Round vs Rectangular Ducting: Which Should You Choose?

Choosing the correct ductwork is an important part of planning an effective ventilation system. The size, shape and route of the ducting can all affect airflow, pressure loss, noise and the space required for installation.

Two of the most common options are round ducting and rectangular ducting, sometimes referred to as flat ducting or flat channel. Both can be used to move air between ventilation equipment, room terminals and external vents, but each profile is better suited to different installation conditions.

So, when comparing round vs rectangular ducting, which option should you choose?

What is round ducting?

Round ducting uses circular pipework to transport air through a ventilation system. Rigid round PVC ducting is commonly used with extractor fans, MEV units and other mechanical ventilation equipment.

At Ventilation Superstore, our round PVC ducting range includes straight pipework alongside bends, T-pieces, Y-pieces, connectors, reducers and adaptors. Popular diameters include 100mm, 125mm and 150mm, allowing the ductwork to be matched to a range of fans, air valves and ventilation units.

Advantages of round ducting

Efficient airflow

A round internal profile provides a straightforward route for moving air. For comparable airflow requirements, round ducting will often provide efficient performance with relatively low resistance.

However, the total pressure loss still depends on the duct diameter, length, airflow rate and number of bends or fittings used. Every section and fitting adds resistance that the connected fan or ventilation unit must overcome.

Suitable for longer duct routes

Where sufficient installation depth is available, round rigid ducting can be a practical option for longer ventilation routes.

It can be installed through:

• Loft spaces
• Service cupboards
• Wall voids
• Ceiling voids
• Utility rooms
• Boxed-in service areas

A correctly sized round duct is often preferable where maintaining airflow is the main priority and the route does not have strict height restrictions.

Wide choice of fittings

Round ducting systems are available with a broad selection of compatible components, including:

• 45-degree and 90-degree bends
• Straight connectors
• T-pieces and Y-pieces
• Reducers
• Wall plates
• Airbrick adaptors
• Round-to-rectangular adaptors

This allows the route to be adapted around the property while maintaining compatible duct dimensions.

Straightforward connection to equipment

Many extractor fans, ventilation units, air valves and external grilles use circular spigots. Round pipework can therefore connect directly to the equipment without needing to change the duct profile.

Disadvantages of round ducting

The main limitation of round ducting is the amount of installation depth it requires.

A 125mm circular duct needs more vertical clearance than a low-profile rectangular channel. This can make round ductwork difficult to conceal beneath shallow ceilings, above kitchen units or within restricted floor voids.

This does not make round ducting unsuitable; it simply means that the building layout must provide enough room for the chosen diameter and any required insulation.

What is rectangular ducting?

Rectangular ducting has a wide, low-profile shape. It is often called flat ducting because it can be installed where the available vertical space is too restricted for an equivalent round pipe.

Ventilation Superstore stocks rectangular PVC ducting in popular sizes including 110 x 54mm, 204 x 60mm and 220 x 90mm. The range also includes connectors, horizontal and vertical bends, T-pieces and adaptors.

Advantages of rectangular ducting

Fits into restricted spaces

The main advantage of rectangular ducting is its reduced installation height.

Flat ducting can be useful when routing ventilation:

• Above kitchen cupboards
• Beneath floorboards
• Inside shallow ceiling voids
• Behind wall linings
• Through boxed-in sections
• Along the top of fitted furniture

Its flatter shape can allow ductwork to pass through areas where round pipework of the required diameter would not physically fit.

Easier to conceal

Rectangular ductwork can be easier to hide within the structure or finishes of a property. This is particularly useful during renovations where there is limited space available for new building services.

The duct can often be positioned against a flat surface, reducing the size of the boxing or service void required.

Range of routing options

Rectangular ducting is available with horizontal bends, vertical bends, connectors, T-pieces and profile adaptors. These fittings make it possible to change direction while keeping the duct close to walls, floors and ceilings.

Higher-capacity rectangular ducting is also available for applications requiring additional airflow without significantly increasing the installation height. For example, 220 x 90mm ducting is designed to accommodate greater airflow while retaining a relatively low-profile shape.

Disadvantages of rectangular ducting

Rectangular ducting should not be selected using physical dimensions alone. A wide but shallow duct may not provide the same airflow capacity as a round duct that appears to be a similar size.

Its performance depends on:

• Internal cross-sectional area
• Shape and aspect ratio
• Required airflow rate
• Total duct length
• Number and type of bends
• Connectors and adaptors
• The performance of the connected fan

An undersized flat duct or a route containing numerous tight bends may create excessive resistance. This can reduce airflow or require the fan to operate at a higher setting.

The appropriate duct size should therefore be confirmed from the ventilation design or the performance information supplied by the ducting and equipment manufacturer.

Round vs rectangular ducting comparison

Consideration	Round ducting	Rectangular ducting
Installation depth	Requires more height	Suitable for shallow spaces
Airflow	Often provides efficient airflow	Depends heavily on correct sizing
Best location	Lofts and larger service voids	Floors, shallow ceilings and boxing
Connections	Connects directly to many circular spigots	May require a profile adaptor
Fittings	Wide range of bends and branches	Horizontal and vertical fittings available
Visibility	May require larger boxing	Easier to conceal against flat surfaces
Common sizes	100mm, 125mm and 150mm	110 x 54mm, 204 x 60mm and 220 x 90mm

Installation considerations for both duct types

Whether you choose round or rectangular ducting, the full route should be planned before installation.

Select the correct size

The duct must be suitable for the required airflow. A smaller duct is not automatically better simply because it is easier to conceal.

Undersized ductwork can create greater resistance, increase air velocity and contribute to system noise.

Minimise bends

Every bend changes the direction of the airflow and adds resistance. The duct route should be kept as direct as reasonably possible, with unnecessary bends and changes in profile avoided.

Keep the route short

Longer duct routes create additional pressure loss. Positioning the fan or ventilation unit carefully can help reduce the total amount of ducting required.

Secure and seal the connections

Connections should be mechanically secure and sealed using a method suitable for the chosen duct system. Air leakage can reduce the amount of air reaching the intended terminal.

Consider insulation

Ducting that passes through an unheated space may require suitable insulation to reduce condensation or unwanted heat loss. The requirements will depend on the ventilation system, duct location and whether the route carries warm, cold, intake or exhaust air.

Follow the ventilation design

For projects in England, Approved Document F advises using rigid ducts wherever possible and designing ductwork to minimise overall length, bends and pressure losses. It also states that ducts should be appropriately sized for the required airflow and that connections should be secured and adequately sealed.

The applicable requirements differ across England, Wales, Scotland, Northern Ireland and Ireland, so always follow the regulations and technical guidance relevant to the project location.

Which ductwork should you choose?

Choose round ducting when:

• There is enough depth for the required diameter.
• The equipment has circular connections.
• You want to minimise unnecessary profile changes.
• The route passes through a loft or larger service void.
• Airflow performance is the main consideration.

Choose rectangular ducting when:

• Ceiling or floor depth is restricted.
• The route must pass above cabinets or fitted furniture.
• The ductwork needs to sit close to a wall or flat surface.
• Large round boxing would be difficult to conceal.
• A low-profile route is required for part of the system.

In many properties, the most practical answer is to use both. Round ducting can be used for the main route, while rectangular ducting helps overcome a particular space restriction.

The most important factors are correct sizing, compatible fittings and a route designed to minimise resistance.

Explore our full range of rigid PVC ductwork, including round PVC ducting, rectangular PVC ducting and compatible fittings at Ventilation Superstore.