NATE Air Distribution Service cheat sheet

Supply air path: equipment → trunk duct → branch ducts → runouts → boots → diffusers; return path reverses through grilles, return ducts, and the air handler.

A diffuser spreads supply air at the ceiling; a register combines a grille with a damper or deflector; a grille is a louvered covering without a damper.

A volume damper (opposed-blade or single-blade) regulates branch airflow; a fire damper closes on heat to protect rated assemblies; a smoke damper responds to detectors.

A turning vane is installed inside square elbows to reduce turbulence and pressure loss; missing vanes raise static pressure and cut airflow.

Flexible duct is rated by diameter and maximum positive/negative pressure; sagging or over-compressed flex reduces effective diameter and capacity.

A plenum is a box or cavity distributing air to multiple outlets; an extended plenum is a trunk section serving several branch takeoffs.

Total external static pressure (TESP) equals the sum of supply and return static pressures measured at the equipment, typically read with a manometer.

A barometric relief damper or bypass damper controls pressure in zoned systems; a zone damper modulates airflow to a conditioned space.

Terminal units include VAV boxes with reheat coils, fan-powered boxes, and constant-volume boxes; each has a primary airflow rating.

Equal-friction method: size each section for a constant friction rate, typically 0.08–0.10 in. w.c. per 100 ft for residential supply trunks.

Static pressure equation: available static = TESP − pressure losses of filters, coils, and accessories; remaining static drives duct sizing.

Friction rate (in. w.c./100 ft) = available static pressure × 100 ÷ total effective length of the longest run.

Velocity method: supply trunk velocities commonly 700–900 fpm, branch runs 500–700 fpm, and return trunks 600–900 fpm.

Duct cross-sectional area (sq ft) = airflow (cfm) ÷ velocity (fpm); equivalent round diameter follows from area = πr².

Equivalent length adds fitting losses (elbows, tees, takeoffs) to measured length; each fitting has a published equivalent-length value in feet.

Total effective length (TEL) = straight-run length + sum of equivalent lengths of all fittings on the longest path.

Friction loss rises approximately with the square of velocity; doubling velocity roughly quadruples friction loss per unit length.

Rectangular equivalent of a round duct uses the equal-friction or equal-velocity conversion tables; aspect ratios above 4:1 increase friction and noise.

Airflow (cfm) = velocity (fpm) × area (sq ft); a duct traverse uses a pitot tube or anemometer averaged across the cross-section.

Balancing sequence: set the air handler to design airflow, balance the branch farthest from the fan first, then work toward the trunk.

Use a flow hood (balometer) at diffusers for delivered cfm; use a manometer and a duct traverse for trunk airflow.

Temperature-rise method: cfm ≈ (sensible Btu/h) ÷ (1.08 × ΔT °F) across the equipment; compare with design airflow.

A static pressure profile across the system locates restrictions: a large drop across a filter or coil signals blockage.

Low airflow with high static pressure indicates restriction; low airflow with low static pressure indicates fan or duct leakage problems.

A blower door test measures envelope leakage; duct leakage testing uses a calibrated fan and pressure gauge with all registers sealed.

Delta T across the evaporator coil outside 16–22 °F often signals low charge, restricted airflow, or duct losses.

A manometer reading taken at the return and supply plenums separately isolates whether the restriction is on the supply or return side.

Duct sealant classes: UL 181A for rigid fibrous glass, UL 181B for flexible duct and closures; mastic or listed tape must be used.

Seal all transverse joints, longitudinal seams, takeoffs, and boots; the highest leakage typically occurs at the plenum and boot connections.

Flexible duct bends should have a centerline radius of at least one duct diameter; avoid sharp 90° turns and compression.

Support flex duct at intervals no greater than 4 ft and within 2 ft of fittings; use broad saddles, not narrow straps that crush the duct.

Duct insulation R-value commonly ranges from R-4 to R-8; the vapor barrier must face the warm side to prevent condensation.

External insulation on cold ducts requires a continuous vapor retarder; joints are sealed and taped to prevent moisture intrusion.

Duct leakage classes in SMACNA range from 3 to 30 cfm per 100 sq ft of duct area at 1 in. w.c. test pressure.

Fire-rated assemblies require listed fire dampers at rated wall and floor penetrations; duct must not reduce the assembly rating.

Clearance to combustibles per listing (commonly 1 in. for listed appliances and connectors) prevents pyrolysis and fire spread.

MERV ratings run 1–16; MERV 8 captures ~20% of 3–10 µm particles, MERV 11 ~65%, MERV 13 ~85%, and MERV 16 >95% for fine particles.

Higher MERV filters raise static pressure; check that the equipment's rated TESP allows the filter's pressure drop at design airflow.

ASHRAE 62.2 sets residential ventilation rates: whole-house ventilation ≈ 0.03 cfm/sq ft plus 7.5 cfm per occupant.

ASHRAE 62.1 governs commercial ventilation; required outdoor air = breathing zone rate × zone population plus area rate.

Pressure drop across a dirty filter rises; a filter change indicator or scheduled static pressure check prevents airflow loss.

Carbon dioxide above ~1,000 ppm indicates inadequate ventilation relative to occupancy and suggests outdoor air adjustment.

Relative humidity above 60% supports mold and dust mites; below 30% causes dryness and static; target 30–50% indoors.

A heat recovery ventilator (HRV) transfers sensible heat; an energy recovery ventilator (ERV) transfers both sensible and latent energy.

Combustion appliances require adequate makeup air; backdrafting is tested with a draft gauge or smoke pencil at the flue.