Round Duct vs Rectangular Duct – Which Ducting Is Best for Your HVAC System?
When specifying ducting for a ventilation or HVAC system, one of the first decisions is the duct shape. The two most common options are round ducting and rectangular ducting. Both are widely used across commercial, industrial, and residential projects, but each has clear strengths depending on the application.
This guide breaks down the differences between round ducting vs rectangular ducting, covering airflow performance, space requirements, installation, cost, and typical UK use cases.
If you are buying ventilation ducting online, this will help you choose the right solution for your project.
What Is Round Ducting?
Round ducting, often referred to as spiral ducting, is a circular ventilation duct commonly used in HVAC systems. It is typically manufactured from galvanised steel, aluminium, or plastic depending on the system requirements.
Round ducting is widely used in:
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Commercial HVAC systems
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Industrial ventilation systems
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Warehouses and factories
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Offices and retail units
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Modern residential ventilation
Advantages of Round Ducting
Better airflow efficiency
Round ducting allows air to flow more smoothly than rectangular ducting. The circular shape reduces friction and pressure loss, which improves system efficiency and airflow performance.
Lower air leakage
Spiral ducting has fewer joints and seams, meaning less air loss. This helps HVAC systems meet UK building regulations and energy efficiency standards.
Reduced noise levels
Because air flows more evenly, round ducting produces less turbulence. This leads to quieter ventilation systems, especially important in offices, apartments, and public buildings.
Stronger structure
Round metal ducting is naturally more rigid. It resists deformation and performs well over long spans without additional reinforcement.
Lower long-term cost
Although round ducting may cost slightly more upfront, the improved airflow efficiency can reduce fan sizes, running costs, and maintenance over time.
What Is Rectangular Ducting?
Rectangular ducting is a flat-sided ventilation duct commonly used where space is limited. It is often chosen when ceiling voids, wall cavities, or structural constraints make round ducting impractical.
Rectangular ducting is common in:
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Residential ventilation systems
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Flats and apartments
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Kitchen and bathroom extract systems
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Refurbishment projects
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Tight ceiling void installations
Advantages of Rectangular Ducting
Space-saving design
Rectangular ducting fits neatly into shallow ceiling voids or along walls. This makes it ideal for projects with height restrictions.
Easier to conceal
Flat ducting can be boxed in or hidden more easily than round ducting, which is often preferred in residential builds.
Flexible layout options
Rectangular ducting allows tighter bends and transitions, useful when working around structural obstacles.
Lower upfront cost
In smaller systems, rectangular plastic ducting can be more cost-effective, especially for domestic ventilation.
Round Duct vs Rectangular Duct – Key Differences
Airflow Performance
Round ducting delivers better airflow with lower resistance. Rectangular ducting creates more friction, which can reduce efficiency unless the system is carefully designed.
Installation
Rectangular ducting is often easier to install in confined spaces. Round ducting is simpler to align and seal in open ceiling environments.
Noise
Round ducting is quieter due to smoother airflow. Rectangular ducting may generate more noise if air velocity is high.
Strength and Durability
Round metal ducting is stronger and more resistant to damage. Rectangular ducting may require additional support on longer runs.
Energy Efficiency
Round ducting supports more energy-efficient HVAC systems due to reduced air leakage and pressure drop.
Which Ducting Should You Use?
Choose Round Ducting If:
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You are installing a commercial HVAC system
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Energy efficiency is a priority
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Noise control matters
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There is sufficient ceiling or service space
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You want long-term performance and reliability
Choose Rectangular Ducting If:
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Ceiling height or space is limited
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You are working on a residential or retrofit project
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Ducting needs to be hidden or boxed in
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Budget is tight for a small system
In many UK projects, a combined ducting system is used. Main runs are installed using round ducting, with rectangular ducting used for branches or tight areas.
UK Building Regulations and Ducting Choice
Both round and rectangular ducting can comply with UK building regulations when correctly specified. Factors such as airflow rate, pressure loss, fire rating, and insulation all matter more than duct shape alone.
Using quality HVAC ducting components, proper sealing, and correct sizing is essential for compliance and performance.
Buying Ducting Online in the UK
When sourcing ducting materials online, look for:
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Clear duct sizing information
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Compatible fittings and connectors
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Galvanised or insulated ducting options
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Fast UK delivery
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Trade-grade materials
Whether you need round ducting, rectangular ducting, spiral ducting, or full HVAC ducting systems, choosing the right supplier makes a difference to installation speed and system reliability.
Final Thoughts
There is no single best answer in the round duct vs rectangular duct debate. The right choice depends on space, airflow requirements, budget, and the type of building.
For most commercial HVAC systems, round ducting offers better performance and efficiency. For residential and tight installations, rectangular ducting remains a practical option.
If you are unsure which ducting is right for your project, speaking to a specialist supplier can save time and cost down the line.
