75mm vs 90mm Radial Ducting: Which Should You Use?
75mm vs 90mm Radial Ducting: Which Should You Use?
When designing an MVHR or mechanical ventilation system, one of the decisions you may need to make is whether to use 75mm or 90mm radial ducting.
Both sizes work on the same basic principle: individual semi-rigid ducts run from a central distribution manifold to supply or extract points around the property.
The main difference is the size of the duct. This affects how much air can be moved through each run, the resistance created by the ductwork and the amount of installation space required.
So, should you choose 75mm or 90mm? The answer depends on the airflow requirements and layout of the project.
What Is Radial Ducting?
A radial ventilation system uses individual duct runs between a central manifold and each room terminal or plenum.
Instead of repeatedly branching a main duct into smaller sections, each room is connected back to the distribution point using its own semi-rigid pipework.
This type of system is commonly used with MVHR and other mechanical ventilation systems because it can provide:
- Simple room-by-room duct routes
- Fewer joints within individual runs
- Flexible installation through floors and ceiling voids
- Airtight connections when correctly installed
- Good acoustic performance
- Easier system balancing
Quiet-Vent offers radial ducting in both 75mm and 90mm sizes, alongside compatible manifolds, plenums and installation accessories.
What's the Difference Between 75mm and 90mm Radial Ducting?
The main difference is straightforward: 90mm ducting has a larger cross-sectional area than 75mm ducting.
That larger internal area can allow a given volume of air to travel at a lower velocity, depending on the specific duct construction.
Generally, this means 90mm ductwork can be useful where higher airflow is required or where reducing pressure resistance through an individual run is particularly important.
75mm ducting, however, is physically smaller and can be easier to route through restricted floor and ceiling spaces.
Neither option is automatically better. The correct choice depends on the system design.
75mm Radial Ducting
75mm semi-rigid ducting is widely used for domestic radial ventilation systems.
Its smaller physical size can make it particularly useful where installation space is limited.
Advantages of 75mm ducting
- Compact diameter
- Easier to route through restricted voids
- Suitable for many domestic MVHR installations
- Flexible routing from manifolds to individual rooms
- Multiple ducts can be used where a room requires greater airflow
For rooms requiring more air than a single duct run can comfortably provide, two or more 75mm ducts may be connected to the same room plenum.
This allows the required airflow to be divided across multiple ducts rather than forcing too much air through one run.
90mm Radial Ducting
90mm ducting provides a larger airflow path.
For the same airflow, a larger duct will generally operate at a lower air velocity than a smaller equivalent duct, which can help reduce pressure resistance.
This can make 90mm particularly useful for longer runs or rooms with greater airflow requirements.
Advantages of 90mm ducting
- Greater available airflow area
- Can help reduce resistance through individual runs
- May require fewer individual runs for some room airflow requirements
- Useful for larger or more demanding ventilation layouts
- Can contribute to lower airflow velocities when correctly designed
Quiet-Vent's 90mm antibacterial radial pipe, for example, is designed to minimise pressure and reduce noise transfer, with installation suitable through walls and ceilings.
75mm vs 90mm at a Glance
| Consideration | 75mm Radial Ducting | 90mm Radial Ducting |
|---|---|---|
| Physical Size | More compact | Larger |
| Installation Space | Useful in tighter voids | Requires more space |
| Airflow Capacity per Run | Lower | Generally higher |
| Pressure Resistance | Can increase sooner as airflow rises | Generally lower for the same airflow |
| Multiple Runs | More commonly required for higher-airflow rooms | May reduce the number of runs required |
| Typical Use | Compact domestic radial systems | Higher airflow or lower-resistance layouts |
| Routing | Easier where space is restricted | Requires more room for bends and routes |
Is 90mm Always Better Because It's Larger?
No.
A larger duct does not automatically make a ventilation system better.
If the required room airflow can be delivered effectively through 75mm ductwork, moving to 90mm may provide little practical benefit while requiring additional installation space.
Likewise, choosing 75mm purely because it fits more easily could create unnecessary resistance if too much airflow is expected from a single run.
The aim is to match the duct size and number of runs to the airflow requirement.
Think About the Available Installation Space
Space is often one of the biggest factors when deciding between 75mm and 90mm.
Semi-rigid radial ducting can be routed through:
- Floor zones
- Ceiling voids
- Service cavities
- Walls
- Roof spaces
75mm ducting requires less depth, which can make routing easier in projects with limited space.
90mm ducting requires additional clearance, particularly where the pipe changes direction. Minimum bend radius should also be considered rather than forcing the duct into an excessively tight turn.
The Quiet-Vent 90mm radial pipe, for example, specifies a minimum bend radius of 150mm. Manufacturer installation requirements should always be checked for the exact pipe being used.
Consider the Length of Each Duct Run
Duct length also matters.
As air travels through ductwork, resistance is created. Longer runs generally produce greater pressure loss than shorter ones.
Bends and changes in direction add further resistance.
A longer 75mm run may therefore behave differently from a short 75mm run carrying the same airflow.
Where routes are particularly long or airflow requirements are higher, a larger duct may provide an advantage.
This is why the entire duct route should be considered rather than selecting a diameter based on room type alone.
How Many Ducts Does Each Room Need?
There is no single answer.
The number of radial ducts required depends on:
- Required room airflow
- Duct diameter
- Length of each run
- Pressure loss
- Supply or extract requirement
- Plenum arrangement
- Performance characteristics of the selected ductwork
A bedroom with a relatively modest supply airflow may require a different arrangement from a kitchen or another area with a higher extract requirement.
Where the required airflow exceeds what one radial run should carry, additional runs can be used.
This is why room-by-room airflow calculations should be completed before deciding how many 75mm or 90mm connections are required.
Don't Mix Duct Size With Outside Diameter Calculations
One important consideration with semi-rigid ductwork is that 75mm and 90mm normally refer to the outside diameter of the pipe.
The usable internal area will be smaller and can vary according to the construction of the product.
For airflow and pressure calculations, always use the performance information provided for the actual duct system rather than calculating performance using the nominal outside diameter alone.
Noise and Air Velocity
One of the advantages of a properly designed radial system is its potential for quiet operation.
However, forcing too much air through an individual duct can increase air velocity and resistance, which may contribute to unwanted airflow noise.
A quieter system therefore depends on more than simply choosing 75mm or 90mm.
The design should also consider:
- Airflow through each run
- Duct length
- Number of bends
- Manifold design
- Plenum selection
- Air valve sizing
- Ventilation unit performance
- System balancing
Correct duct selection forms part of the complete ventilation design.
Can You Use Both 75mm and 90mm in the Same Project?
Different duct sizes can potentially be used within a wider ventilation installation, but this should be planned rather than improvised during installation.
Manifolds, plenums, seals and accessories need to be compatible with the selected ductwork, and airflow calculations should reflect each route.
For most radial systems, keeping the room distribution network based around one duct size can make specification, installation and balancing simpler.
Which Size Should You Choose?
As a general guide:
Consider 75mm radial ducting when:
- Installation space is restricted
- Compact duct routes are important
- Individual room airflows can be achieved efficiently using 75mm runs
- Multiple ducts can be used where higher airflow is required
Consider 90mm radial ducting when:
- Higher airflow is required through individual runs
- Longer duct routes need to be accommodated
- Lower resistance is a priority
- There is sufficient installation space for the larger pipe
The final decision should always be based on the calculated airflow and pressure requirements of the ventilation system.
Need Help Planning Your Radial Ductwork?
Choosing between 75mm and 90mm ducting is only one part of planning an effective ventilation system.
The unit, manifolds, plenums, duct routes, room airflows and number of radial connections all need to work together.
Through the Quiet-Vent Design Request service, you can submit your project details for help with ventilation system layout, compatible equipment and bespoke ducting kit design.
Planning the system before installation can help ensure the correct duct size, number of runs and distribution components are selected for the project.
Explore Quiet-Vent Radial Ducting
Both 75mm and 90mm radial ducting can provide an effective solution when correctly designed.
The best choice isn't simply the largest pipe available. It is the duct size that can deliver the required airflow with appropriate resistance while fitting practically within the building.
Explore the Quiet-Vent range of 75mm and 90mm radial ducting, manifolds, plenums and accessories to build a complete radial ventilation system.