HVAC takeoff

Ductwork is a single geometry that the standard methods quantify in three different ways, and the mechanism changes, not just the unit. United States practice reports pounds of sheet metal, found by multiplying the developed cross-section by the run length and then by a gauge weight factor. United Kingdom practice under RICS NRM2 Work Section 38 measures linear metres along the duct centre line, with fittings deemed included unless separately measured. Australia and New Zealand practice under the ANZSMM, along with much continental practice, measures square metres of duct surface area, with fittings taken extra over. No single source names one worldwide mechanism, so the governing method of measurement decides it, and it must be settled before any quantity is taken.

Straight duct is measured along the duct centre line, down the middle of a rectangular trunk and along the axis of round or spiral branches. Each straight segment runs from one fitting or equipment face to the next, and the run stops at the equipment flange, not inside the unit. Route the centreline orthogonally and sum the legs, since a straight-line diagonal under-measures, and add the vertical work a floor plan cannot show: risers, offsets over beams, and drops to each diffuser, read off sections and riser diagrams.

Two traps recur. Equivalent length is design, not takeoff: the Total Effective Length method in ACCA Manual D and the fitting-loss method in the ASHRAE duct-design references inflate a fitting to a length of straight duct only to size the duct, so adding it grossly over-measures the run. Count each fitting as one piece and measure only the actual straight duct. Fittings are also pieces, not deductions: in per-piece practice the length a fitting occupies leaves the straight run but returns as a counted piece, so the developed length is conserved, while deemed-included practice runs the centreline straight through. There is no void or opening deduction on a linear run in either method.

Where ductwork is priced by weight, the pounds quantity is built in steps. SMACNA fixes the minimum galvanized gauge from the duct's greatest dimension and the static-pressure class, and that greater dimension governs all four sides, though a project specification can override it with a heavier gauge. The stretch-out is the developed perimeter of the cross-section, two times width plus height for rectangular duct and pi times diameter for round and spiral, multiplied by the run length to give the sheet-metal area, with round duct using about 15 percent less metal than the equivalent rectangular. A material allowance for seams, joints, and reinforcement, commonly around 15 percent, is added before the weight factor and is estimating practice rather than a numbered clause.

The weight factor converts area to pounds, taken from the Manufacturers' Standard Gauge for galvanized steel, which already includes the zinc-coating allowance. The canonical reference is 26 gauge at 0.906 pounds per square foot, with heavier gauges stepping up from there, while metric regions key the factor to sheet thickness in millimetres times steel density rather than a gauge number. Labour follows the same split: SMACNA reports pounds per hour for straight duct and hours per piece for fittings, which is why fittings are taken per piece.

The regional difference runs through the primary quantity, the fitting treatment, and the construction standard together. The United States measures pounds per piece and builds to SMACNA, and Canada follows the same convention on metric drawings. The United Kingdom measures linear metres along the centre line, deems fittings included under NRM2 rule 38.7, and builds to DW/144. Australia and New Zealand measure surface area with fittings extra over and build to AS 4254, and continental Europe also measures by surface area, where the EN standards govern construction and leakage, not the method of measurement. Whichever applies, ductwork is segregated into separate lines by shape, gauge, pressure class, leakage class, material, and lining, since merging incompatible rates corrupts the estimate.

Air terminals are counted, each once, segregated by service and by type and size. The ASHRAE terms distinguish them: a grille has no damper, a register is a grille plus a volume damper, and a diffuser discharges radially. Count supply, return, and exhaust separately, keyed to the airflow tag, and do not count light fixtures, access doors, or dampers as terminals. A linear slot diffuser is taken by length where continuous and per unit where scheduled as a discrete assembly.

Equipment such as air handlers, rooftop units, variable-air-volume boxes, fans, and split systems is enumerated each, matched plan symbol to schedule by tag so a unit shown on several sheets is not double-counted. Controls devices such as thermostats, sensors, and actuators are a distinct enumerated line that often straddles divisions 23 and 25.

Dampers and access doors are each enumerated by type: fire, combination fire and smoke, volume or balancing, and backdraft, plus the duct access doors at each in-duct device. Fire and smoke damper locations are fixed by rated-barrier penetrations under the International Mechanical Code, so they are read off the fire-rating plans rather than the mechanical plan alone. Flexible duct is measured in linear feet by run, never weighed; the Air Diffusion Council flexible-duct standard supports it at no more than 4 feet, limits sag to half an inch per foot, and requires it installed fully extended, not compressed.

Insulation, whether wrap or liner, is measured by the surface area of the duct it covers, the same stretch-out, and segregated by R-value, thickness, and wrap versus liner. Duct hangers and supports are derived from the run at a standard spacing, on the order of 8 to 10 feet for horizontal rectangular duct, about 4 feet for flex, and about 3 metres in metric practice. Waste is convention rather than a published standard, since the trade associations set construction and labour standards, not waste allowances; the cited bands run roughly 8 to 12 percent for rectangular and spiral, lower for flex, and higher for duct board and complex shop work. Refrigerant line sets and condensate drains on split systems are a separate linear item by size.

The same duct yields different quantities by purpose, and reporting one as another causes over-billing or under-billing. A bid uses the net measured quantity, procurement uses the net plus scrap and seam allowance, and progress billing uses the net measured in place, where waste never inflates a payment because under NRM2 the waste lives in the rate, not the quantity. Exayard reads the drawings and applies these rules automatically, tracing each run along its centreline, settling the weight, length, or area mechanism for the region, and counting fittings, terminals, equipment, and supports as separate lines.