Ventilation-duct quantity calculation and classified measurement: are supply/exhaust/fresh-air ducts computed separately, the developed-area rules, BOQ listing, and quota tiering calibers

Topic Positioning

Answering the measurement question "are fresh-air, supply, and exhaust ducts computed separately": the unified developed-area and length calibers (GB/T 50856-2024 Appendix G), the boundaries of "separation" at the BOQ and quota levels, with the corresponding Beijing DB11/T 2382-2024 clauses and coefficients.

Core Conclusions

1. Unified measurement (GB/T 50856-2024 Table G.2.1, 030702, notes verbatim):

1. Developed area, no deduction for inspection ports, measuring ports, supply and suction openings; length by design centerline (main/branch divided at their centerline intersections), including elbows, tees, reducers, adapters, excluding component lengths; developed area excludes overlapping duct and mouth areas. Tapered ducts: round by mean diameter, rectangular by mean perimeter.
2. Wall-penetration sleeves computed by developed area, into the corresponding duct quantities by material and thickness.
3. Where galvanizing of shaped steel in clean ducts is required, note it in work content.
4. Factory-made ducts listed under the corresponding duct codes.
5. Stainless smoke-exhaust ducts under the corresponding stainless-steel duct codes.
6. Duct items include support/hanger fabrication and installation; if listed separately, use Appendix N codes.

2. "Separate computation" at three levels:

• Measurement: supply, exhaust, and fresh-air ducts share one table (G.2.1) and one rule set — the code prescribes no per-system algorithm; separation is not a measurement requirement.
• BOQ: Table G.2.1 requires listing by characteristics; practice splits by system use (supply/exhaust/fresh air). GB 50243-2016 Clause 4.1.4: "duct systems are classified by working pressure into micro-pressure, low, medium, and high classes, using ducts of the corresponding class" — different systems often differ in pressure class, thickness, and airtightness, directly affecting unit prices and tiers — listing separately is advisable.
• Quota: Beijing DB11/T 2382-2024 Clause 6.7.25: "thin-steel duct and accessory fabrication/installation set items by joint form, duct size, and other factors per relevant codes" — tiering factors are joint form and size etc.; system (supply/exhaust/fresh) is not among them; same material, joint, and size tier may share one sub-item set (fabrication and installation separately); any differing dimension requires splitting the quantities.

3. Beijing DB11/T 2382-2024 Section 6.7 companion calibers:

• 6.7.29: fabrication/installation by design inner dimensions as developed area, no deduction for inspection/measuring ports and openings of supply/return grilles.
• 6.7.30: length by design centerline, not deducting elbows, tees, reducers and other fittings, deducting valves and components; centerlines terminate at their intersections.
• 6.7.28: tapered ducts for uniform supply execute fabrication/installation by mean diameter or large side with labor x 2.50; air-curtain ducts by mean large side with labor x 3.00; curved ducts labor and materials x 1.12.
• 6.7.27: galvanized thin-steel common-flange rectangular ducts fabrication/installation labor 40%/60%; stainless 72%/28%.
• 6.7.32: wall-penetration sleeves by developed area and end caps by the capped duct section area, both under duct fabrication/installation items.
• 6.7.36: plenum boxes by developed area (by large side), no deduction of connected duct openings.
• 6.7.39/6.7.41: FRP and composite ducts by design outer dimensions as developed area (unlike thin steel by inner dimensions).
• HVAC testing, adjusting, commissioning are separate BOQ items per Appendix G Table G.4.1 (030704).

Applicable Boundaries

• Parts 1-2 are national GB/T 50856-2024 calibers; Part 3 is Beijing local (DB11/T 2382-2024) — other provinces use their own tiering factors and coefficients.
• Mind material-specific area calibers: thin steel by inner, FRP/composite by outer dimensions — do not mix.
• "Separate listing" is not a mandatory clause but a characteristics-management and pricing-clarity practice: with identical tiers, merging is not a violation, though splitting eases settlement, reconciliation, and changes.
• Only measurement and tiering logic here — no local unit prices or fees; follow local quota notes and fee schedules.

Further Reading

• GB/T 50856-2024 Appendix G: Table G.2.1 (030702) and notes 1-6; Table G.4.1 (030704).
• GB 50243-2016 Code for Construction Quality Acceptance of Ventilation and Air Conditioning Works, Clause 4.1.4 and Table 4.1.4 (pressure classes).
• DB11/T 2382-2024 Section 6.7, especially Clauses 6.7.25-6.7.41.
• When assembling, match local sub-item tables dimension by dimension: joint form (seam/welded), size (round diameter or rectangular large side), plate thickness.