British Columbia
WHERE TO FIND THE PRESSURE CLASS AND HOW IT SETS THE GAUGE
Send the pressure class from the drawings, and the gauge follows from the SMACNA tables. A gauge picked by habit can come out too thin, or heavier than the job needs.
The duct pressure class tells the fabricator how much air pressure each run is built to hold. With the duct's size and the spacing of its stiffeners, it sets how thick the metal has to be. Stiffeners are the steel angles that keep the sides from flexing. Most drawing sets give the class in a duct construction table or the mechanical spec. This guide shows where to look, how the tables turn it into a gauge, and what to write when nobody has picked one yet.
The engineer sets the class, and the drawings are supposed to say which part of the system gets which one. Check these places in order, and stop at the first one that answers.
| Where to look | What it looks like | If it isn't there |
|---|---|---|
| A duct construction table on the mechanical drawings | A row for each system, with a pressure class such as "+2 in. w.g." or "-500 Pa" | Move on to the duct plan notes |
| Notes on the duct plan | A line such as "Supply duct upstream of boxes, 500 Pa" | Move on to the written specification |
| The written mechanical specification, in the section on metal ducts | A list of systems with their pressure in pascals, and a line saying construction follows SMACNA | Ask the engineer who wrote the mechanical spec |
| A question to the engineer, sent as an RFI | A written answer naming the class for each system | Hold the order for that run until the answer comes back |
Where the pressure class sits on the drawings
A duct construction table, as it often appears on a mechanical sheet. Example values, not from a job.
| System | Pressure class | Seal class | Construction |
|---|---|---|---|
| Supply, fan to VAV boxes | +500 Pa (2 in.) | Per spec | SMACNA |
| Supply, after VAV boxes | +250 Pa (1 in.) | Per spec | SMACNA |
| Return | -250 Pa (1 in.) | Per spec | SMACNA |
| General exhaust | -500 Pa (2 in.) | Per spec | SMACNA |
Copy this column onto the order, with its plus or minus sign
Canadian project documents often give the class in pascals. SMACNA's tables are labelled in inches of water, written "in. w.g.", and 500 Pa and 2 in. of water name the same class.
A pressure class is the pressure a duct is built to hold, not a reading on a pressure meter on site. Each run is built for a class at least as high as the most pressure it will see in use.
| Class in inches of water | The same class in pascals | Pick this class when the run's highest pressure is |
|---|---|---|
| ½ in. | 125 Pa | Up to ½ in. |
| 1 in. | 250 Pa | Over ½ in., up to 1 in. |
| 2 in. | 500 Pa | Over 1 in., up to 2 in. |
| 3 in. | 750 Pa | Over 2 in., up to 3 in. |
| 4 in. | 1000 Pa | Over 3 in., up to 4 in. |
| 6 in. | 1500 Pa | Over 4 in., up to 6 in. |
| 10 in. | 2500 Pa | Over 6 in., up to 10 in. |
Words like "low pressure" and "medium pressure" don't name a class. One project spec puts medium pressure at 1500 Pa, and another at 1000 Pa (4 in. of water). Write the number instead.
A class can be positive or negative. A plus sign means the fan pushes air out through the duct, and a minus sign means the fan pulls air in.
Positive pressure pushes the sides outward and tries to pull the joints apart. If a run is built too light for it, joints and seams can open, and screws or rivets can pull out. Write "+" beside the class so the run is built to hold together.
Negative pressure sucks the sides inward, and a run built too light can buckle at the corners or collapse. SMACNA's rectangular tables use one gauge for both signs, while round duct for suction needs heavier metal. Write "-" beside the class.
Every job we take on follows SMACNA duct construction standards, and their tables do the work once the class is known. Stiffeners, which SMACNA calls reinforcement, are steel angles or formed joints fastened across the sides to keep them from flexing.
Find the class for the run. Use the table on the drawings, or the engineer's answer, with its plus or minus sign.
Take the larger side. On a 24 x 12 in. duct, the 24 in. side sets the gauge for all four sides. On lined duct, use the metal size, not the airway inside the lining.
Pick a stiffener spacing. The tables list spacings from 10 ft (about 3 m) down to 2 ft (about 0.6 m). Ducts 10 in. and under need none at 2 in. of water or less.
Read the minimum gauge. Each spacing has its own minimum gauge and a grade of stiffener to match. A lower gauge number means thicker metal.
That's why a gauge on its own doesn't describe a run. The same run can be thick and plain, or thinner with stiffeners every few feet.
The tables let you trade thicker metal for more stiffeners, as this 24 in. side at two classes shows. The figures come from SMACNA's 1995 tables. If your spec names a later edition with different figures, we read that edition instead.
| Stiffener spacing | Minimum gauge at 1 in. of water (250 Pa) | Minimum gauge at 2 in. of water (500 Pa) |
|---|---|---|
| None | 18 | 16 |
| 10 ft | 24 | 20 |
| 8 ft | 24 | 22 |
| 6 ft | 26 | 24 |
| 4 ft | 26 | 26 |
The same duct, two ways to meet one pressure class
A 24 in. duct side at 2 in. (500 Pa). Example from the 1995 SMACNA tables, not a spec for your job.
Thicker metal, no stiffeners
Heavier sheet, nothing sticking out from the sides
Thinner metal, stiffeners every 4 ft
Lighter sheet, with a stiffener across the duct every 4 ft
Orange bars are stiffeners, which SMACNA calls reinforcement. A lower gauge number means thicker metal.
The tables stay in gauge numbers, the way SMACNA prints them. Converting gauge numbers to millimetres has its own guide, on sheet metal gauge vs thickness.
Round duct holds pressure better than flat sides do and can usually be lighter at the same size. Suction changes that, and the same round duct needs heavier metal on the return or exhaust side.
| Duct, 24 in. across | Gauge on the supply side, +2 in. of water | Gauge on the return or exhaust side, -2 in. of water |
|---|---|---|
| Round duct with a spiral seam | 26 | 22 |
| Round duct with a straight seam along its length | 24 | 20 |
| Rectangular duct with no stiffeners | 16 | 16 |
These figures come from SMACNA's 1995 tables, as an example and not a spec for your job. We buy spiral in and resell it rather than make it. On a return or exhaust run, check its gauge against the suction column.
The class doesn't stop at the metal. It also settles three more details of how the run goes together. An order that gives the class answers all of them at once.
SMACNA ties sealing to pressure. Class C, at 2 in. of water and under, seals the joints between sections. Class B, at 3 in., adds the seams, and Class A, at 4 in. and up, adds every hole. If your spec asks for more, it wins. The classes are explained in how to specify a duct fitting.
Some joints only hold at low pressure. SMACNA limits plain S-slips to 2 in. of water and allows no drive slips above 4 in. Above that, expect flanged joints, and tell us what the existing duct uses.
The class sets how strong each stiffener must be for the spacing chosen. If the ceiling space is tight, say so. Heavier metal can mean fewer stiffeners sticking out from the sides.
A missing class is common on small jobs and on sketches. Each of these situations has a clear next step.
If there's an engineer on the job, send a short RFI asking for the class of each system. A written answer protects you if a run is questioned at inspection, and it takes the guessing off the order.
SMACNA's 1995 edition assumes 1 in. of water when no class is given, and 2 in. for supply duct feeding VAV boxes, the units that vary airflow to each zone. Some Canadian project specs assume 500 Pa (2 in.) for everything. When defaults disagree, ask.
If you build every run to the fan's outlet pressure, the far end of the system gets heavier metal than it needs. SMACNA says building the whole system to the fan's pressure costs more than it should. Ask for a class per system instead.
If you're adding to or replacing duct on a working system, tell us what the run connects to and which side of the fan it sits on. We'll write the class SMACNA assumes on the quote for you to confirm before we cut.
Each of these orders gives a gauge without the class behind it. Adding the class to the order clears every one of them.
| What the order says | What goes wrong | What to write instead |
|---|---|---|
| "Heavy gauge" | Nobody can tell which gauge, so the order comes back with a question | "Pressure class from drawing M-4, supply +2 in. w.g." |
| "26 gauge throughout" | Wide runs at a higher class need metal heavier than 26, or stiffeners close together | "Gauge and stiffeners per SMACNA for each run's class" |
| "Same as the last job" | The last job may have run at a different pressure, with a different spec | "Classes per this job's duct construction table" |
| "Go heavier so it's quieter" | SMACNA's own testing found thinner duct carrying fast air was no louder | "Gauge per class, and lining where the drawings show it" |
| "Grease duct" listed with the supply runs | Grease duct follows NFPA 96, not these tables, and sets its minimum at No. 16 gauge mild steel or No. 18 gauge stainless | "Grease duct per NFPA 96, listed separately" |
The drawing numbers in this table are examples, not from a job. When noise is the worry, the answer is usually lining, and duct lining and double-wall duct covers what to specify.
Every order is read for its pressure classes before any sheet is cut, whether it's one transition or a full HVAC ductwork package.
Classes read off the drawings. We match each run on your list to its class in the duct construction table or the spec.
Owner rules in the spec checked. Some owners ask for heavier metal than SMACNA does, and the spec wins where it does.
Gauge and stiffeners picked per SMACNA. We choose the pairing for each run unless your drawings name it. If the ceiling space limits it, we tell you.
Questions before the cut. If a run has no class, we ask first. Asking then costs a phone call instead of a remake.
Checked before release. The finished work is checked against the drawings before it leaves Pitt Meadows. If a run is wrong and the mistake was ours, we fix or remake it at no charge.
We fabricate galvanized, aluminum, and mild steel from 30 to 14 gauge, and stainless from 26 to 14. The engineer's spec sets the class, and the local building official has the final say.
SMACNA's tables cover standard sheet metal duct for heating, cooling, and ventilation. Some duct follows other rules, and some orders need a design before they need a gauge. The gauge rules for grease duct have their own guide, on grease duct specifications.
If the system itself isn't designed yet, it needs an engineer before it needs a fabricator. We'll tell you so before quoting, rather than price a guess.
Tick these off, and the gauge on every run is decided before we price it. The checklist for the rest of the job is in What Duct Duct Goose Needs to Quote Custom Metalwork. More decisions like these sit in HVAC Ductwork and Fittings.
A full set of drawings and a sketch with a question mark beside the pressure are both fine places to start. We'll read what you have, ask about the gaps, and come back with a price once each run is defined.
Standard ductwork and fittings can be priced and ordered on WebDuct, the online pricing and ordering portal, once your company account is set up. Lined and double-wall duct, grease duct, kitchen stainless, curbs, installs, and other custom work start with a request or a call.
At Duct Duct Goose, we specialize in sheet metal fabrication and stainless steel services for commercial and industrial applications, including HVAC systems, ducts, and custom metalwork. Please note that we do not offer services for automotive-related projects, such as vehicle parts, repairs, or modifications.
