NATE Air Distribution Installation cheat sheet

Lockout/tagout applies to air handlers, VFDs, and any stored-energy source; verify zero energy at the disconnect before opening a control panel.

Fall protection is required at 6 ft (1.8 m) in general industry and 4 ft (1.2 m) in shipyard/construction contexts; duct hangers above walking surfaces count as overhead work.

Sheet metal cuts are the leading hand injury on duct jobs; cut-resistant gloves are required when handling galvanized edges, and deburr before assembly.

Ladders: 4:1 setup angle, 3 ft (0.9 m) extension above the landing, and never stand on the top two rungs; duct runs at ceiling height often require a rolling scaffold instead.

Confined-space entry (plenums, crawl spaces, large ducts) requires atmospheric testing for O2, LEL, CO, and H2S before entry and a standby attendant.

Silicone sealants, duct mastics, and spray adhesives require ventilation and SDS review; solvent-based products are flammable during cure.

Rigging: know the working load limit of slings and shackles; never exceed the hoist rating, and tag lines control duct sections in the air.

Aerial lift work requires a fall-arrest harness in a boom lift and a trained operator card; keep the platform inside the manufacturer's load chart.

Sensible heat equation: Qs = 1.08 × CFM × ΔT (English units at sea level); latent: QL = 0.68 × CFM × ΔW (grains). Use 4.5 × CFM × Δh for total heat.

Room load = conduction through envelope + solar + internal (people 250–450 Btu/h sensible, lights 3.41 Btu/h per watt) + infiltration/ventilation.

Equal friction method sizes duct for a constant pressure drop per 100 ft (typically 0.08–0.10 in. w.c.); static regain method uses velocity pressure recovery at each branch.

Velocity limits: 1,200–1,500 fpm in main trunks near occupied space for noise, 600–900 fpm in branch runs, 300–500 fpm at diffuser necks.

Friction chart: pressure drop rises with the square of velocity; doubling velocity roughly quadruples friction loss per unit length.

Equivalent length adds fitting losses to straight-run length; a 90° radius elbow can equal 30–60 ft of straight duct.

Return air path matters: a starved return raises space pressure and reduces delivered CFM; transfer grilles or door undercuts are required.

Duct leakage classes (per SMACNA) range from 3% to 30% of design flow depending on class and pressure; seal class A covers all joints and seams.

Galvanized steel G90 (0.90 oz/ft² zinc coating) is standard for HVAC; G60 is acceptable for indoor dry locations, G210 for coastal/industrial.

Gauge selection follows SMACNA pressure class: 1 in. w.c. and lower uses 26–28 ga; 2 in. w.c. uses 24 ga; 4 in. w.c. uses 22 ga for larger sizes.

Standard duct sizes step in 2 in. increments up to 84 in.; joints include TDC, TDF, slip-and-drive, and Pittsburgh lock seams.

Flexible duct: R-4.2 minimum, R-8 for high efficiency; keep installed length ≤ 5 ft per run and sag ≤ 0.5 in./ft to control pressure loss.

Fiberglass duct board is fabricated with a 2-in. shiplap closure and pressure-sensitive tape plus mastic; UL 181 listed closure systems are mandatory.

Spiral and oval duct offer lower friction; rectangular duct has higher friction but fits tight spaces.

Fire-rated duct (1 or 2 hr) requires listed assemblies such as 3M or Unifrax systems with specific wrap thickness and joint treatment.

Duct liner (acoustic) reduces radiated noise but adds friction; use only where the velocity is below ~4,000 fpm to prevent erosion.

Hanger spacing: rectangular duct 8 ft max, round duct 10 ft max, and within 2 ft of each joint or fitting.

Support rectangular duct per SMACNA: 20 ga and lighter uses 1-in. × 18-ga straps; heavier gauges may use 1-in. × 16-ga.

Elbows: use radiused (R/D ≥ 1.5) elbows where space allows; square throated elbows need turning vanes to avoid turbulence.

Branch takeoffs: 45° entries reduce pressure loss versus 90° straight taps; use volume dampers at each branch for balancing.

Flex duct must be fully extended, supported every 4–5 ft with a saddle hanger (not a strap that compresses), and terminate with a listed collar.

Duct penetrations through fire-rated walls/ceilings require listed fire dampers with access doors for inspection and reset.

Smoke dampers are required where ducts penetrate smoke barriers; they must be accessible and tested per NFPA 105.

Coordinate with other trades: keep 1–2 in. clearance around duct for insulation, and never compress insulation at hangers.

Throw (T) is the distance from the diffuser to where terminal velocity falls to 50 fpm; spacing diffusers at 1.0–1.5 throw gives good mixing.

Neck velocity: 300–500 fpm for ceiling diffusers in residential, 500–800 fpm for commercial; higher velocity increases noise (NC).

NC (Noise Criteria) target: NC 25–30 in residential, NC 35–40 in offices, NC 45 in industrial.

Balancing: proportional balance method adjusts terminal devices in sequence from the farthest branch; total airflow is set at the fan first.

A 10% airflow tolerance is typical for balancing reports; use a calibrated flow hood or pitot tube traverse.

Return grilles should be sized for ≤ 500 fpm face velocity to avoid whistle; transfer grilles need free area ≥ 1.5× the door gap.

Static pressure profile: measure at fan discharge, after each major fitting, and at the last branch to find restrictions.

Dampers: opposed-blade for modulating, parallel-blade for two-position; volume dampers should be locked after balancing.

Mastic (UL 181A-M) is for rigid duct joints; UL 181B-M tape is for flex and duct board closures. Never use cloth duct tape as a permanent seal.

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

Fiberglass insulation R-values: R-6 for ducts in conditioned space, R-8 for unconditioned attics, R-13 for exterior runs.

Vapor barrier goes on the warm side (exterior of insulation in cooling climates); a compromised barrier causes condensation and mold.

Insulation must be continuous through hangers; use insulation shields or oversized hangers to prevent compression.

Closed-cell elastomeric insulation is used for refrigerant lines; the vapor barrier is integral and requires sealed seams.

Duct leakage testing per SMACNA or ASHRAE: pressurize the duct and measure leakage in CFM per 100 ft² of duct surface.

Mastic must cure before airflow starts; solvent-based mastic off-gasses VOCs and needs 24 hours of ventilation.

Pitot tube traverse: take readings at 10 points across two perpendicular diameters; average velocity pressure, then V = 4005 × √VP (fpm).

Flow hood calibration should be checked annually; field accuracy is ±3–5% for hoods and ±5–10% for pitot traverses.

Static pressure readings use a manometer or digital gauge; measure across filters, coils, and duct sections to build a pressure profile.

Fan laws: CFM varies linearly with speed, static pressure with speed², and horsepower with speed³.

TAB sequence: verify fan rotation and airflow direction, then set total airflow, then balance branches, then adjust terminal devices.

Commissioning includes functional performance testing: verify economizer operation, damper stroke, and control sequences.

Record final readings on a TAB report with design vs. actual CFM, static pressures, and ambient conditions.

A 10% deviation from design airflow typically triggers a re-check of the system before sign-off.

SMACNA HVAC Duct Construction Standards defines pressure classes, gauge tables, and hanger spacing.

ASHRAE 62.1 sets ventilation rates (cfm/person and cfm/ft²); ASHRAE 90.1 sets duct insulation and leakage requirements.

IECC/IRC Section M1601 covers duct installation, sealing, and insulation for residential; M1502 covers exhaust duct termination.

NFPA 90A governs air distribution in commercial buildings: fire dampers, smoke dampers, and duct clearance from combustibles.

UL 181 lists duct materials and closure systems; UL 181A for rigid, UL 181B for flexible.

Documentation: submittals include duct construction schedule, insulation R-values, and TAB report; as-builts reflect field changes.

Permits and inspections: mechanical permit required for new ductwork; rough-in inspection before closing walls.

Energy code duct leakage: IECC limits duct leakage to 4 cfm/100 ft² (or 8 cfm/100 ft² for tested systems) in some climate zones.

Lockout/tagout applies to air handlers, VFDs, and any stored-energy source; verify zero energy at the disconnect before opening a control panel.

Fall protection is required at 6 ft (1.8 m) in general industry and 4 ft (1.2 m) in shipyard/construction contexts; duct hangers above walking surfaces count as overhead work.

Sheet metal cuts are the leading hand injury on duct jobs; cut-resistant gloves are required when handling galvanized edges, and deburr before assembly.

Ladders: 4:1 setup angle, 3 ft (0.9 m) extension above the landing, and never stand on the top two rungs; duct runs at ceiling height often require a rolling scaffold instead.

Confined-space entry (plenums, crawl spaces, large ducts) requires atmospheric testing for O2, LEL, CO, and H2S before entry and a standby attendant.

Silicone sealants, duct mastics, and spray adhesives require ventilation and SDS review; solvent-based products are flammable during cure.

Rigging: know the working load limit of slings and shackles; never exceed the hoist rating, and tag lines control duct sections in the air.

Aerial lift work requires a fall-arrest harness in a boom lift and a trained operator card; keep the platform inside the manufacturer's load chart.

Sensible heat equation: Qs = 1.08 × CFM × ΔT (English units at sea level); latent: QL = 0.68 × CFM × ΔW (grains). Use 4.5 × CFM × Δh for total heat.

Room load = conduction through envelope + solar + internal (people 250–450 Btu/h sensible, lights 3.41 Btu/h per watt) + infiltration/ventilation.

Equal friction method sizes duct for a constant pressure drop per 100 ft (typically 0.08–0.10 in. w.c.); static regain method uses velocity pressure recovery at each branch.

Velocity limits: 1,200–1,500 fpm in main trunks near occupied space for noise, 600–900 fpm in branch runs, 300–500 fpm at diffuser necks.

Friction chart: pressure drop rises with the square of velocity; doubling velocity roughly quadruples friction loss per unit length.

Equivalent length adds fitting losses to straight-run length; a 90° radius elbow can equal 30–60 ft of straight duct.

Return air path matters: a starved return raises space pressure and reduces delivered CFM; transfer grilles or door undercuts are required.

Duct leakage classes (per SMACNA) range from 3% to 30% of design flow depending on class and pressure; seal class A covers all joints and seams.

Galvanized steel G90 (0.90 oz/ft² zinc coating) is standard for HVAC; G60 is acceptable for indoor dry locations, G210 for coastal/industrial.

Gauge selection follows SMACNA pressure class: 1 in. w.c. and lower uses 26–28 ga; 2 in. w.c. uses 24 ga; 4 in. w.c. uses 22 ga for larger sizes.

Standard duct sizes step in 2 in. increments up to 84 in.; joints include TDC, TDF, slip-and-drive, and Pittsburgh lock seams.

Flexible duct: R-4.2 minimum, R-8 for high efficiency; keep installed length ≤ 5 ft per run and sag ≤ 0.5 in./ft to control pressure loss.

Fiberglass duct board is fabricated with a 2-in. shiplap closure and pressure-sensitive tape plus mastic; UL 181 listed closure systems are mandatory.

Spiral and oval duct offer lower friction; rectangular duct has higher friction but fits tight spaces.

Fire-rated duct (1 or 2 hr) requires listed assemblies such as 3M or Unifrax systems with specific wrap thickness and joint treatment.

Duct liner (acoustic) reduces radiated noise but adds friction; use only where the velocity is below ~4,000 fpm to prevent erosion.

Hanger spacing: rectangular duct 8 ft max, round duct 10 ft max, and within 2 ft of each joint or fitting.

Support rectangular duct per SMACNA: 20 ga and lighter uses 1-in. × 18-ga straps; heavier gauges may use 1-in. × 16-ga.

Elbows: use radiused (R/D ≥ 1.5) elbows where space allows; square throated elbows need turning vanes to avoid turbulence.

Branch takeoffs: 45° entries reduce pressure loss versus 90° straight taps; use volume dampers at each branch for balancing.

Flex duct must be fully extended, supported every 4–5 ft with a saddle hanger (not a strap that compresses), and terminate with a listed collar.

Duct penetrations through fire-rated walls/ceilings require listed fire dampers with access doors for inspection and reset.

Smoke dampers are required where ducts penetrate smoke barriers; they must be accessible and tested per NFPA 105.

Coordinate with other trades: keep 1–2 in. clearance around duct for insulation, and never compress insulation at hangers.

Throw (T) is the distance from the diffuser to where terminal velocity falls to 50 fpm; spacing diffusers at 1.0–1.5 throw gives good mixing.

Neck velocity: 300–500 fpm for ceiling diffusers in residential, 500–800 fpm for commercial; higher velocity increases noise (NC).

NC (Noise Criteria) target: NC 25–30 in residential, NC 35–40 in offices, NC 45 in industrial.

Balancing: proportional balance method adjusts terminal devices in sequence from the farthest branch; total airflow is set at the fan first.

A 10% airflow tolerance is typical for balancing reports; use a calibrated flow hood or pitot tube traverse.

Return grilles should be sized for ≤ 500 fpm face velocity to avoid whistle; transfer grilles need free area ≥ 1.5× the door gap.

Static pressure profile: measure at fan discharge, after each major fitting, and at the last branch to find restrictions.

Dampers: opposed-blade for modulating, parallel-blade for two-position; volume dampers should be locked after balancing.

Mastic (UL 181A-M) is for rigid duct joints; UL 181B-M tape is for flex and duct board closures. Never use cloth duct tape as a permanent seal.

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

Fiberglass insulation R-values: R-6 for ducts in conditioned space, R-8 for unconditioned attics, R-13 for exterior runs.

Vapor barrier goes on the warm side (exterior of insulation in cooling climates); a compromised barrier causes condensation and mold.

Insulation must be continuous through hangers; use insulation shields or oversized hangers to prevent compression.

Closed-cell elastomeric insulation is used for refrigerant lines; the vapor barrier is integral and requires sealed seams.

Duct leakage testing per SMACNA or ASHRAE: pressurize the duct and measure leakage in CFM per 100 ft² of duct surface.

Mastic must cure before airflow starts; solvent-based mastic off-gasses VOCs and needs 24 hours of ventilation.

Pitot tube traverse: take readings at 10 points across two perpendicular diameters; average velocity pressure, then V = 4005 × √VP (fpm).

Flow hood calibration should be checked annually; field accuracy is ±3–5% for hoods and ±5–10% for pitot traverses.

Static pressure readings use a manometer or digital gauge; measure across filters, coils, and duct sections to build a pressure profile.

Fan laws: CFM varies linearly with speed, static pressure with speed², and horsepower with speed³.

TAB sequence: verify fan rotation and airflow direction, then set total airflow, then balance branches, then adjust terminal devices.

Commissioning includes functional performance testing: verify economizer operation, damper stroke, and control sequences.

Record final readings on a TAB report with design vs. actual CFM, static pressures, and ambient conditions.

A 10% deviation from design airflow typically triggers a re-check of the system before sign-off.

SMACNA HVAC Duct Construction Standards defines pressure classes, gauge tables, and hanger spacing.

ASHRAE 62.1 sets ventilation rates (cfm/person and cfm/ft²); ASHRAE 90.1 sets duct insulation and leakage requirements.

IECC/IRC Section M1601 covers duct installation, sealing, and insulation for residential; M1502 covers exhaust duct termination.

NFPA 90A governs air distribution in commercial buildings: fire dampers, smoke dampers, and duct clearance from combustibles.

UL 181 lists duct materials and closure systems; UL 181A for rigid, UL 181B for flexible.

Documentation: submittals include duct construction schedule, insulation R-values, and TAB report; as-builts reflect field changes.

Permits and inspections: mechanical permit required for new ductwork; rough-in inspection before closing walls.

Energy code duct leakage: IECC limits duct leakage to 4 cfm/100 ft² (or 8 cfm/100 ft² for tested systems) in some climate zones.

Lockout/tagout applies to air handlers, VFDs, and any stored-energy source; verify zero energy at the disconnect before opening a control panel.

Fall protection is required at 6 ft (1.8 m) in general industry and 4 ft (1.2 m) in shipyard/construction contexts; duct hangers above walking surfaces count as overhead work.

Sheet metal cuts are the leading hand injury on duct jobs; cut-resistant gloves are required when handling galvanized edges, and deburr before assembly.

Ladders: 4:1 setup angle, 3 ft (0.9 m) extension above the landing, and never stand on the top two rungs; duct runs at ceiling height often require a rolling scaffold instead.

Confined-space entry (plenums, crawl spaces, large ducts) requires atmospheric testing for O2, LEL, CO, and H2S before entry and a standby attendant.

Silicone sealants, duct mastics, and spray adhesives require ventilation and SDS review; solvent-based products are flammable during cure.

Rigging: know the working load limit of slings and shackles; never exceed the hoist rating, and tag lines control duct sections in the air.

Aerial lift work requires a fall-arrest harness in a boom lift and a trained operator card; keep the platform inside the manufacturer's load chart.

Sensible heat equation: Qs = 1.08 × CFM × ΔT (English units at sea level); latent: QL = 0.68 × CFM × ΔW (grains). Use 4.5 × CFM × Δh for total heat.

Room load = conduction through envelope + solar + internal (people 250–450 Btu/h sensible, lights 3.41 Btu/h per watt) + infiltration/ventilation.

Equal friction method sizes duct for a constant pressure drop per 100 ft (typically 0.08–0.10 in. w.c.); static regain method uses velocity pressure recovery at each branch.

Velocity limits: 1,200–1,500 fpm in main trunks near occupied space for noise, 600–900 fpm in branch runs, 300–500 fpm at diffuser necks.

Friction chart: pressure drop rises with the square of velocity; doubling velocity roughly quadruples friction loss per unit length.

Equivalent length adds fitting losses to straight-run length; a 90° radius elbow can equal 30–60 ft of straight duct.

Return air path matters: a starved return raises space pressure and reduces delivered CFM; transfer grilles or door undercuts are required.

Duct leakage classes (per SMACNA) range from 3% to 30% of design flow depending on class and pressure; seal class A covers all joints and seams.

Galvanized steel G90 (0.90 oz/ft² zinc coating) is standard for HVAC; G60 is acceptable for indoor dry locations, G210 for coastal/industrial.

Gauge selection follows SMACNA pressure class: 1 in. w.c. and lower uses 26–28 ga; 2 in. w.c. uses 24 ga; 4 in. w.c. uses 22 ga for larger sizes.

Standard duct sizes step in 2 in. increments up to 84 in.; joints include TDC, TDF, slip-and-drive, and Pittsburgh lock seams.

Flexible duct: R-4.2 minimum, R-8 for high efficiency; keep installed length ≤ 5 ft per run and sag ≤ 0.5 in./ft to control pressure loss.

Fiberglass duct board is fabricated with a 2-in. shiplap closure and pressure-sensitive tape plus mastic; UL 181 listed closure systems are mandatory.

Spiral and oval duct offer lower friction; rectangular duct has higher friction but fits tight spaces.

Fire-rated duct (1 or 2 hr) requires listed assemblies such as 3M or Unifrax systems with specific wrap thickness and joint treatment.

Duct liner (acoustic) reduces radiated noise but adds friction; use only where the velocity is below ~4,000 fpm to prevent erosion.

Hanger spacing: rectangular duct 8 ft max, round duct 10 ft max, and within 2 ft of each joint or fitting.

Support rectangular duct per SMACNA: 20 ga and lighter uses 1-in. × 18-ga straps; heavier gauges may use 1-in. × 16-ga.

Elbows: use radiused (R/D ≥ 1.5) elbows where space allows; square throated elbows need turning vanes to avoid turbulence.

Branch takeoffs: 45° entries reduce pressure loss versus 90° straight taps; use volume dampers at each branch for balancing.

Flex duct must be fully extended, supported every 4–5 ft with a saddle hanger (not a strap that compresses), and terminate with a listed collar.

Duct penetrations through fire-rated walls/ceilings require listed fire dampers with access doors for inspection and reset.

Smoke dampers are required where ducts penetrate smoke barriers; they must be accessible and tested per NFPA 105.

Coordinate with other trades: keep 1–2 in. clearance around duct for insulation, and never compress insulation at hangers.

Throw (T) is the distance from the diffuser to where terminal velocity falls to 50 fpm; spacing diffusers at 1.0–1.5 throw gives good mixing.

Neck velocity: 300–500 fpm for ceiling diffusers in residential, 500–800 fpm for commercial; higher velocity increases noise (NC).

NC (Noise Criteria) target: NC 25–30 in residential, NC 35–40 in offices, NC 45 in industrial.

Balancing: proportional balance method adjusts terminal devices in sequence from the farthest branch; total airflow is set at the fan first.

A 10% airflow tolerance is typical for balancing reports; use a calibrated flow hood or pitot tube traverse.

Return grilles should be sized for ≤ 500 fpm face velocity to avoid whistle; transfer grilles need free area ≥ 1.5× the door gap.

Static pressure profile: measure at fan discharge, after each major fitting, and at the last branch to find restrictions.

Dampers: opposed-blade for modulating, parallel-blade for two-position; volume dampers should be locked after balancing.

Mastic (UL 181A-M) is for rigid duct joints; UL 181B-M tape is for flex and duct board closures. Never use cloth duct tape as a permanent seal.

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

Fiberglass insulation R-values: R-6 for ducts in conditioned space, R-8 for unconditioned attics, R-13 for exterior runs.

Vapor barrier goes on the warm side (exterior of insulation in cooling climates); a compromised barrier causes condensation and mold.

Insulation must be continuous through hangers; use insulation shields or oversized hangers to prevent compression.

Closed-cell elastomeric insulation is used for refrigerant lines; the vapor barrier is integral and requires sealed seams.

Duct leakage testing per SMACNA or ASHRAE: pressurize the duct and measure leakage in CFM per 100 ft² of duct surface.

Mastic must cure before airflow starts; solvent-based mastic off-gasses VOCs and needs 24 hours of ventilation.

Pitot tube traverse: take readings at 10 points across two perpendicular diameters; average velocity pressure, then V = 4005 × √VP (fpm).

Flow hood calibration should be checked annually; field accuracy is ±3–5% for hoods and ±5–10% for pitot traverses.

Static pressure readings use a manometer or digital gauge; measure across filters, coils, and duct sections to build a pressure profile.

Fan laws: CFM varies linearly with speed, static pressure with speed², and horsepower with speed³.

TAB sequence: verify fan rotation and airflow direction, then set total airflow, then balance branches, then adjust terminal devices.

Commissioning includes functional performance testing: verify economizer operation, damper stroke, and control sequences.

Record final readings on a TAB report with design vs. actual CFM, static pressures, and ambient conditions.

A 10% deviation from design airflow typically triggers a re-check of the system before sign-off.

SMACNA HVAC Duct Construction Standards defines pressure classes, gauge tables, and hanger spacing.

ASHRAE 62.1 sets ventilation rates (cfm/person and cfm/ft²); ASHRAE 90.1 sets duct insulation and leakage requirements.

IECC/IRC Section M1601 covers duct installation, sealing, and insulation for residential; M1502 covers exhaust duct termination.

NFPA 90A governs air distribution in commercial buildings: fire dampers, smoke dampers, and duct clearance from combustibles.

UL 181 lists duct materials and closure systems; UL 181A for rigid, UL 181B for flexible.

Documentation: submittals include duct construction schedule, insulation R-values, and TAB report; as-builts reflect field changes.

Permits and inspections: mechanical permit required for new ductwork; rough-in inspection before closing walls.

Energy code duct leakage: IECC limits duct leakage to 4 cfm/100 ft² (or 8 cfm/100 ft² for tested systems) in some climate zones.

Lockout/tagout applies to air handlers, VFDs, and any stored-energy source; verify zero energy at the disconnect before opening a control panel.

Fall protection is required at 6 ft (1.8 m) in general industry and 4 ft (1.2 m) in shipyard/construction contexts; duct hangers above walking surfaces count as overhead work.

Sheet metal cuts are the leading hand injury on duct jobs; cut-resistant gloves are required when handling galvanized edges, and deburr before assembly.

Ladders: 4:1 setup angle, 3 ft (0.9 m) extension above the landing, and never stand on the top two rungs; duct runs at ceiling height often require a rolling scaffold instead.

Confined-space entry (plenums, crawl spaces, large ducts) requires atmospheric testing for O2, LEL, CO, and H2S before entry and a standby attendant.

Silicone sealants, duct mastics, and spray adhesives require ventilation and SDS review; solvent-based products are flammable during cure.

Rigging: know the working load limit of slings and shackles; never exceed the hoist rating, and tag lines control duct sections in the air.

Aerial lift work requires a fall-arrest harness in a boom lift and a trained operator card; keep the platform inside the manufacturer's load chart.

Sensible heat equation: Qs = 1.08 × CFM × ΔT (English units at sea level); latent: QL = 0.68 × CFM × ΔW (grains). Use 4.5 × CFM × Δh for total heat.

Room load = conduction through envelope + solar + internal (people 250–450 Btu/h sensible, lights 3.41 Btu/h per watt) + infiltration/ventilation.

Equal friction method sizes duct for a constant pressure drop per 100 ft (typically 0.08–0.10 in. w.c.); static regain method uses velocity pressure recovery at each branch.

Velocity limits: 1,200–1,500 fpm in main trunks near occupied space for noise, 600–900 fpm in branch runs, 300–500 fpm at diffuser necks.

Friction chart: pressure drop rises with the square of velocity; doubling velocity roughly quadruples friction loss per unit length.

Equivalent length adds fitting losses to straight-run length; a 90° radius elbow can equal 30–60 ft of straight duct.

Return air path matters: a starved return raises space pressure and reduces delivered CFM; transfer grilles or door undercuts are required.

Duct leakage classes (per SMACNA) range from 3% to 30% of design flow depending on class and pressure; seal class A covers all joints and seams.

Galvanized steel G90 (0.90 oz/ft² zinc coating) is standard for HVAC; G60 is acceptable for indoor dry locations, G210 for coastal/industrial.

Gauge selection follows SMACNA pressure class: 1 in. w.c. and lower uses 26–28 ga; 2 in. w.c. uses 24 ga; 4 in. w.c. uses 22 ga for larger sizes.

Standard duct sizes step in 2 in. increments up to 84 in.; joints include TDC, TDF, slip-and-drive, and Pittsburgh lock seams.

Flexible duct: R-4.2 minimum, R-8 for high efficiency; keep installed length ≤ 5 ft per run and sag ≤ 0.5 in./ft to control pressure loss.

Fiberglass duct board is fabricated with a 2-in. shiplap closure and pressure-sensitive tape plus mastic; UL 181 listed closure systems are mandatory.

Spiral and oval duct offer lower friction; rectangular duct has higher friction but fits tight spaces.

Fire-rated duct (1 or 2 hr) requires listed assemblies such as 3M or Unifrax systems with specific wrap thickness and joint treatment.

Duct liner (acoustic) reduces radiated noise but adds friction; use only where the velocity is below ~4,000 fpm to prevent erosion.

Hanger spacing: rectangular duct 8 ft max, round duct 10 ft max, and within 2 ft of each joint or fitting.

Support rectangular duct per SMACNA: 20 ga and lighter uses 1-in. × 18-ga straps; heavier gauges may use 1-in. × 16-ga.

Elbows: use radiused (R/D ≥ 1.5) elbows where space allows; square throated elbows need turning vanes to avoid turbulence.

Branch takeoffs: 45° entries reduce pressure loss versus 90° straight taps; use volume dampers at each branch for balancing.

Flex duct must be fully extended, supported every 4–5 ft with a saddle hanger (not a strap that compresses), and terminate with a listed collar.

Duct penetrations through fire-rated walls/ceilings require listed fire dampers with access doors for inspection and reset.

Smoke dampers are required where ducts penetrate smoke barriers; they must be accessible and tested per NFPA 105.

Coordinate with other trades: keep 1–2 in. clearance around duct for insulation, and never compress insulation at hangers.

Throw (T) is the distance from the diffuser to where terminal velocity falls to 50 fpm; spacing diffusers at 1.0–1.5 throw gives good mixing.

Neck velocity: 300–500 fpm for ceiling diffusers in residential, 500–800 fpm for commercial; higher velocity increases noise (NC).

NC (Noise Criteria) target: NC 25–30 in residential, NC 35–40 in offices, NC 45 in industrial.

Balancing: proportional balance method adjusts terminal devices in sequence from the farthest branch; total airflow is set at the fan first.

A 10% airflow tolerance is typical for balancing reports; use a calibrated flow hood or pitot tube traverse.

Return grilles should be sized for ≤ 500 fpm face velocity to avoid whistle; transfer grilles need free area ≥ 1.5× the door gap.

Static pressure profile: measure at fan discharge, after each major fitting, and at the last branch to find restrictions.

Dampers: opposed-blade for modulating, parallel-blade for two-position; volume dampers should be locked after balancing.

Mastic (UL 181A-M) is for rigid duct joints; UL 181B-M tape is for flex and duct board closures. Never use cloth duct tape as a permanent seal.

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

Fiberglass insulation R-values: R-6 for ducts in conditioned space, R-8 for unconditioned attics, R-13 for exterior runs.

Vapor barrier goes on the warm side (exterior of insulation in cooling climates); a compromised barrier causes condensation and mold.

Insulation must be continuous through hangers; use insulation shields or oversized hangers to prevent compression.

Closed-cell elastomeric insulation is used for refrigerant lines; the vapor barrier is integral and requires sealed seams.

Duct leakage testing per SMACNA or ASHRAE: pressurize the duct and measure leakage in CFM per 100 ft² of duct surface.

Mastic must cure before airflow starts; solvent-based mastic off-gasses VOCs and needs 24 hours of ventilation.

Pitot tube traverse: take readings at 10 points across two perpendicular diameters; average velocity pressure, then V = 4005 × √VP (fpm).

Flow hood calibration should be checked annually; field accuracy is ±3–5% for hoods and ±5–10% for pitot traverses.

Static pressure readings use a manometer or digital gauge; measure across filters, coils, and duct sections to build a pressure profile.

Fan laws: CFM varies linearly with speed, static pressure with speed², and horsepower with speed³.

TAB sequence: verify fan rotation and airflow direction, then set total airflow, then balance branches, then adjust terminal devices.

Commissioning includes functional performance testing: verify economizer operation, damper stroke, and control sequences.

Record final readings on a TAB report with design vs. actual CFM, static pressures, and ambient conditions.

A 10% deviation from design airflow typically triggers a re-check of the system before sign-off.

SMACNA HVAC Duct Construction Standards defines pressure classes, gauge tables, and hanger spacing.

ASHRAE 62.1 sets ventilation rates (cfm/person and cfm/ft²); ASHRAE 90.1 sets duct insulation and leakage requirements.

IECC/IRC Section M1601 covers duct installation, sealing, and insulation for residential; M1502 covers exhaust duct termination.

NFPA 90A governs air distribution in commercial buildings: fire dampers, smoke dampers, and duct clearance from combustibles.

UL 181 lists duct materials and closure systems; UL 181A for rigid, UL 181B for flexible.

Documentation: submittals include duct construction schedule, insulation R-values, and TAB report; as-builts reflect field changes.

Permits and inspections: mechanical permit required for new ductwork; rough-in inspection before closing walls.

Energy code duct leakage: IECC limits duct leakage to 4 cfm/100 ft² (or 8 cfm/100 ft² for tested systems) in some climate zones.