NATE Air Distribution Installation flashcards

35 cards. Read the term and the definition here. The deck at the top of this page is optional.

Lockout/tagout verification

Before servicing an air handler or VFD, isolate all energy sources, apply locks and tags, and confirm zero voltage at the disconnect with a tested meter. Stored energy in capacitors and springs must be released.

Fall protection threshold

General industry requires fall protection at 6 feet (1.8 m); construction and shipyard work at 4 feet (1.2 m). Duct installation above walking surfaces triggers the requirement for guardrails, harnesses, or nets.

Ladder setup angle

Use a 4:1 ratio: one foot of horizontal distance for every four feet of working height. Extend the ladder three feet above the landing and secure the top before climbing.

Confined space atmospheric testing

Test for oxygen (19.5–23.5%), flammable gas (below 10% LEL), carbon monoxide, and hydrogen sulfide before entry. A standby attendant remains outside and maintains communication with entrants.

Sensible heat equation

Qs = 1.08 × CFM × ΔT in English units at sea level. For example, 1,000 CFM with a 20°F rise carries 21,600 Btu/h of sensible heat.

Latent heat equation

QL = 0.68 × CFM × ΔW, where ΔW is the moisture difference in grains per pound. Total heat uses 4.5 × CFM × Δh with enthalpy in Btu/lb.

Equal friction sizing method

Size each duct section for a constant pressure drop per 100 feet, typically 0.08–0.10 in. w.c. This method is simple and self-balancing for many systems.

Static regain method

Size duct so that velocity pressure recovered at each branch offsets friction loss, keeping static pressure nearly constant. It requires larger downstream duct but lowers fan energy.

Duct velocity limits

Main trunks near occupied space: 1,200–1,500 fpm. Branch runs: 600–900 fpm. Diffuser necks: 300–500 fpm. Higher velocities raise noise and friction loss.

Duct leakage class

SMACNA classes range from 3% to 30% of design airflow depending on pressure class and seal class. Lower leakage class numbers indicate tighter, better-sealed duct systems.

Galvanized coating designations

G90 means 0.90 oz of zinc per square foot of steel. G90 is standard HVAC; G60 suits dry indoor use; G210 is used in coastal or industrial corrosive environments.

SMACNA pressure classes

Pressure classes include 0.5, 1, 2, 3, 4, 6, and 10 in. w.c. Gauge, reinforcement, and hanger spacing are selected from tables based on the class and duct size.

Flexible duct installation limits

Keep flex runs as short as possible, fully extended, with sag no more than 0.5 in./ft. Support every 4–5 ft with saddle hangers; compressed or sagging flex sharply increases pressure loss.

UL 181 closure systems

UL 181A covers rigid duct closures; UL 181B covers flexible duct and duct board. Tape and mastic must be listed for the specific duct material and application.

Duct board shiplap closure

Fiberglass duct board joints use a 2-inch shiplap with pressure-sensitive tape and mastic. The closure system must be UL 181 listed for the board and application.

Hanger spacing rules

Rectangular duct: 8 ft maximum spacing. Round duct: 10 ft maximum. Place a hanger within 2 ft of each joint or fitting to prevent sag and joint separation.

Radiused elbow ratio

Use elbows with centerline radius at least 1.5 times the duct diameter (R/D ≥ 1.5) to minimize pressure loss. Square throated elbows require turning vanes.

Branch takeoff angle

A 45° branch entry has lower pressure loss than a 90° straight tap. Install a volume damper in each branch for balancing and lock it after adjustment.

Fire damper access

Fire dampers installed in rated walls or floors require an accessible inspection door. They must be inspected and tested after installation and periodically per NFPA 80.

Smoke damper requirements

Smoke dampers are installed where ducts cross smoke barriers and must be accessible for testing. NFPA 105 governs their installation and periodic testing.

Diffuser throw

Throw is the distance from the diffuser to where air velocity drops to 50 fpm. Space diffusers at roughly 1.0–1.5 throw for uniform mixing without drafts.

NC noise criteria targets

NC 25–30 suits residential spaces, NC 35–40 offices, and NC 45 industrial areas. Higher neck velocity and turbulent fittings raise the NC level.

Proportional balancing method

Set total fan airflow first, then adjust branch dampers starting from the farthest terminal. This method divides airflow proportionally and reduces repeated re-adjustment.

Flow hood accuracy

A calibrated flow hood reads terminal airflow within roughly ±3–5%. Calibration should be verified annually and the hood zeroed before each balancing session.

Opposed vs parallel dampers

Opposed-blade dampers modulate airflow linearly and are used for control. Parallel-blade dampers are used for two-position or shutoff service because they concentrate flow.

Seal class A

Seal class A covers all transverse joints, longitudinal seams, and duct penetrations. Class B covers joints and seams; class C covers transverse joints only.

Vapor barrier placement

Place the vapor barrier on the warm side of the insulation. In cooling climates that is the exterior; a torn or compressed barrier causes condensation and mold.

Duct insulation R-values

R-6 for ducts in conditioned space, R-8 for unconditioned attics, and R-13 for exterior runs are common targets. Insulation must stay continuous through hangers.

Pitot tube traverse

Take velocity pressure readings at multiple points across two perpendicular diameters. Average the readings, then V = 4005 × √VP to obtain velocity in feet per minute.

Fan affinity laws

Airflow varies linearly with fan speed, static pressure with the square of speed, and horsepower with the cube. A 10% speed increase raises power about 33%.

TAB sequence

Verify fan rotation and airflow direction first, set total airflow at the fan, balance branches from the farthest, then adjust terminal devices. Record all final readings.

SMACNA duct construction standard

SMACNA HVAC Duct Construction Standards defines pressure classes, gauge tables, reinforcement, hanger spacing, and seal classes used to build and install duct systems.

ASHRAE 62.1 ventilation

ASHRAE 62.1 sets minimum outdoor air rates in cfm per person plus cfm per square foot. It also defines the ventilation rate procedure and indoor air quality procedure.

NFPA 90A scope

NFPA 90A governs air distribution in commercial buildings, covering duct materials, fire and smoke dampers, and clearance from combustible construction.

IECC duct leakage limit

The IECC limits duct leakage to about 4 cfm per 100 square feet of duct surface in many climate zones, with a higher allowance for tested systems.