Ductwork installation

How does Ductwork installation work, step by step?

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   Step 1: Coordinate the route before anything is ordered

   The starting point is a coordinated services model that shows the ductwork against the structure, the pipework, the containment, the sprinkler system, the lighting and the finished ceiling line. Ductwork usually takes priority in that model because it is the biggest and the least bendable, but the priority is agreed rather than assumed. Every penetration through a wall, floor or beam is identified and issued to the structural designer as a builder's work item, because a hole cut later without approval is a structural problem as well as a services one. On most projects this coordination stage is the single largest programme risk in the mechanical package - a clash found in the model costs a drawing revision, and the same clash found above a ceiling costs a section of duct, a scaffold, a delay to the ceiling contractor and an argument.

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   Step 2: Fabricate off site to the released drawings

   Once the model is frozen, fabrication is released. Rectangular duct is cut, folded and joined into sections with a flanged jointing system; circular duct is rolled or bought as spiral-wound tube with fittings; flat oval sits between the two and is used where the void is shallow but the airflow suits a round profile. Each section is marked with its drawing reference and its position in the run, and delivered in sequence. Sealants and gaskets are selected against the duct classification the designer has specified, and the fabricator works to the tolerances and stiffening arrangements that classification calls for. Changing anything after release means changing it in the model first and then in the shop - never on the floor with an angle grinder.

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   Step 3: Set out and install the supports

   Support positions are set out from the coordinated drawings before any duct goes up, and the supports are installed as a system: drop rods or threaded hangers fixed into the structure, bearers under the duct, and anti-vibration provision where the design calls for it. Spacing, bearer size and fixing type all come from the specification and the supplier's support design against the duct size and weight, not from what looks about right. Fixings into concrete are proof-tested where the specification requires it. Long horizontal runs, vertical risers and anything carrying insulation and cladding on top of the metal are all separate support cases, and the risers in particular need support at the floor structure rather than hanging off the run below.

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   Step 4: Erect the runs and make the joints

   Duct sections are lifted into the supports and jointed working from the plant outwards, so that the main runs establish the level and the branches pick up from them. Joints are pulled up with the specified cleats or clamps, gasketted or sealed, and checked as they are made - a joint above a ceiling that is already boarded is not going to be improved later. Open ends are capped at the end of every shift, both to keep the system clean and to stop the run becoming a rubbish chute for the following trades. Flexible connections go in at plant items so the fan does not shake the duct system, and terminal connections are left off until the ceiling grid is set out.

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   Step 5: Fit fire dampers, access panels and insulation

   Wherever a duct crosses a fire compartment line, a damper is installed in the wall or floor construction in the arrangement the fire strategy and the damper manufacturer require, with its builder's work opening made good around it and its position recorded. Access panels are fitted at the positions the designer has scheduled - at dampers, at fans, at attenuators and at intervals along runs so that the system can be inspected and cleaned in service. Panels above a ceiling are worthless if the ceiling has no matching access, so the panel positions are coordinated with the ceiling contractor. Thermal and acoustic insulation, and any external cladding, then go on to the completed and inspected metalwork.

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   Step 6: Test for leakage and clean the system

   Ductwork leaks, and the specification states how much leakage is acceptable for the class of duct installed. Testing is carried out on sections as they are completed rather than on the whole system at the end, so that a failure can be traced to a manageable length of run and put right while access still exists. The specialist contractor pressurises the sealed section with a test fan and measures the flow needed to hold pressure, and the result is assessed against the criteria the designer set. The system is then cleaned to the standard of cleanliness the specification calls for, sealed, and handed to the commissioning team.

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   Step 7: Balance, commission and record

   With terminals fitted and the ceiling in, the commissioning engineer sets the dampers and terminal devices so that each part of the system delivers the airflow the design intends, and records what was achieved against what was specified. Fire dampers are proved to operate and their positions and references are logged. The record package handed over includes as-installed drawings, the duct classification, the leakage test results, the damper schedule with locations, the balancing figures and the cleaning report. On a building that will be maintained for decades, an accurate damper and access panel schedule is worth more than almost anything else in the file.