Duct pressure class groups sheet-metal ductwork by the maximum static pressure it must withstand, which determines gauge, seam type, and reinforcement. In SMACNA guidance the three common classes are low pressure (up to 1 in. w.g., about 250 Pa), medium pressure (1 to 3 in. w.g., roughly 250 to 750 Pa), and high pressure (3 to 10 in. w.g., roughly 750 to 2,500 Pa). Selecting the wrong class causes rattling, leaks, and failed commissioning, while over-specifying wastes steel and labor on every joint in the building.
Why Pressure Class Drives the Whole Duct Design
Static pressure is the force the fan pushes against inside the duct. A low-pressure supply duct may run at only 0.5 in. w.g. (about 125 Pa), while a high-velocity high-pressure system can exceed 6 in. w.g. (1,500 Pa). The higher the class, the heavier the sheet gauge, the tighter the longitudinal seam, and the more frequent the transverse joint reinforcement must be. A duct that is one class too light will telegraph every pressure spike as a whistle or a bulge at the nearest weak seam.
Pressure Class Comparison
The table below contrasts the three classes on the parameters that affect cost and constructability. Use it as a first-pass selection before running a detailed SMACNA calculation, then confirm the exact gauge against the joint spacing the local code requires for the chosen class.
| Pressure Class | Typical Static Range | Sheet Gauge (galv.) | Seam / Joint Type |
|---|---|---|---|
| Low pressure | 0 – 1 in. w.g. (0 – 250 Pa) | 26 – 22 ga | Slide / Pittsburgh, T-bars |
| Medium pressure | 1 – 3 in. w.g. (250 – 750 Pa) | 22 – 18 ga | Standing seam, flanged |
| High pressure | 3 – 10 in. w.g. (750 – 2,500 Pa) | 18 – 14 ga | Welded / flanged + gasket |
Leakage Limits Scale With Class
SMACNA defines allowable leakage in cubic feet per minute per 100 ft² of surface. A low-pressure Class 3 duct may be permitted up to about 0.65 cfm per 100 ft², while a high-pressure system is held to a fraction of that. At 2,500 Pa a leaking joint can lose 5% to 15% of total airflow, directly raising fan energy by a comparable margin, so sealing class must match the pressure class rather than being treated as an afterthought at installation.
Velocity and Noise Trade-offs
Higher pressure classes usually run at higher velocities. Low-pressure ducts sit around 1,000 to 1,500 fpm (5 to 7.5 m/s); high-pressure systems reach 2,500 to 4,000 fpm (12 to 20 m/s). Above 2,000 fpm (10 m/s) duct breakout noise becomes significant, often requiring internal lining or external wrap to keep occupied spaces below 35 to 45 dBA. The pressure class you pick therefore commits you to a noise-control strategy almost as much as to a gauge schedule.
Reinforcement and Hanger Spacing
As pressure rises, transverse joints need stiffeners at closer centers. A 750 Pa medium-pressure duct typically carries a tie-rod or angle ring every 1.2 to 1.8 m, while a 2,500 Pa high-pressure run may need stiffeners every 0.9 m and a heavier hanger schedule to stop the duct from walking under pulsation. Ignoring this is the usual cause of a system that passes a static test yet hums audibly once the fan reaches speed.
When to Choose Each Class
- Choose low pressure for constant-volume comfort HVAC in offices and homes where velocities under 1,500 fpm are acceptable and first cost matters most.
- Choose medium pressure when space is tight and you need 1,500 to 2,500 fpm velocities without the cost of welded construction.
- Choose high pressure for long duct runs, high-rise vertical shafts, or cleanrooms where 750 to 2,500 Pa is needed and leakage must stay minimal.
Keruite manufactures this equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment.
Commissioning Checks
During testing, verify static pressure at three points: fan discharge, furthest outlet, and return. A well-designed medium-pressure system loses no more than 0.1 in. w.g. (25 Pa) per 100 ft of straight run. If measured pressure exceeds the class rating at any joint, upgrade the gauge or reseal before balancing, because operating above class invites fatigue cracks within 3 to 5 years. A ductular leakage test at 1.5 times operating pressure is the simplest way to catch weak seams before occupancy.
Pressure Testing and Acceptance
Before a duct system is balanced it should pass a static-pressure test at 1.5 times its operating pressure for at least 10 minutes, with no visible deflection at joints and leakage under the SMACNA class limit. A low-pressure system tested at 375 Pa (1.5 × 250 Pa) that holds its reading confirms the seams and transverse joints were detailed correctly. Medium-pressure duct is tested at 1,125 Pa and high-pressure at up to 3,750 Pa, and any joint that weeps at these pressures must be re-rolled or re-welded rather than patched, because a patch rarely survives the thermal cycling of the first heating season.
Documenting the test pressure and the measured leakage per 100 ft² also gives the commissioning agent a baseline to judge future degradation over the 15 to 25 year service life of the duct. A system that was measured at 0.20 cfm per 100 ft² when new and later reads 0.60 cfm per 100 ft² is telling you a seam has opened, often at a vibration node near a fan or a damper actuator. Catching that early, before the fan has quietly added 5% to 10% to its energy draw, is the whole point of the test record, so the measurement should be filed with the as-built drawings rather than discarded with the commissioning paperwork.
Coordination With Other Trades
Duct pressure class also drives coordination with structural and architectural teams, because medium and high-pressure runs need heavier hangers and larger penetration sleeves through rated walls. A 750 Pa duct with 18 gauge sheet and stiffeners every 0.9 m can weigh 15 to 25 kg/m, so the structural engineer must allow for that load at each trapeze, and fire-stop ratings at the penetration must suit the class so a 2,500 Pa run does not lose its rating at the first concrete wall it crosses.
FAQ
What is the static pressure limit for low-pressure duct?
Low-pressure ductwork is rated for up to 1 inch of water gauge, equal to about 250 Pa. Within this class, sheet gauge typically runs 26 to 22 and joints use Pittsburgh or slip-and-drive seams. Staying at or below this pressure keeps construction economical while still serving most comfort-cooling airflows, and it avoids the stiffener density that higher classes demand at
When should I specify medium-pressure duct?
Specify medium pressure when the system needs 1 to 3 in. w.g. (250 to 750 Pa) and velocities of 1,500 to 2,500 fpm to fit limited ceiling space. It uses heavier 22 to 18 gauge sheet and flanged or standing seams, balancing cost against the tighter leakage control that high pressure would demand.
How much leakage is allowed in high-pressure duct?
SMACNA allows only a small fraction of the leakage permitted in low-pressure duct, because at 750 to 2,500 Pa even tiny gaps waste 5% to 15% of airflow. High-pressure systems therefore use welded or gasketed flanged joints and are tested to a tight Class limit before acceptance.
Does higher pressure class mean more noise?
Yes, generally. Velocities climb from about 1,000 fpm in low pressure to 2,500 to 4,000 fpm in high pressure, and breakout noise grows with velocity. Keeping occupied rooms under 35 to 45 dBA usually requires lining or wrap once velocities pass 2,000 fpm, adding both cost and thickness.