How to Size Ductwork for Extract & Supply Systems
How to Size Ductwork for Extract & Supply Systems
Correctly sizing ventilation ductwork is an important part of designing an effective extract or supply ventilation system. The ductwork needs to carry the required volume of air around the property without creating excessive resistance, noise or unnecessary pressure losses.
Ductwork that is too small can restrict airflow and force the ventilation unit to work harder. On the other hand, simply choosing the largest duct available is not always the answer either. Space, routing, airflow requirements, system type and installation constraints all need to be considered.
Whether you are planning an MVHR, MEV or another whole-house ventilation system, understanding the basics of duct sizing can help you create a quieter, more efficient installation.
Why Does Duct Size Matter?
A ventilation system relies on ductwork to move air between the ventilation unit and individual rooms.
In a typical whole-house system, extract air is removed from areas such as:
- Kitchens
- Bathrooms
- Ensuites
- Utility rooms
- WCs
With an MVHR system, fresh filtered supply air is then delivered to rooms including:
- Bedrooms
- Living rooms
- Dining areas
- Home offices
- Other habitable spaces
Each room has an airflow requirement. The ductwork connected to that room must be capable of carrying that airflow while keeping resistance and noise at acceptable levels.
Poor duct sizing can contribute to problems such as reduced airflow, excessive fan speeds, increased energy consumption, noisy air valves and difficulty balancing the completed system.
Start With the Required Airflow
Before choosing a duct diameter, you first need to know how much air needs to travel through it.
Ventilation requirements are normally calculated according to the property size, number and type of rooms, system design and the relevant building regulations or standards for the project.
For example, a bathroom extract duct may only need to carry the airflow allocated to that bathroom. A larger main duct close to the ventilation unit, however, may need to carry the combined airflow from several rooms.
This means duct sizes can change throughout the system.
For projects in England, the ventilation requirements set out within Approved Document F are a useful regulatory reference. Requirements can differ elsewhere in the UK and Ireland, so the relevant regulations for the project location should always be checked.
Airflow, Duct Area and Air Velocity
Once the required airflow is known, duct size can be assessed by looking at the relationship between airflow and air velocity.
In simple terms:
Air velocity = airflow ÷ duct cross-sectional area
If the same volume of air is forced through a smaller duct, the air has to travel faster.
Higher air velocities can increase pressure loss and may make the system noisier, particularly around bends, restrictions, valves and the ventilation unit itself.
Increasing the duct area allows the same airflow to travel at a lower velocity.
However, duct sizing should not be based on velocity alone. The complete route, pressure loss and performance characteristics of the selected ductwork also need to be considered.
Pressure Drop Is Just as Important as Diameter
Every part of a ventilation system creates some resistance to airflow.
This is known as pressure drop or pressure loss.
Pressure losses can be caused by:
- The length of the duct run
- Duct diameter
- Bends and changes in direction
- Tees and branches
- Reducers
- Manifolds and distribution boxes
- Air valves
- External grilles
- Filters
- Silencers
- Other system components
A short, straight duct generally produces less resistance than a long route containing several tight bends.
This is why two duct runs with the same diameter and airflow requirement may not necessarily perform in exactly the same way.
When designing a system, pressure losses need to remain within the performance capabilities of the ventilation unit.
Sizing Main Supply and Extract Ductwork
The main ducts connected directly to a ventilation unit normally carry the greatest volume of air.
For an MVHR system, these can include:
- Fresh air intake
- Exhaust air to outside
- Supply air from the unit
- Extract air returning to the unit
Because these ducts handle the combined airflow of multiple rooms, they are normally larger than the individual room connections.
As the ductwork branches towards different areas of the property, the amount of air travelling through each section reduces. Smaller ducts may therefore be suitable further along the system.
The exact diameter depends on the airflow requirement, ventilation unit, system configuration and acceptable pressure loss.
Sizing Branch Ductwork
Imagine a main supply duct serving three rooms.
Before the first branch, the duct needs to carry the combined airflow for all three rooms. After air branches off into the first room, the remaining section only has to carry the airflow required by rooms two and three.
After the second branch, the final section only carries the airflow required by the last room.
A correctly designed branched system therefore takes account of the airflow travelling through each section of duct, rather than choosing one diameter and using it throughout the entire installation.
Careful routing is also important. Unnecessary bends and long duct runs can increase pressure losses even when the duct diameter itself appears suitable.
What About Semi-Rigid Radial Ductwork?
Semi-rigid radial systems work slightly differently.
Rather than using one main duct that repeatedly branches into smaller ducts, a radial system normally uses a manifold or distribution box with individual duct runs travelling towards each room terminal.
BPC Ventilation supplies a range of semi-rigid radial ductwork for MVHR, MEV and domestic ventilation projects, including 75mm and 90mm options alongside manifolds, plenums, connectors and installation accessories.
In a radial system, each individual run is sized according to the airflow it needs to provide or extract.
Where a room has a higher airflow requirement, the design may use a larger duct size or multiple radial ducts connected to the same room terminal or plenum. The correct arrangement will depend on the duct manufacturer's airflow and pressure-loss information.
One important point is that the stated outside diameter of semi-rigid ductwork should not automatically be used when carrying out airflow calculations. Internal dimensions and construction vary between products, so manufacturer performance data should always be checked.
75mm vs 90mm Radial Ducting
Both 75mm and 90mm semi-rigid ductwork are commonly used within residential ventilation systems.
The best choice depends on factors including:
- Required airflow
- Length of the duct run
- Available installation space
- Number of ducts serving the room
- Pressure loss
- Manifold and plenum connections
- Overall system design
A larger duct can generally carry a given airflow at a lower velocity, which can be useful for reducing pressure resistance. However, larger ductwork also requires more installation space.
For projects with restricted floor or ceiling voids, the duct layout needs to be considered early in the building process.
Keep Duct Runs as Simple as Possible
Duct sizing is only one part of creating an efficient ventilation system.
Good duct routing can often make just as much difference.
Where possible:
- Keep duct runs reasonably short
- Avoid unnecessary bends
- Use gradual changes in direction
- Avoid crushing or deforming flexible and semi-rigid ductwork
- Follow the manufacturer's minimum bend radius
- Make sure connections are properly sealed
- Support ductwork correctly
- Avoid unnecessary reductions in duct diameter
- Plan routes before first fix
Reducing resistance throughout the installation can make the system easier to balance and allows the ventilation unit to operate more efficiently.
Does the Longest Duct Run Matter?
Yes.
When assessing a ventilation system, particular attention should be paid to routes with high resistance.
A room with a long duct run containing several bends may be harder to supply or extract from than a room located directly beside the ventilation unit.
The ventilation unit must be capable of overcoming the resistance created by the system while still providing the required airflow.
This is one reason why duct layout and ventilation unit selection should be considered together rather than treating them as two separate parts of the project.
Don't Forget Noise
A ventilation system should provide the required airflow without creating unnecessary noise within the property.
Incorrect duct sizing can result in higher air velocities, which may contribute to unwanted airflow noise.
Noise can also be affected by:
- Fan speed
- Duct configuration
- Air valve selection
- Sharp bends
- Turbulence around fittings
- Poorly balanced airflow
- Unit location
- Lack of appropriate attenuation
Bedrooms and other quiet areas deserve particular attention during the design process.
Using suitable duct sizes, sensible routes and correctly selected components can help create a quieter overall system.
Different Duct Types Require Different Design Considerations
Circular rigid duct, rectangular duct, semi-rigid radial ductwork and flexible connections all have different characteristics.
They should not automatically be treated as interchangeable simply because two products appear to have a similar physical size.
For a closer look at the available options and where they are commonly used, read our guide to the different types of ventilation ducting.
Selecting the correct duct type early in the project also makes it easier to coordinate ventilation routes with joists, ceilings, insulation, services and other parts of the building.
Common Duct Sizing Mistakes
Some of the most common problems occur when ductwork is selected without considering the complete system.
Choosing ductwork based only on the ventilation unit connection
The diameter of the spigot on a ventilation unit does not mean every duct throughout the property should be that size.
Different sections of the system carry different quantities of air.
Reducing the duct too early
Reducing a duct diameter while it is still carrying a high airflow can significantly increase air velocity and resistance.
Ignoring pressure losses
Diameter alone does not tell you how a duct system will perform. Length, fittings, bends, valves and other components all contribute to the overall resistance.
Using too many tight bends
A technically suitable duct size can still perform poorly if the route is unnecessarily restrictive.
Designing the ductwork after construction has started
Leaving ventilation routes until late in a project can result in awkward runs, additional bends and restricted spaces.
Planning the system from the drawings before first fix can make installation considerably easier.
BPC Ventilation Design & Technical Support
If you are unsure how to size or route the ductwork for your project, BPC Ventilation can help.
Our ventilation design service allows you to send us your building plans so that the project can be reviewed before ordering. The team can help determine suitable airflow requirements, ventilation equipment, ductwork and component requirements for the property.
Planning the system at this stage can help identify suitable unit locations, supply and extract positions and practical duct routes before installation begins.
BPC Ventilation also provides technical support, so if you have questions about your ventilation system, ductwork or components during the project, our technical team can provide guidance.
Whether you are working on a new build, renovation or retrofit, getting the ductwork right from the beginning can make the completed ventilation system quieter, easier to balance and more effective.
