Duct takeoffs by system, drafted from the mechanical drawings and the duct specification, for your estimator to check.

A mechanical bid lands as a drawing set and a Division 23 book. Your estimator sorts the job into systems, then works out what each one has to be built from: the pressure class, the metal and gauge, where sealant goes, what gets wrapped or lined, and which dampers, access doors and other accessories hang on it. Some engineers print a duct construction schedule on the drawings and others leave the same information in the specification, and that is where the two begin to disagree. The University of Michigan's master section asks for a project-specific fabrication schedule giving gauge and reinforcement for each pressure class and duct size, which is the same thinking a careful takeoff starts from.

During the test, AI works from copies of that bid set and drafts the takeoff by system: pressure class, material and gauge, seal class, insulation or liner, dampers and accessories, each with the sheet or paragraph it came from. A second list shows every system where the drawings and the specification don't agree. Nothing gets priced and no live bid is touched. Your reviewer sets each draft beside the takeoff your team really used.

A duct section is really a list of systems. The University of Houston's master Section 23 31 13, revised in January 2018, gives each system its own paragraph with bracketed defaults for the engineer to edit: VAV supply upstream of the terminal units up to plus 3 inches of water gauge, downstream up to plus 2, return minus 1, garage supply plus 4 and exhaust minus 4, lab hood exhaust minus 6 in welded 316L stainless or fiberglass reinforced plastic. Stairwell pressurization and garage duct go uninsulated, while general exhaust gets wrapped only through nonconditioned space and on horizontal runs from toilets. The draft keeps one line per system and names the paragraph it read.

Gauge can come from three places. The medium-pressure section for Massillon West Elementary in Ohio, dated July 2023, prints its own table and says to use it or the latest SMACNA table, whichever is more stringent: 22 gauge for rectangular duct up to 12 inches, stepping to 16 gauge at 49 to 60 inches, with separate columns for spiral and longitudinal seam round. Michigan's master starts at 24 gauge through 12 inches and reaches 18 at 84 inches and larger. Houston treats the published gauges as minimums and expects the contractor to go heavier if the duct has to survive installation. So the draft shows where each gauge came from, uses your shop's own gauge schedule wherever the specification only points to a standard, and marks lines where two sources differ.

Seal class and liner hide in the words. Michigan calls for Seal Class C at 2 inches or less, Class B for exhaust at that level, and Class A outdoors, above 2 inches and for any duct made inaccessible in a shaft or above a hard ceiling, so one run can change class where it enters a shaft. Washington State's energy code counts spin-ins, taps, access door frames and equipment connections as transverse joints, which widens what a joint-sealing line has to carry. Sizes bite too. Houston's drawings show clear inside dimensions and its section says to increase each dimension where liner is specified; the Massillon section says to hold sizes inside the lining. Every lined system gets a mark so the metal size gets a second look.

The material changes where the drawing doesn't. Michigan wants aluminum or 316L stainless, continuously welded, for dishwasher exhaust, uninsulated duct on a roof and anything within 20 feet of a shower area exhaust register, and stainless only from sterilizers and glass washers. It bars tie rods from vivarium duct and from return or exhaust duct in health care facilities. Houston's stainless exhaust is 20 gauge 316L at minimum, with liquid-tight continuous external welds by certified welders, an access panel at each change of direction and every 50 feet of horizontal run, and no fire or control dampers apart from exhaust terminal units and fan isolation dampers. A single line on a plan can carry all of that.

Accessories are where owners part ways. The University of Arizona's manual requires balancing dampers at every branch duct, bans extractors and rejects button punch snap lock seams. Houston accepts button punch at corner seams, and the Massillon section wants flexible connections made with fittings that have an integral air extractor and balancing damper. Flexible duct to a terminal unit stops at 1 foot 6 inches in Houston and two feet at the school, while Houston allows 8 feet to an air device and Arizona 48 inches on low pressure. What your estimator will check first: a system left off, a class or gauge read from the wrong paragraph, liner missed, damper and access door counts against the schedules, and each conflict the draft listed. These published rules are examples, not legal or engineering advice.

After your reviewer says yes, new bid sets go through the same reading for about 30 days.

Your estimator approves each takeoff before it goes anywhere, and every price stays with your team.

The comparison carries on against live work. A next job would be a separate decision at its own price.