Technical guide

Sizing a ventilation duct network

Sizing a duct rests on two parameters: the airflow to be carried (in m³/h) and the maximum acceptable air velocity (in m/s). The diameter of a circular duct follows from D = √(4 × Q / (π × V × 3600)), where Q is the airflow in m³/h and V the velocity in m/s. Usual velocities are of the order of 3 to 5 m/s on main branches in residential work, 5 to 8 m/s in commercial buildings and 8 to 12 m/s in industry. The higher the velocity you choose, the more compact the duct — but the noisier it is, and the higher the pressure drop and therefore the fan's energy consumption.

Recommended air velocities

These ranges are design orders of magnitude: a noise-sensitive space (meeting room, bedroom, consulting room) means working at the bottom of the range, whereas a dust capture network needs a high velocity precisely so particles do not settle out inside the duct.

Usual velocity ranges by building type and position in the network
Building type	Main branch	Secondary branch	At the terminals
Residential	3 – 5 m/s	2.5 – 4 m/s	1.5 – 2.5 m/s
Commercial (offices, retail)	5 – 8 m/s	4 – 6 m/s	2 – 3 m/s
Industrial	8 – 12 m/s	6 – 10 m/s	3 – 5 m/s

Round or rectangular duct?

For an equivalent cross-section, circular duct generates less pressure drop, behaves better acoustically and is easier to seal. Rectangular duct becomes necessary where the ceiling void available is limited; you then aim for an aspect ratio close to 2:1 (width ≈ 2 × height) at equal free area, because a very elongated aspect ratio degrades performance badly.

Circular: minimum pressure drop, better sealing, quick to install — the default choice.
Rectangular: ceiling void constraints, large cross-sections, connection onto a unit.
Flat-oval: a compromise between the two, useful in refurbishment work.

Materials and accessories

Galvanised steel: the market standard, with a low friction factor (of the order of 0.02 for a smooth wall).
Stainless steel: humid or corrosive environments, food processing, extraction of cooking vapours.
Aluminium: lightweight, for short flexible and semi-rigid runs.
Balancing dampers: essential if every terminal is to deliver the airflow the design intends.
Bends, tees and reducers: every fitting adds a pressure drop; favour large-radius bends.
Flexible connectors: acoustic decoupling between the fan unit and the ductwork.

Pressure drop and balancing

Calculating pressure drop means adding the linear losses (straight runs) to the fitting losses (bends, tees, dampers, filters, terminals). The total determines the static pressure the fan unit will have to provide. A poorly balanced network delivers too much air to the terminals near the fan and not enough to the far ones: balancing dampers, designed in from the start, are what let you correct that at commissioning.

Duct leakage

A leak in the ductwork is airflow paid for but never delivered. The care taken over the joints (lip seals, suitable tapes, screw fixings), especially on rectangular ductwork, directly determines the fan's energy consumption and whether the design airflows actually reach the terminals.

Frequently asked questions

How do you calculate the diameter of a ventilation duct?

The diameter follows from the airflow and the chosen air velocity, through D = √(4 × Q / (π × V × 3600)), where Q is the airflow in m³/h and V the velocity in m/s. You first pick an acceptable velocity based on the building type and the position in the network, derive the cross-section needed, then take the next standard diameter up.

What air velocity should be used in a duct?

In residential work, 3 to 5 m/s on main branches, 2.5 to 4 m/s on secondary branches and 1.5 to 2.5 m/s at the terminals. In commercial buildings it rises to 5 to 8 m/s on main branches, and in industry up to 8 to 12 m/s. A high velocity reduces the space taken but increases noise and pressure drop.

Is circular or rectangular duct better?

For an equivalent cross-section, circular duct offers lower pressure drop, better acoustic behaviour and easier sealing: it is the default choice. Rectangular duct becomes necessary where the ceiling void available is limited; you then aim for an aspect ratio close to 2:1 to limit the loss of performance.

Why fit balancing dampers on a ventilation system?

Without dampers, the terminals near the fan get too much air and the far ones not enough. Balancing dampers let you balance the network at commissioning so every terminal delivers the airflow the design calls for. They must be designed in from the start, in positions that remain accessible for maintenance.

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