ASHRAE Terminology

A Comprehensive Glossary of Terms for the Built Environment
ashrae.org/ashraeterms

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air filter bypass

unfiltered air that passes through the air-handling unit (AHU) filter installation but remains unfiltered because it bypasses the installed air filters.

air-handling unit (AHU)

Assembly consisting of sections containing a fan or fans and other necessary equipment to perform one or more of the following functions: air circulation, filtration, heating, cooling, heat recovery, humidifying, dehumidifying and mixing of air, and necessary control functions.

blowthrough unit

air-handling unit with a section or sections downstream of the supply air fan.

combined section of an air-handling unit

section within which two or more functions are combined.

filter mixing box

in air-handling units, the section containing the filters and the outside air/return air dampers and mixing plenum.

flow equalizer

(1) a perforated plate installed downstream of a fan in an air-handling unit designed to diffuse airflow equally across a downstream coil. Also called a diffuser plate. (2) in a multiple cooling tower installation, a pipe installed below and interconnecting the cold basin of multiple cooling towers to equalize the basin water levels and prevent the sump suction in one tower from drawing air or creating a vortex. (3) in refrigeration, a line or lines installed between compressors to equalize oil levels. (4) the bypass pipe in a primary/secondary flow system that hydronically decouples the primary from the secondary pumping system. See cross connection.

makeup air unit

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.

primary air temperature

air temperature of the primary airflow (e.g., supply airflow from an air-handling unit).

relief air

(1) all return air that is discharged directly to the outside or exhausted by separate exhaust fans. (2) building return air discharged by the air-handling unit (AHU) equipment to control building pressure when an HVAC system is operating in the economizer cycle.

static pressure loss

(1) external static pressure loss for forced-air systems using packaged air-handling equipment, the static pressure loss resulting from airflow through the supply and return ductwork and other elements external to the packaged air-handling unit. Compare to internal static pressure loss and total static pressure loss. (2) internal static pressure loss for forced-air systems, the static pressure loss resulting from airflow through the internal elements of the packaged air-handling unit. These internal elements may include such items as filters, coils, dampers, mixing sections, etc. Compare to internal static pressure loss and total static pressure loss. (3) total static pressure loss for forced-air systems, the sum of the external static pressure loss and the internal static pressure loss. See internal static pressure loss and external static pressure loss. The term is not applicable to an individual fan or a fan located within a builtup air-handling unit. Fans are rated by the term fan total static pressure, which is not the same as fan total pressure. Compare to fan total pressure.