Specifications of Ducting work of HVAC

The below details contains specifications of Ducting work of HVAC i.e. designing, supplying, fabricating, installing, and testing all sheet metal ducting systems, ensuring seamless air distribution through proper balancing of grills and diffusers. This scope aligns with specified requirements detailed in the bill of quantities and project specifications. After contract award, the contractor is tasked with preparing detailed working and shop drawings that will:

• Outline air flow rates through each diffuser and grill.
• Specify system static pressure available for ducting and air distribution.
• Include the dry bulb temperature of air at the main branch and grill/diffuser outlet.

The design, manufacture, inspection, and testing of ducting systems must comply with rigorous standards to ensure durability, performance, and alignment with project goals.

Rectangular Duct Construction Guidelines

Duct fabrication involves the creation of straight sections, tapers, elbows, branches, terminal boxes, and other necessary components. All items must be factory-fabricated and adhere to DW 143 standards.

1. Longitudinal Joints (Seams)

• These joints must be confined to two diagonally opposite edges.
• Machine-formed joint types permitted:
  • Pittsburgh Lock Type.
  • Button Punch Snap Lock Type.
• Each joint must withstand 1.5 times the maximum operating pressure without experiencing deformation or failure.

2. Transverse Joints

• Transverse joints should also endure 1.5 times the maximum operating pressure without deformation or failure.
• Where transverse joints act as reinforcement, allowable deflection is limited to:
  • 0.25 inches (6.25mm) for ducts up to 48 inches (1220mm) in width.
  • W/200 for ducts exceeding 48 inches in width (W = duct width).

3. Reinforcement Spacing

The spacing of transverse joints and reinforcements shall comply with project-specific guidelines, ensuring structural stability and system performance.

The spacing of transverse joints and reinforcements shall be as furnished below:

Where angle iron flanges are used:

• The metal duct shall be lapped by minimum of 6 mm across the flanges.
• All flanges shall be applied with 2 coats of Zinc Chromate, silver, or Zinc paint before erection. (In case our engineer found usage of Red Oxide, then the MS Flanges will not be paid for at all.)
• Food grade rubber or neoprene gasket of minimum 3-mm thickness shall be used for ducts with up to 1500mm longer side and for larger ducts minimum 4mm or more thick gaskets shall be used between duct flanges in all duct installation.
• Flanges shall be welded electrically, and all holes shall be drilled. Maximum drill hole spacing shall be 150mm.
• Factory-made slip-on Flanges shall be used.

CROSS BRACING OR BEADING

Beading is preferred to cross bracing. These shall be provided where no bracings are provided.

SEALING OF DUCT

Silicon sealant to be used as fillers in grooves of longitudinal seams. Permissible norms for

Leakage’s shall be as per DW 143.

Standard Duct Sealing requirement.

Support Systems for Ducting:

To ensure the structural integrity and reliability of the ducting system, the support materials and their configurations must adhere to the following guidelines:

General Requirements

• Material:
  • All support materials must be either galvanized or coated with aluminum paint to prevent corrosion and extend the system’s lifespan.
• Design:
  • Supports must be designed to handle the weight of the ductwork, including the effects of airflow and potential thermal expansion.
• Spacing and Placement:
  • The supports must be installed in accordance with the specifications outlined in the following table to ensure stability.

Comprehensive Guidelines for Fabrication, Installation, Testing, and Maintenance of Ducting Systems:

Fabrication Equipment and Processes:

All operations, including cutting, folding, notching, beading, and shearing, must be performed using precision machinery to ensure consistency, accuracy, and durability in ductwork fabrication.

Duct Installation Guidelines

• Adherence to Drawings:
  Ducts must be installed in strict accordance with approved execution drawings. Any deviation due to site conditions must be resolved with the Client/Consultant’s site representative to ensure compliance and functionality.
• Erection Standards:
  • All ducts must be installed neatly and securely, with independent supports fixed to the building structure.
  • Horizontal ducts should have MS hangers coated with silver paint or zinc chromate for corrosion resistance, spaced closely to prevent sagging.
• Flexibility and Adjustments:
  • Provisions must be made to accommodate beams, pipes, and other obstructions.
  • Ducts should be transformed, divided, or curved as needed, maintaining the required cross-sectional area.
• Vibration Isolation:
  Ducts connected to fans or air handling units (AHUs) must include flexible fire-resistant connections (PTFE double canvas sleeves of at least 150 mm width) to mitigate vibration and allow temperature monitoring.
• Leakage Testing:
  All duct systems must undergo smoke/pressure testing in accordance with DW 144 standards, ensuring minimal leakage and optimal performance.
• Supporting Arrangements:
  The contractor must provide detailed methods for support systems, adhering to approved standards.

Duct Cleaning Procedure

To maintain hygiene and performance, the following steps must be followed:

• Store GI sheets in a clean, dry area, elevated from the ground.
• Clean ducts thoroughly using lint-free dry mops; use wet mopping if necessary.
• Block open duct ends with non-particle-shedding materials (e.g., plywood) to prevent dust ingress after erection.
• Obtain approval for cleaning and installation from the client’s representative, maintaining detailed records.
• Provide visual inspections for approval and attach manufacturer test certificates with each consignment.

Testing of Ducts

The installed duct system must be tested for leakage as per DW 143 standards:

• Maximum Allowable Leakage: Less than 1% of supply air quantity or 0.17 liters/sec/m² at 500 Pascal pressure, whichever is higher.
• Use fans and flow-measuring devices for testing supply and return ducts

Air Balancing Procedure

• Open collar dampers for grilles and diffusers.
• Operate AHUs and measure airflow with a digital anemometer.
• Adjust air quantities using dampers and, if necessary, modify the drive set.
• Fine-tune airflow through branch dampers, collar dampers, and diffusers.
• Record and tabulate air quantities for all diffusers and grilles.
• Measure and record noise and vibration levels near the AHU and diffusers.
• Update drawings with final air quantity, temperature, relative humidity (RH), and sound levels.

Notes:

• Reading Guide- For duct sizes between, say, 651 mm and 700 mm, when the pressure class is 1″ WG. static, we require a Standard” E’ class flange and duct gauge of 26. For the same size range but with static pressure at 4″ WG. a Standard ‘H’ class flange with duct gauge of 24 should be used.

• The Standard flange classes available are designated E, H and J. For E & H class of Standard gasket size 10 mm wide and 4.5 mm thick. For Standard J-class use 15 mm wide and 6 mm thick gasket.

• For pressure class 6″ WG static and above contact Standard to confirm the gasket type & size requirement.

• (Not Applicable for Current Specifications) For non-critical comfort cooling applications (1″ WG pressure class), optional “C & S” or “C & SS” cleat joints can be used.

• • 451 to 750 mm duct size use “C & SS” cleats.
  • Over 750 mm duct size use Standard flanges.
  • Upto 450 mm duct size use “C & S” cleats.
• A higher class flange can always be substituted for a lower class (e.g. class” J” for class “H”, class “H” for class “E”).

Specifications for the Duct Construction:

The fabricated duct dimensions should be as per approved drawings and all connecting sections are dimensionally matched to avoid any gaps.

• Dimensional Tolerances: All fabricated dimensions will be within +/- 1.0mm of specified dimension. To obtain required perpendicularity, permissible diagonal tolerances shall be +/-1.0 mm per meter.
• Each duct piece should be identified by color coded sticker which shows specific part numbers, job name, drawing number, duct sizes and gauge.
• Ducts shall be straight and smooth on the inside. Longitudinal seams shall be airtight and at corners only, which shall be either Pittsburgh or Snap Button Punch as per DW 143 practice, to ensure air tightness.
• Changes in dimensions and shape of ducts shall be gradual (between 1:4 and 1:7). Turning Vanes or air splitters shall be installed in all bends and duct collars designed to permit the air to make the turn without appreciable turbulence.
• Plenums shall be shop/factory fabricated panel type and assembled at site.
• The deflection of transverse joints should be within specified limit for rectangular duct deflection as given DW 143.
• Reinforcement of ducts shall be achieved by either cross breaking or straight beading dep ending on length of ducts.

“Duct Sizes 19″ (483 mm) wide and larger which have more than 10 sq.ft. of unbraced panel shall be beaded or cross broken unless ducts will have insulation covering or acoustical liner. This requirement is applicable to 20 g (1.00 mm) or less and 3″ W.G. (750 Pa) pressure or less. Ducts for 4″ W.G. (1000 Pa) or more do not require beads or cross-breaks.”

What causes noise in duct

Installation Instructions for the Site work

Tools and tackles for site work

• The duct installation shall conform to DW 144 norms. For duct assembly and installation, the use of suitable tools and tackles should be used to give the required duct quality and speed of installation including (but not restricted to).

• Electric Pittsburgh Seamer – used for closing Pittsburgh joints.
  • Electric Slitting shear – to make cut-outs.
  • Drilling machine with drill bits – for drilling holes in sheet metal work.
  • Hammer drill machine with drill bits – for drilling holes in building structures for anchors.
  • Hoisting system – for lifting the duct assembly up to mounting heights.

INSTALLATION PRACTICE AT SITE:

• All ducts shall be installed as per tender drawings and in strict accordance with approved shop drawings to be prepared by the Contractor.

• The Contractor shall provide and neatly erect all sheet metal work as may be required to carry out the intent of these specifications and drawings. The work shall meet with the approval of Owner’s site representative in all its parts and details.
• All necessary allowances and provisions shall be made by the Contractor for beams, pipes, or other obstructions in the building whether the same are shown on the drawings. Where there is interference/fouling with other beams, structural work, plumbing and conduits, the ducts shall be suitably modified as per actual site conditions.
• Ducting over false ceilings shall be supported from the slab above, or from beams. In no case shall any duct be supported from false ceilings hangers or be permitted to rest on false ceiling. All metal work in dead or furred down spaces shall be erected in time to occasion with no delay to other contractor’s work in the building.
• Where ducts pass through brick or masonry openings, it shall be provided with 25mm thick appropriate insulation around the duct and totally covered with fire barrier mortar for complete sealing.
• All ducts shall be totally free from vibration under all conditions of operation. Whenever ductwork is connected to fans, air handling units or blower coil units that may cause vibration in the ducts, ducts shall be provided with a flexible connection, located at the unit discharge.
• The truck arrives with duct profiles.
• Unload and stock the profiles in the near marked place as recommended in Instruction to Contractors.
• Identify the profiles and take the profiles for boxing as per priority.
• Use Electric Seam closer and other tools recommended on above for closing the longitudinal seams.
• Fix MS flange to the ducting as recommended in the MS Flange brochure.
• Stack the ducts vertically.
• Make opening for collars with electric slitting shear/nibbler on the ducting.
• Fix round collars for connecting to diffuser boxes in the L shaped duct profiles prior to boxing up the ducting.
• Install support system as recommended in the earlier Support system.
• Install the ducting and do leveling.
• Make opening on the ducting with electric slitting shear/nibbler for branch duct connection and balance collars if any.
• Install branch ducting and collars along with progress of false ceiling.
• Install all duct accessories as shown on the above wherever applicable.
• Install diffuser boxes and level.
• Connect the diffuser boxes through flexible ducting and fix up the clamps.
• Install collar dampers and fix the grilles.
• Install the collar dampers for diffusers within the diffuser box, fix the diffuser to the diffuser boxes and align with false ceiling.
• Take joint measurements and bill progressively. Commission the system.

FRP coating over MS Flange Joints of Ductings

• 1 mm thick Glass Fiber Cloth of width to cover the flange joints with suitable overlap over ducting surface on either side to be applied on the duly cleaned dust free flange joints, bonded to the ducting surface together with resin and hardener, to form impervious leak proof seal across the flange lip.

GRILLS AND DIFFUSERS.

• All grilles (SA & RA), diffusers (SA & RA) shall be made from 1.0 mm extruded Aluminum sections, powder coated or anodized to match the interiors. All duct collars terminating on to grille or diffuser shall be given two coats of black paint for a minimum length of 300 mm. Grilles and diffusers shall be selected for an aerodynamic noise power value not more than NC35. All supply and return air grilles/ diffusers will be provided with opposed blade dampers fabricated from aluminum section painted in two coats of matt black stoving enamel paint. The dampers should be suitable for operation from the front face of grille / diffuser. The Supply Grille/ Diffuser should match with Return Grille/ Diffuser with respect to color, size & finish.

• All supply air diffusers shall be equipped with fixed air distribution grids, removable key operated volume control dampers and anti-smudge rings. (The Grills and Diffuser can be fabricated in SS 304 18swg Quality SS Sheets). Sample of each Grille and Damper to be fixed at site for client / consultant approval. Manufacturing of Grill and Damper to be done only after approval from client/ consultant.

• Except outer frame center core of diffuser / Grill should be removable with any tool for cleaning purpose.

Linear Grilles and Diffusers shall be of extruded aluminum construction, flushed mounted type with single or double directional air flow. The diffuser/ Grilles shall be in a flange with minimum 20 mm margin. Capped air-flow connections shall be provided as necessary for testing and balancing of air distribution

INSULATION FOR DUCTING WORK

Insulation with Cross-Linked Polyethylene Foam

When insulating ducts with closed-cell Cross-Linked Polyethylene Foam, the following steps and specifications should be adhered to:

• Material Specifications:
  • The insulation material must be closed-cell Cross-Linked Polyethylene Foam with a thermal conductivity not exceeding 0.036 W/Mk at an average temperature of 40°C.
  • The material should meet the bending trial and dimensional stability requirements as per DIN 51949 and DIN 53431 for an operating temperature range of -40°C to +110°C.
  • The density of the material must be 30 ± 3 Kg/m³ or 0.30 g/cc.
• Performance Standards:
  • The smoke density of the material must not exceed 1 as per AS-1530.3.
  • The water vapor permeability should remain below 0.15 ng/m·s·Pa, as per DIN 52615.
  • The material must have a fire safety approval from CBRI, Fire Advisor (Govt. of India), or the Chief Fire Officer.
• Cladding for UV Protection:
  • To ensure UV protection, the insulation must be cladded with a minimum 30-micron aluminum P.E. foil.
  • The cladding should be factory-finished to minimize site work.
  • The insulation material can be provided with a self-adhesive layer covered by a peel-off paper as an alternative.

Insulation with Nitrile Rubber for Duct Applications

For ducts insulated with Nitrile Rubber, the following application process must be followed:

• Preparation:
  • Thoroughly clean the duct surfaces before applying the insulation.
  • If the insulation is not self-adhesive, an appropriate adhesive must be uniformly applied to the insulation and cured before attaching it to the duct.
• Application:
  • The insulation should be wrapped around the duct as a single piece whenever possible.
  • For larger ducts, cut the insulation to the exact width or height of the duct and apply it neatly.
• Joint Sealing:
  • Overlap insulation joints with self-adhesive tape made of the same material.
  • The tape must be at least 2.5 mm thick and 50 mm wide to ensure a secure seal.
• Flange Insulation:
  • Apply a self-adhesive strip of suitable thickness from the same material around the entire height of the flanges.
  • Cover the flange joints with insulation to minimize heat loss and seal the connection securely.

These processes ensure effective thermal insulation, reduce energy loss, and enhance the overall efficiency of ducting systems.

LIST OF BUREAU OF INDIAN STANDARDS CODES

• Following relevant IS codes shall apply read in concurrence with there latest amendments.
  IS:226-1975 Specification for structural steel
  IS:277-1992 Specification for galvanised sheet (plain and corrugated)
  IS:325-1978 Specification for three phase induction motors
  IS:554 – 1975 Dimensions for pipe threads where pressure tight joints are required on the
  threads.
  IS:655-1963 Specification for metal duct
  IS 659-1964 (1991) Safety code for air-conditioning (resived)
  IS:660-1963 (1991) Safety code for mechanical refrigeration
  IS:778-1984 Specification for copper alloy and gate , globe & check valves for water
  works
  IS:780-1984 Specification for sluice valves for water works (50 to 300 mm size)
  IS:800-1984 Code of practice for general construction in steel
  IS:808-1964 Specification for rolled steel beam channel and angle section
  IS:816-1969 Code of practice for metal arc welding for general purpose in mild steel
  IS:823-1964 Code of procedure for manual metal arc welding of mild steel
  IS:1239-1979 (Part 1) MS tubes,tubulars and other wrought steel fittings
  1990
• IS:1239-(Part 2) –1992 MS tubes tubulars and other wrought steel fittings
  IS:1536 – 1976 Flanges configuration
  IS:1554-(Part 1) –1976 Specs for PVC insulated (heavy duty electrical cables)
  IS:2253-1974 Designation for types of construction and mounting arrangement of rotating
  electric machine.
  IS:2312-1967 Specs for propeller type AC ventilating fans
  IS:2379 – 1963 Colour code for the identification of pipelines
  IS : 3103-1975 Code of practice for Industrial Ventilation
  IS 4064 – (Part -II) 1978 Specific requirements for the direct switching of individual motors.
  IS: 4736 – 1968 Hot-dip zinc coatings on steel tubes
  IS: 4894-1987 Test Code for Centrifugal Fan.
  IS : 7240-1981 Application & Finishing of thermal insulation material
  IS:8544 (Part-I to IV)
  1979 Starters
  IS:9224 (Part II) – 1979 HRC cartridge fuse links upto 650 volts.
  IS:3069-1965 Glossary of terms, symbols and unit relating to thermal insulation material
  IS:3346-1980 Method for the determination of thermal conductivity thermal I
  insulation materal ( two slab, guarded hot plate method)
  IS:3588-1966 Specification for electric axial flow fans
  IS:3589-1981 and 1991 Seamless or electrically welded steel pipes for water, gas and sewage (168.3
  to 2032 mm outside dia)
  IS:3724-1966 Specs for cartridge type heating elements (non embedded type)
  IS:4158-1967 Specs for solid embedded type electric heating elements
  IIS:4671-1984 Specs for expanded polystyrene for thermal insulation purpose
  IS:4691-1984 Degree of protection provided by enclosure for rotating electrical machine
  IS:4722-1968 Specs for rotating electrical machine
  IS:4729-1968 Measurement and evaluation of vibration of rotating electrical machine.
  IS:4831-1968: Recommendation on units and symbols for Refrigeration
  IS:4894-1987 Specs for centrifugal fans
  IS:5111 -1993 Testing of Refrigerating compressors.
  IS:5512:(Part 1) –1984 Specs for swing check type (non return) for water works purposes.
  IS:6272-1971: Specs of industrial cooling fans
  IS: 6392-1971 Specs for steel pipe flanges
  IS:6168-1976 Code of practice for treatment of water for industrial cooling system
  IS:7616-1975 Method of testing panel type air filters for air conditioning and ventilation
  purposes
  IS;8623 1977 Specs of factory built switch / control section.
  IS:8623(Part3) 1993: Specs for low voltage switchgear and control gear assemblies
  IS: 8789- 1978 Values of performance characteristics for three phase induction motor
  IS:9137-1978 Code for acceptable tests for centrifugal, mixed flow and axial pumps class
  C

Frequently Asked Questions

What is ductwork in HVAC?

Ductwork in HVAC refers to the system of metal or flexible conduits used to distribute and circulate air—such as heated or cooled air—throughout a building. It is an integral part of heating, ventilation, and air conditioning systems, connecting air handlers or furnaces to supply registers and return grilles.

What are standard HVAC duct dimensions?

Standard duct dimensions vary based on system requirements but commonly include:

• Rectangular ducts: Sizes like 12″x6″, 24″x12″, and 36″x18″.
• Round ducts: Diameters such as 4″, 6″, 8″, and 12″.
  Duct dimensions are typically selected based on airflow requirements and space constraints.

What is the rule of thumb for HVAC ducting?

A general rule of thumb is to provide 1 CFM per square foot of space. This assumes standard ceiling heights and average insulation levels. The duct size is adjusted to ensure proper airflow with minimal pressure loss.

How many types of ducts are in HVAC?

HVAC ducts are categorized into:

• Rigid ducts: Made of galvanized steel, aluminum, or fiberglass.
• Flexible ducts: Lightweight and easily adjustable, often used in tight spaces.
• Fabric ducts: Used for even air distribution, typically in commercial applications.

How to repair an HVAC duct?

• Locate the damaged section.
• Clean the area and remove loose materials.
• Patch holes using duct tape, mastic sealant, or metal patches.
• Insulate the repaired section if needed to prevent heat loss or condensation.

How to calculate duct CFM?

CFM (Cubic Feet per Minute) is calculated using:
CFM=Air Velocity (FPM)×Duct Area (sq. ft.)
where duct area is derived from the duct dimensions.

What is CFM in HVAC?

CFM stands for Cubic Feet per Minute and measures the volume of air flowing through a system. It is a key parameter for ensuring adequate ventilation and maintaining indoor air quality.

How to measure HVAC duct?

Measure the height and width of rectangular ducts or the diameter of round ducts using a measuring tape. For area calculation, use the formula for the shape (rectangular or circular).

How to calculate duct area?

• Rectangular duct: Area (sq. ft.)=Height (ft)×Width (ft)\text{Area (sq. ft.)} = \text{Height (ft)} \times \text{Width (ft)}Area (sq. ft.)=Height (ft)×Width (ft)
• Round duct: Area (sq. ft.)=π×(Diameter (ft)2)2\text{Area (sq. ft.)} = \pi \times \left(\frac{\text{Diameter (ft)}}{2}\right)^2Area (sq. ft.)=π×(2Diameter (ft)​)2

How many CFM per sq ft?

The typical guideline is 1 CFM per square foot, although this can vary based on ceiling height, insulation, and climate.

How do you calculate HVAC requirements?

• Determine the room area in square feet.
• Calculate the total heat load (sensible and latent).
• Use the Manual J calculation or equivalent to size the HVAC system appropriately.

How do you test HVAC ducts?

• Use a duct blaster test to measure leakage.
• Perform a smoke test to detect leaks.
• Measure airflow at registers with a balometer or anemometer.

What is used to seal HVAC ductwork?

• Mastic sealant
• Metal foil tape (not standard duct tape)
• Butyl rubber tape

How do you install HVAC ducts?

• Plan the duct layout and prepare drawings.
• Cut and assemble ducts using connectors.
• Secure ducts to the building structure with supports.
• Seal and insulate as required.

What is the CFM formula?

CFM=FPM (Velocity)×Area (sq. ft.)

What is the formula for duct size?

Duct Size (sq. ft.)=CFM / Velocity (FPM)

What is FPM in HVAC?

FPM (Feet Per Minute) is the measure of air velocity through ducts. It is used to calculate airflow and determine duct sizes.

How to convert ACH to CFM?

CFM=ACH × Room Volume (cubic feet)
ACH is the number of air changes per hour.

What is the formula for duct flow?

Flow (CFM)=Area (sq. ft.)×Velocity (FPM)

What is the full form of CFM?

CFM stands for Cubic Feet per Minute.

How many CFM per ton?

A typical rule is 400 CFM per ton of cooling capacity.

How to calculate ESP for duct?

ESP (External Static Pressure) is calculated as the sum of pressure losses in the duct system, fittings, and equipment.

How to convert FPM to CFM?

CFM=FPM × Duct Area (sq. ft.))

How many CFM is a 12×12 duct?

For a 12″x12″ duct:
Area=1 sq. ft.
If velocity is 500 FPM:
CFM=500×1=500 CFM

How to calculate ACH in HVAC?

ACH= CFM × 60 / Room Volume (cubic feet)

What is static pressure in HVAC?

Static pressure is the resistance to airflow in the duct system, typically measured in inches of water column (in. w.c.).

What is the formula for CFM?

CFM=Velocity (FPM)×Area (sq. ft.)

How to select duct size?

Duct size is selected based on the required CFM, available static pressure, and allowable velocity using duct sizing charts or software.

How to test duct airflow?

Use a balometer, anemometer, or pitot tube to measure airflow at registers and ducts.

What is the formula for ducting calculation?

Duct Area (sq. ft.)=CFM / Velocity (FPM)

How many CFM for 1000 sq ft?

For typical HVAC sizing:
CFM=1 CFM/sq. ft.×1000 sq. ft.=1000 CFM

How to calculate AHU CFM?

Calculate AHU CFM by summing the airflows for all connected ducts and terminals:
AHU CFM=Sum of terminal CFM values values

ASRAE’s DUCT SYSTEMS DESIGN GUIDE