ASHRAE Terminology

ventilation airflow divided by room volume. It indicates how many times, during one hour, the air volume from a space is replaced with outdoor air.

duct and dampers arrangement with an automatic control system that together allow a cooling system to supply outdoor air to reduce or eliminate the need for mechanical cooling during mild or cold weather.

exchange of outdoor air with the air already in a building can be divided into two broad classifications: ventilation and infiltration.

the uncontrolled inward airflow through openings in the building envelope caused by the pressure effects of wind, the effect of differences in indoor and outdoor air density, or both (cfm) [m3/s].

see outdoor air.

Air surrounding a building or object, the source of outdoor air brought into a building, etc. (Usually outdoor air or the air in an enclosure under study.)

introduction of cool, outdoor air into an attic by exhausting its warm air to the outdoors.

a class interval or grouping, typically for outdoor air temperature.

that portion of the year that the outdoor air temperature is above 18.3°C (65°F).

a ventilation system that delivers 100% outdoor air to each individual space in a building.

1. A dedicated outdoor air system (DOAS) uses separate equipment to condition all of the outdoor air brought into a building for ventilation and delivers it to each occupied space, either directly or in conjunction with local or central HVAC units serving those same spaces. The local or central HVAC units are used to maintain space temperature. 2. Primary Air System

Outdoor air unit that provides ventilation directly to a space or sensible heating/cooling units. If equipped with an exhaust fan it can control building pressure. Can have an air-to-air recovery device to save energy . Other options include custom air cleaning, enhanced dehumidification devices, compressors, condensors. 2. Make up air unit.

system that provides outdoor air directly to a unit for combustion and for draft relief and which provides for discharge of all flue gases to the outside atmosphere.

the constant outdoor air change rate that would result in the same average pollutant concentration over the same period of time as actually occurs under varying conditions.

the temperature that the wet-bulb portion of a psychrometer would measure if exposed to air entering the evaporator coil. For a draw-through fan with no heat gains or losses in the ductwork and no outdoor air mixed with return air, this would also be the zone air wet-bulb temperature. For a similar configuration, but when outdoor air is mixed with return air, EWB equals the mixed-air wet-bulb temperature. For mixtures of water vapor and dry air at atmospheric temperatures and pressures, the wet-bulb temperature is approximately equal to the adiabatic saturation temperature (temperature of the air after undergoing a theoretical adiabatic saturation process). The wet-bulb temperature given in psychrometric charts is really the adiabatic saturation temperature.

a system consisting of one or more fans that remove air from the building, causing outdoor air to enter by ventilation inlets or normal leakage paths through the building envelope.

volume of outdoor air introduced into a space.

(1) (also known as infiltration losses) energy required to warm outdoor air leaking in through cracks and crevices around doors and windows, through open doors and windows, and through porous building materials. (2) (also known as transmission losses) heat transferred through confining walls, glass, ceilings, floors, or other surfaces. (3) See also heat gain (heat uptake).

thermodynamic heating/refrigerating system to transfer heat. The condenser and evaporator may change roles to transfer heat in either direction. By receiving the flow of air or other fluid, a heat pump is used to cool or heat. Heat pumps may be the air source with heat transfer between the indoor air stream to outdoor air or water source with heat transfer between the indoor air stream and a hydronic source (ground loop, evaporative cooler, cooling tower, or domestic water).

a seasonal efficiency number that is a combined value based on the formula listed in AHRI Standard 920 of the two COP values for the heating season of a direct expansion dedicated outdoor air system (DX-DOAS) unit water or air source heat pump, expressed in W/W.

(1) dedicated replacement air. (2) air brought into a building from the outdoors to replace air that is exhausted. Makeup air may or may not be conditioned. (3) any combination of outdoor and transfer air intended to replace exhaust air and exfiltration. (4) in a cleanroom, air introduced to the secondary air system for ventilation, pressurization, and replacement of exhaust air. (5) in a laboratory or kitchen, outdoor air deliberately brought into the building from the outside and supplied to the vicinity of an exhaust hood to replace air, vapor, and contaminants being exhausted. Makeup air is generally filtered and fan forced, and it may be heated or cooled depending on the requirements of the application. Makeup air may be delivered through outlets integral to the exhaust hood or through outlets in the same room.

outdoor air deliberately brought into the building from the outside and supplied to the vicinity of an exhaust hood to replace air, vapor, and contaminants being exhausted. Makeup air is generally filtered and fan-forced, and it may be heated or cooled depending on the requirements of the application. Makeup air may be delivered through outlets integral to the exhaust hood or through outlets in the same room.

air-handling unit that provides 100% outdoor air to offset air that is exhausted and exfiltrated, often providing conditioning or treatment of the outdoor air.

a modulating damper or set of dampers used to control the outdoor airflow to the system in excess of minimum ventilation outdoor air for free cooling (airside economizer). Also called economizer outdoor air damper. May also serve to provide the minimum outside airflow control.

temperature is based on the arithmetic average of the mean daily minimum and mean daily maximum outdoor (dry-bulb) temperatures for the month in question.

a damper in parallel with the maximum outdoor air damper to provide the minimum outdoor air required for ventilation.

(1) air that contains two or more streams of air. (2) combined outdoor air and recirculated air.

(1) in a building, pressure lower than pressure outdoors. Note: as the negative pressure increases, outdoor air is drawn in through any openings in the building envelope. (2) in building spaces, pressure lower than pressure in adjacent spaces or rooms.

the indoor air volume of a space or building divided by the rate of outdoor air supply, the nominal time constant also equals the average age of air exiting from the space or building. The reciprocal of the nominal time constant is called the nominal air change rate.

(1) air outside a building or taken from the external atmosphere and, therefore, not previously circulated through the system. (2) ambient air that enters a building through a mechanical ventilation system, through intentional openings for natural ventilation, or by infiltration. (3) compare to outside air.

the ratio of the volumetric rate at which outdoor air enters a building space to the building volume with identical volume units (normally expressed in units of air changes per hour).

the outdoor air fraction is the ratio of the volumetric flow rate of outdoor air brought in by the air handler to the total supply airflow rate.

volumetric outside airflow rate expressed as a percentage of total supply airflow rate.

a type of smoke control system in which stair shafts are mechanically pressurized with outdoor air to keep smoke from contaminating them during a fire incident.

outdoor air that is used to replace air removed from a building through an exhaust system. Replacement air may be derived from one or more of the following: makeup air, supply air, transfer air, and infiltration. However, the ultimate source of all replacement air is outdoor air. When replacement air exceeds exhaust, the result is exfiltration.

all components of the roof/ceiling envelope through which heat flows, thus creating a building transmission heat loss or gain, where such assembly is exposed to outdoor air and encloses a heated and/or mechanically cooled space.

device intended to allow the passage of outdoor air or exhaust air while minimizing the ingress of airborne sand. Also see louver.

a type of smoke control system that is intended to positively pressurize stair and/or elevator shafts with outdoor air by using fans to keep smoke from contaminating the shafts during an alarm condition.

the term may apply to a combination of ailments (a syndrome) associated with a place of work, residence, or assembly. Sick building syndrome can be related to a lack of adequate outdoor air ventilation, improper exhaust, ventilation of odors, chemicals or fumes, or poor indoor-air quality. Other sources of sick buildings may be linked to contaminants produced by outgassing of some types of building materials, VOC, bacteria molds, etc. This syndrome does not conform to a particular illness and is difficult to trace to a specific source. See indoor air quality, volatile organic compounds (VOC).

(1) movement of air into and out of buildings, chimneys, flue gas stacks, or other containers and is driven by buoyancy. Buoyancy occurs due to a difference in indoor to outdoor air density resulting from temperature and moisture differences. The result is either a positive or negative buoyancy force. The greater the thermal difference and the height of the structure, the greater the buoyancy force, and thus the stack effect. The stack effect is also referred to as the chimney effect, and it helps drive natural ventilation and infiltration. (2) movement of air or other gas in a vertical enclosure (e.g., duct, chimney, building), induced by the density difference between the air or other gas in the enclosure and the ambient atmosphere. Note: stack effect is a significant concern in heating-system design for tall buildings in cold climates. Sometimes referred to as chimney effect. (3) pressure difference caused by the difference in density between indoor and outdoor air due to an indoor/outdoor temperature difference. (4) the vertical airflow within buildings caused by temperature differences between the building's interior and exterior.

(1) air delivered by mechanical or natural ventilation to a space, composed of any combination of outdoor air, recirculated air, or transfer air. (2) air entering a space from an air-conditioning, heating, or ventilating apparatus for the purpose of comfort conditioning. Supply air is generally filtered, fan forced, and either heated, cooled, humidified, or dehumidified as necessary to maintain specified conditions. Only the quantity of outdoor air within the supply airflow may be used as replacement air.

(1) discharge piping connected to a safety or relief valve. (2) passages and conduits in a direct vent or direct exhaust system through which gases pass from the combustion chamber to the outdoor air.

the minimum amount of outdoor air required for the purpose of controlling air contaminant levels in buildings.