IS 655 : 2018Code of Practice for Mechanical Ventilation and Air Conditioning of Buildings

IS 655:2018 is the specification for metal air ducts — the sheet-metal ductwork (galvanised steel, aluminium) that distributes conditioned/ventilation air in HVAC systems. It governs sheet thickness (gauge) by duct size and pressure class, construction, joints/seams, stiffening, hangers/supports, and leakage class. It is the fabrication-and-quality standard a services engineer specifies HVAC ducting to and a commissioning team checks against.

*(Note: the code-database title for this entry currently reads 'Code of Practice for Mechanical Ventilation and Air Conditioning of Buildings'. IS 655's established subject is the metal air duct specification; the discrepancy has been flagged for correction. For the system design code, see SP 7 / NBC 2016 Part 8 and IS 659/IS 3103.)*

It is read with the HVAC/ventilation stack:

• NBC 2016 Part 8 Section 3 / SP 7 (NBC) HVAC — air-conditioning and ventilation system design
• IS 659 — safety code for air conditioning · IS 3103 — industrial ventilation
• IS 277 — galvanised steel sheet (the duct material)

The construction rules that drive a compliant duct:

• Sheet gauge by duct dimension and pressure class — larger ducts and higher static pressure require thicker sheet; a schedule maps the longer side / pressure to minimum thickness
• Seams and joints — types of longitudinal seams and transverse joints (e.g. slip-and-drive, flanged/cleat) appropriate to the pressure class, sealed for the leakage class
• Stiffening / cross-breaking — bracing of large flat panels to prevent drumming and deflection
• Hangers and supports — spacing and sizing so the duct does not sag or stress joints
• Transformations, bends, branches — radius/turning-vane requirements to control pressure loss and noise
• Air-leakage class — permissible leakage as a function of pressure (the acceptance test at commissioning)

Gauge selection follows the duct size + pressure — not the fabricator's stock sheet.

Brief: a low-pressure supply duct, longer side ~ 900 mm, rectangular GSS.

Step 1 — pressure class: confirm system static pressure → low-pressure class for AHU supply.

Step 2 — gauge: from the IS 655 thickness schedule, a ~900 mm longer-side low-pressure duct calls for the corresponding minimum sheet thickness (heavier than for a 450 mm duct) — pick the scheduled gauge, not lighter stock.

Step 3 — stiffening: the large flat sides are cross-broken / angle-stiffened at the scheduled spacing to stop panel drumming.

Step 4 — joints: transverse joints of the type permitted for the pressure class, sealed to meet the leakage class.

Step 5 — supports: hangers at the scheduled spacing, sized for the duct weight + insulation, so joints aren't stressed.

Step 6 — acceptance: duct leakage tested at commissioning against the IS 655 leakage class for the system pressure — a visually neat duct can still fail leakage.

1. Under-gauge sheet for the size/pressure. Using a fabricator's standard light gauge on large or higher-pressure ducts → drumming, deflection, leakage and noise.

2. Ignoring the leakage class. Unsealed seams on a 'good-looking' duct lose conditioned air and energy; leakage is an *acceptance test*, not an afterthought.

3. No stiffening/cross-breaking on large panels. Flat sheet sides resonate and deflect under pressure cycling without the scheduled bracing.

4. Inadequate or wrongly-spaced supports. Sagging ducts stress joints and pull seams open over time.

5. Sharp bends/no turning vanes. Poor transformations spike pressure loss and noise — defeating the fan/energy design even if the sheet metal is sound.

• NBC 2016 Part 8 Section 3 — air-conditioning, heating and mechanical ventilation
• SP 7 (National Building Code) — HVAC · IS 659 — safety code for air conditioning
• IS 3103 — industrial ventilation · IS 4894 — centrifugal fans
• IS 277 — galvanized steel sheet (duct material)
• IS 14705 — flexible ducts · IS 11799 — sandwich-type air ducts (insulated panel ducts)
• ECBC — duct insulation and leakage requirements (energy)

IS 655 is the construction/quality spec for sheet-metal ducting and is reaffirmed; on large commercial projects designers often *also* invoke SMACNA duct-construction standards (gauge/seam/leakage classes), which are compatible in approach and frequently written into specs alongside IS 655 — that is acceptable when cross-referenced.

Two practitioner points. First, the page/database title for this entry appears mis-described as a *system design* code of practice — IS 655's actual scope is the metal air duct specification; rely on the duct-spec subject and use SP 7/NBC Part 8 for system design. Second, the costly field outcomes are duct leakage and noise, not collapse — so the disciplines that matter are correct gauge-by-pressure, sealed joints to a stated leakage class with a commissioning leakage test, proper stiffening and supports, and good aerodynamic transformations. Specify the pressure class and leakage class explicitly; 'duct as per IS 655' without them invites the lightest, leakiest interpretation.