from a boiler or furnace.

Furnace: that part of the heating system in which combustion and heat transfer occur.

Fuse: a metal strip in an electrical circuit that melts and breaks the circuit when excessive current flows through it.

Gas Furnace Heat Exchanger: Located in the furnace, the heat exchanger transfers heat to the surrounding air, which is then pumped throughout your home.

Gas valve: a valve that controls fuel flow to the burner in a gas-fired furnace or heater.

Grille: a cage-like panel that covers the end of a duct, permitting the passage of air but not foreign objects.

HCFC: hydrochlorofluorocarbon, a refrigerant. Sometimes abbreviated to HFC.

Head pressure: high-side pressure in a refrigeration system; pressure from the compressor discharge to the metering device.

Heat exchanger: any device that transfers heat from one substance to another substance.

Heat pump: a mechanical-compression cycle refrigeration system that can be reversed to either heat or cool a conditioned space.

High-efficiency gas furnace: a furnace that recycles combustion gases to obtain efficiencies of 85% to 95%.

Horizontal furnace: a short furnace with intake at one end and discharge out the other.

HSPF (Heating Seasonal Performance Factor): ratio of the total seasonal heating requirements (BTU), including supplemental electric heat, during annual usage period for heating, divided by the total electric power consumption in watts during the same period. (measurement for rating heat pump efficiency)

Humidifier: a machine that adds water vapor to air to increase its relative humidity.

Humidistat: a humidity-sensing control that cycles the humidifier on and off.

IAQ: indoor air quality; an HVACR field that deals with developing and maintaining a building's indoor air system so that it is clean, healthful and comfortable for the occupants of the building.

Intelligent building: a structure in which all aspects of air quality and operation are monitored and controlled by digital computers.

Liquid line: the refrigerant tubing extending from the condenser outlet to the metering device. Usually the small warm line.

Locked rotor amperage (LRA): the current drawn by a motor at start-up, before the rotor starts turning.

Low ambient control: a device for maintaining high-side pressure when the temperatures around an air-cooled condenser are abnormally low.

OEM: original equipment manufacturer.

Overcharge: to fill a system with refrigerant beyond its design capacity.

Package unit: a system with all the major components contained in a single cabinet or installed in a single location, as opposed to a split system with remote components.

Preventative maintenance: the scheduled inspection and replacement of short-lived components in order to avoid untimely and expensive emergency repairs.

Pump down: to use the compressor to pump all of the system's refrigerant into the receiver and/or condenser prior to opening the system for service. This is also used in commercial systems to prevent refrigerant migration in the off cycle. 

Refrigerant: A chemical that produces a refrigerating effect while expanding and vaporizing. Most residential air conditioning systems contain R-22 refrigerant. R-22 is regulated by international controls under the Montreal Protocol and in the United States by the Environmental Protection Agency. It is scheduled to be in production until the year 2020. It's used in approximately 95 percent of air conditioning equipment manufactured in the U.S. today.

Refrigerant reclaim: the process of returning recovered refrigerant to new product specifications. This is usually performed at a reprocessing facility.

Refrigerant recovery: the process of removing refrigerant from a system and placing it in a container.

Refrigerant recycling: the process of cleaning recovered refrigerant by reducing contaminant levels with filters, on site.

Register: a grille with adjustable louvers or dampers for controlling the direction of discharge air.

Relay: an electrical switch controlled by the flow of current in a separate or parallel circuit.

Retrofit: the process of changing the design or equipment to incorporate modern features or later improvements.

Return intake: an opening through which air is exhausted from a conditioned space.

Reversing valve: an electrical, four-way valve in a heat pump that diverts refrigerant flow according to whether cooling or heating is needed; also called a four-way valve.

SEER: Seasonal Energy Efficiency Ratio. A measure of cooling efficiency for air conditioners and heat pumps. The higher the seer, the more energy efficient the unit. The government's minimum SEER rating is 10. (It's similar to comparing miles per gallon in automobiles.)

Short cycling: continual starting and stopping of a system over a shorter-than-normal time period, due to a malfunction.

Single-phasing: the dropping out of a leg in a three-phase circuit. The motor continues to run as a single-phase motor, causing the motor to overheat and burn out.

Split system: The combination of an outdoor unit (air conditioner or heat pump) with an indoor unit (furnace or air handler). Split systems must be matched for optimum efficiency.

Squirrel cage: the wheel of a centrifugal fan, so called due to its resemblance to rotary exercise wheels.

Subbase: a thermostat mounting plate that incorporates operating circuitry into its construction.

Suction line: the refrigerant piping from the evaporator outlet to the compressor inlet. (It is the larger, cold insulated refrigerant line.)

Thermocouple: two dissimilar metal conductors welded together. When their junction is heated, a voltage is produced. It is used as a safety device in a gas burner system.

Thermostat: A thermostat consists of a series of sensors and relays that monitor and control the functions of a heating and cooling system.

Ton: A unit of measurement used for determining cooling capacity. One ton is the equivalent of 12,000 BTUs per hour.

Transformer: a set of coils that increases or decreases voltage by induction.

TXV (thermostatic expansion valve): a valve that controls the flow of refrigerant. It is operated by evaporator temperature and pressure.

Undercharged: a refrigeration system that is short of refrigerant.

Universal replacement part: a part that can be used as a replacement part for many different models, including equipment of different manufacturers.

Upflow furnace: a heater in which air is drawn in through the sides or bottom and discharged out the top.

Vacuum: any pressure below atmospheric pressure.

Vacuum pump: a vapor pump capable of creating the degree of vacuum necessary to evaporate moisture near room temperature.

Zoning: A method of dividing a home into different comfort zones so each zone can be independently controlled depending on use and need.

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