What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingductwork
also DW/144, DW/143, low pressure, medium pressure, air leakage class, spiral duct
DW/144 is a single specification covering construction and gauge by pressure class; DW/143 is the leakage testing method. Neither name exists in American practice, and neither is SMACNA.
Australia
Close, not identicalductwork
also AS 4254, flexible duct, rigid duct, duct class
Follows its own standard with flexible duct used far more widely in housing than in Britain, which changes the effective resistance of a system far more than the fittings do.
United Statesyours
Same word, different thingductwork
also duct, SMACNA, pressure class, leakage class, CL, gauge, sheet metal
SMACNA gives construction by pressure class in inches of water gauge; the LEAKAGE class is specified separately and is not implied by the construction class the way DW/144 implies it.
Canada
Close, not identicalductwork
also duct, SMACNA, pressure class, leakage class, sheet metal
As the United States, with pressure still commonly quoted in inches of water gauge even where the rest of the design is metric.
The governing standard, by market
United Kingdom
DW/144 / DW/143
Specification for sheet metal ductwork including gauge, stiffening and pressure class, and the practical guide to ductwork leakage testing.
United States
SMACNA HVAC Duct Construction Standards / ASHRAE 90.1
Metal and flexible duct construction by pressure class, and the energy standard's duct sealing and leakage testing requirements.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- What is a pressure class, and why does it decide the gauge?
- It is the pressure the duct must hold without deforming, and it sets the metal thickness and the stiffening because a duct is a pressure vessel with flat sides. Round duct resists pressure by its shape and needs little help. A rectangular duct's flat panels want to bow outward under positive pressure and suck inward under negative, so the thicker the metal and the closer the stiffening beads or angles, the higher the pressure it can take without drumming, leaking at the seams or visibly deflecting. Both markets therefore specify construction AGAINST a pressure class rather than as a single thickness — and the classes themselves are stated in different units, inches of water gauge in North America and pascals in Britain. The practical trap is that the class is a property of the SYSTEM at that point, not of the fan: duct upstream of a damper can sit at a much higher pressure than the design figure quoted for the branch, and the highest pressure any section sees is what governs its construction.
- Is a leakage class the same thing as a pressure class?
- No, and conflating them is the commonest specification error in ductwork. PRESSURE class says what the duct must withstand. LEAKAGE class says how much air it is permitted to lose at that pressure, expressed as a flow per unit of duct surface area. They are related — leakage rises with pressure — but a duct can be built stiff enough for its pressure and still leak badly, because leakage is a property of the SEAMS and joints rather than of the panels. British practice ties the two together in one document, so quoting a DW/144 class implies a leakage class. American practice specifies them separately, so a drawing that names a SMACNA pressure class and stops has said nothing about leakage at all. That gap matters commercially as well as technically: sealing to a tighter leakage class is labour on every joint, and discovering it after fabrication is a variation rather than a detail.
- Why does a flexible duct ruin a design that calculated correctly?
- Because its resistance depends on how it was installed, and the figure in the calculation assumed it was straight. Flexible duct has a corrugated bore, so even fully extended it has several times the resistance of smooth rigid duct of the same diameter. Left compressed — shipped in a carton at a fraction of its length and simply pushed into place — that resistance multiplies again, and a lazy bend sagging between hangers adds more. A branch specified for a given flow can end up delivering a fraction of it while the system's fan runs at its design speed, which shows up as a room that never reaches temperature and a balancing exercise that cannot close. This is why every flexible connection has a maximum permitted length in the specification, why it should be pulled taut and supported at intervals rather than draped, and why replacing a run of rigid duct with flexible during installation is a design change rather than a substitution.