Mechanical Ductwork, Grilles And Diffusers





1. Mechanical Ductwork

The Contractor shall supply deliver and erect the whole of the mechanical ductwork system complete with dampers, supports, insulation and fixings and shall adhere to the duct runs shown on the drawings except where local conditions prevent or where variations are necessary to suit any revisions to plant layout. All leading dimensions and positions of plant shall be checked on site by the Contractor who will be responsible for ensuring that the ductwork shall conform with the equipment to be installed, also with the building details and structure.

It is the responsibility of the Contractor to submit to the Engineer for his acceptance, fully dimensioned detailed drawings of ductwork before manufacture.

All metal ducting shall be constructed of galvanised cold rolled close annealed, patent flattened steel sheets.

Low velocity low pressure air system rectangular ducts shall be manufactured and installed in accordance with Specification DW 142 as issued by the Heating and Ventilating Contractors Association, London.

Cross joints shall be constructed from profile rolled galvanised steel strip forming complete angle sections with a pressed form reinforcing bracket spot-welded to each corner. An extruded gasket shall be applied to the face of the flanges and the joint made with galvanised rolled section cleats.

High velocity high pressure air system circular ducts shall be manufactured in accordance with Specification DW 142 as issued by the Heating and Ventilating Contractors Association, London.

Bolts shall be 8mm for angles up to and including 32mm and shall be 10mm for angles above 32mm. All joints both longitudinal and lateral shall be made with an accepted sealer and shall be air-tight, to the maximum system pressure.

All ductwork exposed to external atmosphere or corrosive fumes shall be suitably painted or otherwise protected, internally and externally as required. Where ducts are lined internally for acoustic attenuation the internal area shall be maintained and the sheet metal size increased accordingly.

Flexible ducting shall be used only where specially accepted and full details and samples shall be submitted when seeking acceptance. All PVC ducting shall comply with Specification DW 151 as issued by the Heating and Ventilating Contractors Association, London. Where headroom clearances are given on the drawings or elsewhere these shall be to the lowest point of the supporting structure.

2. Mechanical Ductwork Supports

Mechanical Ductwork will not be permitted to rest directly on portions of the building except on concrete slabs or substantial brick walls and then only with the acceptance of the Engineer. Adequate thermal insulation shall be applied as necessary.

The welding or bolting of supports to structural steelwork will not be permitted without written authorisation from the Engineer.

3. Mechanical Dampers - General Requirements

Sufficient mechanical dampers shall be provided to regulate and balance the system. Dampers on grilles or diffusers shall be used for fine or secondary control only. All dampers shall be sufficiently rigid to prevent fluttering. Unless otherwise indicated the air-leakage past dampers in the fully closed position shall not exceed 5% of the maximum design air flow in the duct. All duct dampers except fire dampers and self-closing dampers shall be fitted with locking devices and position indicators.

Each primary control damper shall be fitted with a non-corrodible label stating the actual air flow in m3/s and the cross sectional area; position of a damper as set after final regulation and balancing shall be indelibly marked on the damper quadrant.

4. Flexible Joints for Mechanical Ductwork

Flexible joints shall be provided on fan inlet and outlet connections and elsewhere on the ductwork where indicated. They shall be of the full cross sectional area of the mating fan inlet or outlet or duct section. The ends of the ducts or the duct and the fan connection shall be in line.

Flexible joints shall consist of, or be protected by, material having a fire penetration time of at least fifteen minutes when tested in accordance with BS 476.

5. Air Intakes and Outlets

Fresh air intake and exhaust air louvres shall have fixed blades angled to give adequate weather protection. Unless otherwise indicated louvres shall be constructed of aluminium with blades not less than 3mm thick braced as necessary to ensure complete rigidity. Where louvres are required to be supplied under this Contract they shall be handed to the main Contractor for fixing.

A galvanised steel wire screen of 6mm mesh complete with a frame of galvanised steel rod with securing lugs or of flat iron shall be provided and fixed by the Contractor on the inner side of the louvres.

6. Kitchen Extract Canopies

Canopies are to be provided to the sizes indicated on the drawings. Canopy and valance framework shall be formed from extruded sections of aluminium alloy, jointed by welding into assemblies of convenient size for delivery to site, and/or otherwise connected by non-ferrous fastenings. All internal surfaces of the canopy and supports shall be arranged at an angle greater than 60o to the horizontal to prevent condensation drips.

At junctions between canopies and walls or ceilings the Contractor shall seal all gaps with an accepted sealant to exclude vermin. Hoods for kitchen equipment and for the extraction of condensible vapours shall have a water tight extruded section channel gutter not less than 50mm wide or more than 25mm deep (40mm x 20mm for “single appliance” canopies) at the bottom edge. Draining points shall be formed by a tapped hole into which a cleaning eye and washer (of the type fitted to sink waste traps) is screwed finger tight. Such draining points shall be provided in the gutter at intervals of not more than 6m.

Canopies shall be fixed so that the bottom edge is not more than 2.000m from floor level. They shall be supported from above by suspension rods each having a turn-buckle for levelling and load adjustment. Canopies and connected ductwork shall be so supported that no part of the weight of one is imposed on the other. The Contractor's attention is drawn to any supply ducting which is to be installed within a canopy. All joints between the canopy and ductwork shall be sealed against vermin as previously described.

7. Kitchen Extract Air Ductwork

Adequately sized cleaning doors shall be provided, regardless of whether grease filters are fixed at the entry points, so that access for interior cleaning is possible at intervals of not more than 3.00m. Precautions must be taken against possible leakage of condensate and/or grease from the ducts by avoiding longitudinal seams along the bottom face, and employing suitable gaskets material at the joints between sections.

The ducts shall be fixed with a slight fall, preferably in the direction of air flow, and at low points such as at the bottom of risers there should be plugged drainage points suitable for the later connection of a permanent trapped drainage pipe if operating conditions demonstrate the need.

8. Access Doors

Access doors shall be provided adjacent to all items of equipment requiring maintenance or inspection. Access doors having the dimensions up to and including 460mm x 460mm shall be of the bolt-on-type manufactured from 1.65mm thickness galvanised sheet steel, secured to 32mm x 6mm flat iron frames riveted to the ductwork.

Access doors having dimensions greater than 460mm x 460mm shall be of the hinged type manufactured from 1.65mm thickness galvanised sheet steel suitably stiffened. The doors shall be provided with robust hinges on one vertical side and a suitable handle operated lock on the other vertical side. All hinges and handle operated locks shall be cadmium plated or treated with a similar corrosion resistant finish. All access doors shall be made air tight by the use of plastic or felt strip.

9. Ductwork Test Points

At points throughout the ductwork system, test points suitable for pitot tube insertion shall be provided in accordance with the HVCA DW 142 and the C.I.B.S. commissioning code “A” on two adjacent sides of rectangular ducts as determined by the access available. They shall consist of 25mm diameter holes, fitted with a cover secured by large headed set screws, one hole being slotted to allow the cover to swing clear without being removed. The use of plastic or rubber plugs will not be accepted. Test points shall be positioned at points where the air flow is stable and shall include each branch duct as well as suitable locations in the main duct.

10. Ductwork Protection

804.10 Any part of galvanised ductwork where the galvanising is damaged during manufacture or erection shall be painted with two coats of aluminium, zinc rich or other corrosion-resisting paint. In kitchens and dining rooms the outer-most surface of exposed ductwork or insulation shall be not less than 30mm away from walls and ceilings to permit access for cleaning.

11. Grilles

Grilles shall be of steel or aluminium as indicated. Steel grilles shall be protected against rusting and supplied in fully finished condition. Grilles shall be not less than the size indicated; where no size is given they shall be capable of dealing with the air flow and distribution as indicated. All grilles except those on exposed ducting shall incorporate an edge seal.

The drawings show provisional locations but in each instance outlets shall be installed in accordance with architectural final detail drawings and reflected ceiling plans.

Velocities, nett airways and distribution patterns shall give satisfactory air distribution, and be free of droughts, stratification or noise nuisance. The Contractor shall make final adjustments to air patterns when balancing.

Damper assemblies and internal surfaces of outlets and take-off ductwork where visible through outlet faces, shall be painted matt black.

All outlets shall be fitted truly square and in alignment. Perimeter trims shall seat hard against wall or ceiling finishes as applicable and shall properly mask the trimmed openings. Unless otherwise accepted, all fixings shall be concealed.

Each supply air grille shall be complete with an opposed blade multi-leaf damper. Alternatively, in lieu of the opposed blade multi-leaf damper a rhomboidal air controller may be provided; this air controller shall control both the volume of air passing and the distribution of air across the grille face. Each supply air grille shall have two sets of separately adjustable louvres one set horizontal and one set vertical, all adjustable from the front or the grille. For up to ten grilles, one set of any tools required for adjusting the louvres and dampers or air controllers shall be provided; from eleven to twenty-four grilles, two sets; above twenty-five grilles, three sets.

Return air grilles may have either a single set of louvres or bars (either vertical or horizontal) or a lattice or egg crate front, as indicated in the Schedules. Each return air grille shall be complete with an opposed blade multi-leaf damper or a rhomboidal air controller operable from the front.

For Kitchens and Dining Rooms both supply and return air grille shall have quick-release sub-frames.

12. Diffusers

Diffusers shall be of steel or aluminium and supplied in a fully finished condition. Diffusers, except on exposed ductwork, shall incorporate an edge seal; diffusers mounted on ceilings shall have anti-smudge rings. Pan type diffusers may be provided except where cone type diffusers are indicated.


Typical Installation details of Mechanical Plenum box and Diffusers in Mechanical MEP contract work
Typical Installation details of Mechanical Plenum box and Diffusers in Mechanical MEP contract work


Circular diffusers shall be provided with volume control dampers of the iris, flap or sleeve type. Rectangular diffusers shall be provided with opposed blade dampers. Dampers shall be adjustable from the front of the diffuser. Where the length of a vertical duct to a diffuser is less than twice the diameter of the diffuser an equalizing deflector shall be fitted.

For Kitchens and Dining Rooms diffusers shall have a quick-release centre to facilitate easy cleaning.

Linear diffusers shall include a control damper at the rear of the vanes giving volume control down to complete shut-off and operated from the face of the diffuser. Linear diffusers for supply air shall have adjustable blades to give directional control of air flow. Where linear diffusers are mounted in a continuous line there shall be means of ensuring alignment between consecutive diffusers and of equalizing pressure behind the vanes.

13. Sand Trap Louvres

Sand trap louvres shall be constructed from aluminium sheet, 1.5mm thick for blades and 2.0mm thick for casing, with mill finish. The base member of the box shall incorporate sand drain holes. The louvres shall be of the labyrinth type designed to achieve velocity changes of the air passing through it inducing the sand to drop out.

The louver shall incorporate a galvanized wire mesh insect screen at the rear.

14. Air Valves

Extract air valves shall be of the sizes indicated on the Schedules and be manufactured from pressed steel sheet with stove enamel finish. The valve shall be supplied complete with a mounting ring of the bayonet fitting type, and screw adjustment with locknut for volume control.

15. Fire dampers

Fire dampers shall be provided in each duct passing through all structural fire compartment walls and each floor level slab, and also where shown on the Air Conditioning and Ventilation drawings and shall be of the spring loaded stainless steel folding shutter type, unless otherwise accepted in writing by the Engineer.


Mechanical Fire Damper installation detail in MEP Mechanical contract work
Mechanical Fire Damper installation detail in MEP Mechanical contract work


The fire dampers shall have a fire rating at least equal to the barrier through which they are passing and in any event shall be at least two hours (BS 476 Part 1).

The damper shall comprise a casing fabricated from not less than 1.2mm thick galvanized rolled steel section and 0.8mm stainless interlocking hinged blades tensioned by two constant force coiled band stainless steel springs.

Damper blades shall be held in the open position by a fusible link to fuse at 69oC, care being taken to ensure that on fusing the links and fixings shall not prevent complete seating of the damper blades. The damper blades shall be out of the airflow when open. External indication shall be provided to indicate fire damper "open" and fire damper "closed".

All parts used in the construction of fire dampers shall be galvanized after manufacture or stainless steel.

Access doors shall be provided in the ductwork adjacent to all fire dampers to permit inspection and replacement of fusible links. The fire dampers shall have a free area of not less than 90% of the cross sectional area of the ducts to which they connect, and if necessary, the fire damper casings shall be increased in size above that of the ducts in which they are installed to comply with this requirement. All fire dampers shall be built into the appropriate fire compartment walls strictly in accordance with the manufacturers instructions. All fire dampers shall be to the approval of the relevant local Fire Authority.