Attribution and pricing of round-to-rectangular transition ducts (rectangular-to-round reducers) in ventilation and air-conditioning: they are duct fittings priced with the duct fabrication and installation (030702), not listed as duct components (030703) - avoiding double counting

Topic Positioning

The attribution and pricing of the "round-to-rectangular transition" (rectangular-to-round reducer) in ventilation bills: fitting or component, how its quantities merge into the duct, which code / items price it, and how local consumption standards differ.

Core Conclusions

1. Attribution: the transition is a "fitting", not a "component"

GB/T 50856-2024 Annex G table G.2.1 "Duct Fabrication and Installation" (code 030702), note 1, original:

Duct developed area deducts no inspection, measuring, supply or exhaust openings; duct length follows the design-shown centreline (mains and branches dividing at the centreline intersection), including the lengths of elbows, tees, reducers, round-to-rectangular transitions and other fittings, but excluding the lengths occupied by components. Duct developed area excludes the overlapping areas of ducts and mouths. Tapered ducts: round ducts on mean diameter, rectangular ducts on mean girth.

The transition is expressly listed with elbows, tees and reducers as a "fitting" inside the duct item; while valves, diffusers, registers, hoods, flexible connections, silencers, plenum boxes and other "components" list separately per table G.3.1 (code 030703). The transition therefore must not be listed under a "component" item - the duct quantities already contain the fittings; listing again double counts.

2. Quantities: length merged, area merged, no separate item

• Length: the transition's axial length enters the connected duct's centreline length, mains and branches dividing at the centreline intersection;
• Area: its developed area merges into the duct's developed area; duct area excludes overlapping duct and mouth areas;
• Tapered transition basis: rectangular one end, round the other is a tapered transition - "round ducts on mean diameter, rectangular on mean girth", i.e. developed at design-shown dimensions and merged into the connected duct (practice: mean girth x axial length merged into the duct);
• Non-deductions: inspection, measuring, supply and exhaust openings are not deducted.

Other notes of the same table:

2 Wall sleeves compute on developed area and enter the corresponding duct quantities by material and thickness. 3 Where the rolled sections of clean ducts require galvanising, the work content states "sections galvanised". 4 Factory-made ducts list under the corresponding duct items. 5 Stainless steel smoke-exhaust ducts list under the corresponding stainless sheet duct codes. 6 The duct items include the fabrication and installation of supports and hangers; where listed separately, they take the corresponding items of Annex N, other and ancillary works.

3. Pricing: rides with the duct, no transition item added

1. Match "Duct Fabrication and Installation" (030702) items on material - thickness - shape (round / rectangular) - joint form - size band, merging the transition's area into the connected duct's item quantities;
2. Where the two ends fall in different bands (rectangular long-side band versus round diameter band), price per the design agreement or the host system's band, merging the area into that duct;
3. No standalone "transition" item exists and none may be invented; where a listing is truly required (design or contract), follow the supplementary unit-rate build-up schedule approval route.

4. Local (utility-tunnel) consumption standard example

SJG 111.3-2022 Consumption Quota Standard for Urban Utility Tunnels, Volume 3 (ancillary installation works), clause 4.2.2 "ventilation works":

1 Thin-sheet ducts compute in m2 on the design-shown outer-diameter size developed area. No deduction for inspection, measuring, supply or exhaust openings. Overlapping duct and mouth areas are already inside the items - no further charge. 2 Duct length follows the design-shown centreline (mains and branches dividing at the centreline intersection), including elbows, reducers, round-to-rectangular transitions and other fittings, LESS the lengths occupied by components. Component deductions follow the design; absent design values: butterfly valve L = 150 mm; non-return valve L = 300 mm; round fire damper L = 220 mm; rectangular fire damper L = 210 mm; sealed opposed-blade damper L = 210 mm; sealed inclined-gate valve L = 220 mm.

The local basis likewise treats the transition as a fitting riding with the duct, but expressly deducts component lengths with a deduction table; developed area is on outer diameter, overlaps already inside. Other rules of the clause: flexible ducts per centreline metre; turning-vane fabrication and installation on vane area (tables 4.2.2-1 / 4.2.2-2 absent design values); canvas connections on developed area; duct inspection doors on mass in kg; temperature and flow measuring ports by count per type.

Applicability Boundaries

• The national basis rests on GB/T 50856-2024 Annex G tables G.2.1 and G.3.1; local / sector standards may differ (developed area on inner or outer diameter, overlaps charged or not, presence of a component-deduction table) - price per the measurement / consumption standard the project executes.
• "Not listed as a component" presupposes the transition is a fitting of its connected duct; where the design or contract expressly lists it as a stand-alone factory-made component, the agreement governs - outside this card.
• With no standalone item, invent none; a listing truly needed follows the supplementary schedule approval route.
• This card covers attribution and measurement only, no regional rates or on-cost data.

Further Reading

• GB/T 50856-2024 quantity take-off standard for general building services, Annex G: table G.2.1 (030702) and notes 1-6; table G.3.1 (030703).
• SJG 111.3-2022 Consumption Quota Standard for Urban Utility Tunnels, Volume 3, 4.2.2 ventilation works - fitting merger and the component-deduction table (tables 4.2.2-1, 4.2.2-2 vane areas and counts).
• When pricing, match the local "thin-sheet duct fabrication / installation" item tables on shape (round diameter / rectangular long side), thickness and joint form, then merge the transition's developed area.
• Duct classes, thicknesses and tightness per GB 50243-2016 (ducts classified by working pressure: micro, low, medium, high).