NATE Air Distribution Service key terms

A box or chamber at the air handler that receives supply or return air and distributes it to several ducts. It equalizes pressure before air enters branch runs.

A fitting that starts a branch run from a trunk or plenum. A takeoff may be straight, angled, or boot-style, and its shape strongly affects pressure loss.

A duct layout in which a large main trunk carries most of the air and smaller branches feed individual outlets. Graduated trunks step down in size as branches leave.

A supply layout where the main trunk keeps one size for most of its length before stepping down near the end. It is common in residential installations with a long central run.

The route air follows from conditioned rooms back to the air handler, including return grilles, return ducts, and the filter. Restricted return paths raise negative pressure and starve airflow.

A louvered or bar-type opening that admits room air into the return system. Its free area must be large enough to keep face velocity low and noise acceptable.

An outlet with vanes or cones that spreads supply air across a room and promotes mixing. Selection depends on throw, drop, noise criteria, and the required air pattern.

The horizontal or vertical distance air travels from an outlet before its velocity falls to a stated value, commonly 50 or 100 feet per minute. Throw must suit the room geometry.

The average speed of air measured perpendicular to the face area of a grille, filter, or coil. It is calculated as airflow divided by the free or gross face area.

A factory-made duct with a wire helix and plastic or foil jacket, often insulated. It is easy to route but loses more pressure than rigid duct, especially when sagging or sharply bent.

The ACCA residential duct design procedure that sizes ducts from room airflow, available static pressure, and fitting losses. It uses equivalent length and manufacturer blower data.

The ACCA load calculation method that determines heating and cooling loads for a dwelling. Its room-by-room results set the airflow each duct must deliver under Manual D.

The outward push of air against duct walls, measured in inches of water column. Supply static plus return static, taken near the unit, indicates total external resistance.

The total resistance of the supply and return duct systems measured at the air handler, excluding internal components. It is compared with the blower's rated capacity to confirm airflow.

The portion of blower static pressure left for the duct system after subtracting pressure drops of filters, coils, grilles, and other components. Duct friction budget comes from this value.

The pressure lost per unit length of duct, usually expressed in inches of water column per 100 feet. It is selected so total duct loss fits within available static pressure.

The straight-duct length that would produce the same pressure loss as a fitting. Adding equivalent lengths of fittings to measured run length gives total effective length.

The width-to-height ratio of a rectangular duct. Lower ratios reduce friction and noise, so a square duct is more efficient than a flat one carrying the same airflow.

The speed of air moving through a duct, usually in feet per minute. High velocity raises friction and noise, while very low velocity can cause poor distribution and dirt settling.

The volume of air a room needs, usually in cubic feet per minute, derived from its heating or cooling load. It is the starting point for sizing each branch run.

The pressure created by air in motion, equal to total pressure minus static pressure. It is the value a pitot tube measures to calculate velocity in a duct.

An L-shaped probe with a forward-facing total pressure port and side static ports. Used with a manometer, it yields velocity pressure and thus duct velocity at a traverse point.

A measurement method that averages velocity readings at several points across a duct cross section. It reduces error from uneven velocity profiles near fittings and transitions.

A calibrated fabric or metal hood that captures all air leaving a grille and reads its volume directly. It is the standard tool for balancing supply and return airflow at outlets.

An adjustable blade inside a duct or at a takeoff that varies resistance to change branch airflow. Dampers are adjusted during balancing and then locked in position.

The process of adjusting dampers and blower settings so each room receives its design airflow within tolerance. It follows installation and is verified with measured readings.

A diagnostic measurement of supply and return static pressures at the air handler to find excessive restriction. High total static with low airflow points to undersized ducts or a loaded filter.

The difference between return and supply air temperatures across a furnace or heat pump. Comparing it with the rating plate value helps confirm correct airflow.

Air that escapes or enters through joints, seams, and penetrations instead of reaching conditioned rooms. Leakage wastes capacity and can draw pollutants into the system.

A thick brushable or troweled sealant applied over duct joints and seams. It remains flexible and bonds well to galvanized metal, making it preferred over tape alone.

A calibrated fan and pressure gauge assembly used to pressurize a duct system and measure leakage airflow. Results are reported at a reference pressure, commonly 25 pascals.

A measure of a material's resistance to heat flow. Duct insulation is rated by R-value, and higher values are needed where ducts run through unconditioned space.

A low-permeability layer, usually the facing or jacket on duct insulation, that limits moisture movement. It must stay on the warm side to prevent condensation on cold ducts.

Material wrapped around or lining ducts to limit heat gain or loss and control surface condensation. It also reduces noise and must be sealed and supported properly.

Minimum Efficiency Reporting Value, a rating of a filter's ability to capture particles of given sizes. Higher MERV captures more, but may add static pressure if the filter is restrictive.

A return grille with a built-in frame that holds the air filter at the room side. It simplifies filter changes but adds resistance that must be included in static pressure calculations.

A duct and hood that brings fresh air into the return or supply system for ventilation. It needs a damper, screen, and rain protection, and must be sized to the ventilation requirement.

The volume of outdoor air supplied to a space per unit time, set by code or standard for occupancy. It is separate from the airflow needed for heating and cooling.

A single-number rating of acceptable background sound in a space, used to select outlets and size ducts. Lower NC values mean quieter operation and usually larger ducts or slower velocities.