HVAC Glossary

Alternative HVAC Solutions would like to offer the following HVAC Glossary as a helpful tool for you, our customer, to help with the various terms and technical definitions for the heating, cooling and indoor air quality industry.

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CAE (Combined Annual Efficiency) -  A measure of the amount of heat produced for every dollar of fuel consumed for both home heating and water heating.
Calorie -  One energy calorie is equivalent to 4.2 joules. Thus, it takes 500,000 calories of energy to boil a pot of coffee. One food calorie equals 1,000 energy calories.
Capacitor -  A device used to start a motor or compressor (or to keep it running after start up.)
Capacity -  (or System Capacity ) The output or producing ability of a piece of cooling or heating equipment. Cooling and heating capacity are normally referred to in BTUs. The capacity of an air conditioner is measured by the amount of cooling it can do when running continuously. The total capacity is the sum of the latent capacity (ability to remove moisture from the air) and sensible capacity (ability to reduce the dry-bulb temperature). Each of these capacities is rated in Btus per hour (Btu/h). The capacity depends on the outside and inside conditions. As it gets hotter outside (or cooler inside) the capacity drops. The capacity at a standard set of conditions is often referred to as "tons of cooling."
Carbon Dioxide -  (CO2) A colorless, odorless, non-poisonous gas that is a normal part of the air. Carbon dioxide, also called CO2, is exhaled by humans and animals and is absorbed by green growing things and by the sea. A gas which can at high levels (above 1.5 % or 15,000 parts per million), have physiological effects. Main indoor source is human respiration; measurements used as indicators of ventilation conditions.
Carbon Monoxide -  (CO) An colorless, odorless gas that is the product of incomplete fuel combustion or carbon burns without sufficient air nearby. It is a chemical asphyxiant; in the bloodstream it effectively prevents the transport of oxygen to the body's tissues. CO exposure can affect the lungs, heart, and nervous system, and can cause death. Sources include cooling and heating appliances, tobacco smoke, and entrained exhaust from parking garages and truck idling areas. A gas, made up of carbon and oxygen molecules, produced by incomplete burning of carbon or carbonaceous materials, including carbon-based fuels. including coal, natural gas, gasoline, oil and wood. It is a major air pollutant on the basis of weight. Carbon monoxide is also produced from incomplete combustion of many natural and synthetic products. For instance, cigarette smoke contains carbon monoxide. When carbon monoxide gets into the body, the carbon monoxide combines with chemicals in the blood and prevents the blood from bringing oxygen to cells, tissues and organs. The body's parts need oxygen for energy, so high-level exposures to carbon monoxide can cause serious health effects, with death possible from massive exposures. Symptoms of exposure to carbon monoxide can include vision problems, reduced alertness, and general reduction in mental and physical functions. Carbon monoxide exposures are especially harmful to people with heart, lung and circulatory system diseases.
Carboxyhemogoblin Saturation -  Carbon monoxide poisoning.
Carcinogen -  An agent suspected or known to cause cancer.
Ceiling Plenum -  The space between the suspended and structural ceiling used as part of the air distribution system that accomodates the mechanical and electrical equipment. This space usually accommodates electrical, communications, and mechanical connections as well. The space is kept under negative pressure.
Celsius -  A temperature scale based on the freezing (0 degrees) and boiling (100 degrees) points of water. Abbreviated as C in second and subsequent references in text. Formerly known as Centigrade. To convert Celsius to Fahrenheit, multiply the number by 9, divide by 5, and add 32. For example: 10 degrees Celsius x 9 = 90; 90 / 5 = 12; 18 + 32 = 50 degrees Fahrenheit.
Central Air Handling Unit ( Central AHU ) -  This is the same as an Air Handling Unit, but serves more than one area.
Central Forced-Air Heating System -  A piece of equipment that produces heat in a centralized area, then distributes it throughout the home through a duct system.
CFC -  (Chlorofluorocarbons or Chlorinated Fluorocarbons) A class of refrigerants. Generally refers to the Chlorofluorocarbon family of refrigerants. Sometimes called Freon A family of artificially produced chemicals receiving much attention for their role in stratospheric ozone depletion. Since they were introduced in the mid-1930s, CFCs have been used as refrigerants, solvents and in the production of foam material. These chemicals and some related chemicals have been used in great quantities in industry, for refrigeration and air conditioning, and in consumer products. CFCs and their relatives, when released into the air, rise into the stratosphere, a layer of the atmosphere high above the Earth. In the stratosphere, CFCs and their relatives take part in chemical reactions which result in reduction of the stratospheric ozone layer, which protects the Earth's surface from harmful effects of radiation from the sun. On a per molecule basis, these chemicals are several thousand times more effective as greenhouse gases than carbon dioxide. The 1987 Montreal protocol on CFCs seeks to reduce their production by one-half by the year 1998. The 1990 CLEAN AIR ACT includes provisions for reducing releases (emissions) and eliminating production and use of these ozone-destroying chemicals.
CFM -  (Cubic Feet per Minute) A standard measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system. A typical system produces 400 CFM per ton of air conditioning.
Charge -  Amount of refrigerant placed in a refrigerating unit.
Chemical Sensitization -  Evidence suggests that some people may develop health problems characterized by effects such as dizziness, eye and throat irritation, chest tightness, and nasal congestion that appear whenever they are exposed to certain chemicals. People may react to even trace amounts of chemicals to which they have become "sensitized."
Chiller -  A device that produces chilled water to provide air conditioning for large buildings or cooling for process applications. A device that cools water, usually to between 40 and 50 degrees Fahrenheit for eventual use in cooling air.
Circuit -  One complete run of a set of electric conductors from a power source to various electrical devices (appliances, lights, etc.) and back to the same power source.
Clean Air Act -  The original Clean Air Act was passed in 1963, but the national air pollution control program is actually based on the 1970 version of the law. The 1990 Clean Air Act Amendments are the most far-reaching revisions of the 1970 law. The 1990 amendments are o ften referred to as as the 1990 Clean Air Act.
Clean Air Act Amendments of 1990 -  These amendments represent a major overhaul of the earlier Clean Air Act of 1970. Changes include revised provisions for attainment and maintenance of National Ambient Air Quality Standards, mobile sources, hazardous air pollutants, and other assorted air quality issues. In addition, it establishes guidelines for reductions in air pollution. The Act also specifically limits sulfur dioxide and nitrogen oxide emissions of power plants.
Cogeneration -  Simultaneous production of two or more forms of useable energy from a single fuel source, e.g., heat energy and electrical or mechanical power, in the same facility. Because a typical cogeneration facility uses thermal energy which is generally vented in a traditional power plant, the process can be 50 to 70 percent more efficient. Fuels used in cogeneration facilities may take the form of natural gas, biomass, oil or coal. Most cogeneration systems are designed to simultaneously produce electric power (to be used on site or sold back to an investor-owned utility or both) and thermal heat for industrial processes or the heating and cooling of buildings. Cogeneration projects can be any size, from 10 kilowatts to 1,000 megawatts or more.
Coil -  A cooling or heating element, often including fins, through which treated gas or liquid is passed, exchanging thermal energy with air surrounding it for heating or cooling.
Colony Forming Unit -  (CFU) A laboratory measure of fungal concentration, indicating the quantity of viable organisms collected for a given unit sample.
Comfort Conditioning -  The process of treating air to simultaneously control its temperature, humidity, cleanliness, and distribution to meet the comfort requirements of the occupants of the conditioned space.
Comfort Zone -  The range of temperatures, humidities and air velocities at which the greatest percentage of people feel comfortable. The range of temperatures over which the majority of persons feel comfortable (neither too hot nor too cold).
Commercial -  The commercial sector is generally defined as nonmanufacturing business establishments, including hotels, motels, restaurants, wholesale businesses, retail stores, and health, social, and educational institutions. The utility may classify commercial service as all consumers whose demand or annual use exceeds some specified limit. The limit may be set by the utility based on the rate schedule of the utility.
Commissioning -  The testing of HVAC systems prior to building occupancy to check whether the systems meet the operational needs of the building within the capabilities of the system design. Start-up of a building that includes testing and adjusting HVAC, electrical, plumbing, and other systems to assure proper functioning and adherence to design criteria. Commissioning also includes the instruction of building representatives in the use of the building systems.
Compressor -  The heart of an air conditioning or heat pump system. The large (usually black) part in the condenser (outdoor unit) that pumps refrigerant. The pump of a refrigerating mechanism which draws a low pressure on cooling side of refrigerant cycle and squeezes or compresses the gas into the high pressure or condensing side of the cycle. The compressor maintains adequate pressure to cause refrigerant to flow in sufficient quantities to meet the cooling requirements of the system.
Condenser -  This is the unit that will sit outside and is part of a split system, it contains the compressor which is the heart of your air conditioner or Heat Pump, it pumps the refrigerant through your system. Some people call the condenser the compressor, but the compressor is a component of the condenser along with the fan motor and condenser coil. Coil or outdoor coil dissipates heat from the refrigerant, changing the refrigerant from vapor to liquid.
Condenser Approach Temperature -  The temperature difference between the condenser's refrigerant temperature and the leaving condenser water temperature. An ideal indicator of fouling of condenser tubes, which can significantly degrade chiller efficiency.
Condenser Coil -  [Also see Outdoor Coil] The outdoor portion of a heating or cooling system that either releases or collects heat from the outside air, depending on the time of year. The Condenser Coil is connected directly to the home's Air Handler and is also known as the Outdoor Coil.
Condensing Unit -  Part of a refrigerating mechanism which pumps vaporized refrigerant from the evaporator, compresses it, liquefies it in the condenser and returns it to the refrigerant control. The outdoor portion of a split system air conditioner contains the compressor and outdoor coil ignoring the reverse cycle operation, also the outdoor in a heat pump system.
Conditioned Air -  The air that has been heated, cooled, humidified, or dehumidified to maintain an interior space within the "comfort zone"
Conditioned Floor Area -  The floor area of enclosed conditioned spaces on all floors measured from the interior surfaces of exterior partitions for nonresidential buildings and from the exterior surfaces of exterior partitions for residential buildings.
Conditioned Space -  Enclosed space that is either directly conditioned space or indirectly conditioned space.
Conditioned Space, Directly -  An enclosed space that is provided with heating equipment that has a capacity exceeding 10 Btus/(hr-ft2), or with cooling equipment that has a capacity exceeding 10 Btus/(hr-ft2). An exception is if the heating and cooling equipment is designed and thermostatically controlled to maintain a process environment temperature less than 65 degrees Fahrenheit or greater than 85 degrees Fahrenheit for the whole space the equipment serves.
Conditioned Space, Indirectly -  Enclosed space that: (1) has a greater area weighted heat transfer coefficient (u-value) between it and directly conditioned spaces than between it and the outdoors or unconditioned space; (2) has air transferred from directly conditioned space moving through it at a rate exceeding three air changes per hour.
Conductance -  The quantity of heat, in Btu's, that will flow through one square foot of material in one hour, when there is a 1 degree F temperature difference between both surfaces. Conductance can be expressed in other units as well. Conductance values are given for a specific thickness of material, not per inch thickness.
Conduction -  The transfer of heat through a solid material. The transfer of heat energy through a material (solid, liquid or gas) by the motion of adjacent atoms and molecules without gross displacement of the particles.
Configuration -  This describes the direction in which a furnace outputs heat. A furnace may have an upflow, downflow or crossflow (horizontal) configuration.
Constant Air Volume Systems -  Air handling system that provides a constant air flow while varying the temperature to meet heating and cooling needs.
Contactor -  In the condenser, the main switch that turns the condenser on.
Contaminant -  An unwanted constituent that may or may not be associated with adverse health or comfort effects.
Convection -  The movement of heat by air flow.
Cooling Capacity -  The maximum rate at which cooling equipment removes heat from airflow at operating conditions.
Cooling Capacity, Latent -  Available refrigerating capacity of an air conditioning unit for removing latent heat from the space to be conditioned.
Cooling Capacity, Sensible -  Available refrigerating capacity of an air conditioning unit for removing sensible heat from the space to be conditioned.
Cooling Capacity, Total -  Available refrigerating capacity of an air conditioner for removing sensible heat and latent heat from the space to be conditioned.
Cooling Degree Day -  A unit of measure that indicates how heavy the air conditioning needs are under certain weather conditions.
Cooling Load -  The rate at which heat must be extracted from a space in order to maintain the desired temperature within the space.
Cooling Load Temperature Difference -  (CLTD) A value used in cooling load calculations for the effective temperature difference (delta T) across a wall or ceiling, which accounts for the effect of radiant heat as well as the temperature difference.
Cooling System -  A system of air-to-air, liquid-to-air, liquid-to-liquid, etc., heat exchangers, ducts and/or pipes, etc., for removing head from a system containing heat sources, such as power plants, automobile engines, and homes. Also, an energy Efficiency program promotion aimed at improving the efficiency of the cooling delivery system, including replacement, in the residential, commercial, or industrial sectors.
Cooling tower -  A heat transfer device, which cools warm water using outside air.
COP -  (Coefficient Of Performance) COP compares the heating capacity of a heat pump to the amount of electricity required to operate the heat pump in the heating mode. COPs vary with the outside temperature: as the temperature falls, the COP falls also, since the heat pump is less efficient at lower temperatures. ARI standards compare equipment at two temperatures, 47 F and 17 F, to give you an idea of the COP in both mild and colder temperatures. Geothermal equipment is compared at 32 F enter water temperature. COP & HSPF can not be compared equally. Air Source Equipment is rated by HSPF or COP and Geothermal equipment is rated by COP.
Current -  (Electric) A flow of electrons in an electrical conductor. The strength or rate of movement of the electricity is measured, e.g., in amperes.