Getting MVHR ducting size right is one of the most consequential decisions in any mechanical ventilation with heat recovery installation. Undersized ducts create excessive air velocity, elevated noise levels, and pressure losses that push the fan outside its efficient operating range.
Oversized ducts waste space, cost more, and can cause moisture problems in low-velocity sections. This guide gives contractors, M&E engineers, and ventilation designers the practical sizing knowledge needed to specify MVHR ductwork correctly from the outset.
Why Does MVHR Ducting Size Matter So Much?
MVHR ducting size directly determines whether the system performs to its design specification. Every duct run in an MVHR system carries a calculated volume of air to or from a specific terminal. If the duct bore is too small for that volume, air velocity rises beyond the acceptable limit, pressure loss across the system increases, and the fan must work harder to compensate.
The result is higher energy consumption, increased noise at the terminals, and in some cases, a system that simply cannot achieve the balanced airflow rates required by Approved Document F.
Beyond performance, incorrectly sized ductwork creates problems during commissioning. A system that cannot be balanced to the design airflow rates will fail to meet the minimum requirements of the Domestic Ventilation Compliance Guide and will not produce a valid commissioning certificate.
For contractors responsible for signing off new build and retrofit installations, getting the sizing right before the first fix is significantly cheaper than correcting it after the plasterboard goes up.
Duct size also has a direct relationship with acoustic performance. Higher air velocities generate turbulence and air velocity noise that transmits to occupied rooms. Keeping velocities within the recommended limits for each section of the system is one of the most reliable ways to ensure the finished installation meets the noise expectations of the client and the building specification.
What Are the Standard MVHR Duct Diameters Used in the UK?
The four standard circular duct diameters used in UK residential MVHR installations are 75mm, 100mm, 125mm, and 150mm. Each corresponds to a different range of design airflow rates, and the appropriate diameter for each branch depends on the volume of air being carried and the target velocity for that section of the system.
75mm Ducting
75mm ducts are used in rigid radial distribution systems where individual spurs serve a single terminal at relatively low airflow rates, typically up to 20 to 25 m³/h. This diameter is common in proprietary semi-rigid duct systems such as the Domus Radial system, where a central manifold distributes individual 75mm runs to each room terminal.
The compact bore suits the low flow rates associated with individual bedrooms and living room supply terminals in smaller dwellings.
100mm Ducting
100mm is the most common diameter in residential MVHR installations. It suits branch runs carrying airflow rates in the range of 25 to 55 m³/h when velocity is maintained within the 2 to 3 m/s limit recommended for terminal branches.
Many MVHR units connect to 100mm spigots on the unit itself, and 100mm fittings are widely available from UK ducting suppliers. For single-room heat recovery units, 100mm is the standard connection diameter for both the internal and external duct connections.
125mm Ducting
125mm ducting suits higher-flow branch runs and is commonly used for the main distribution sections of larger residential MVHR systems, as well as for bathroom and kitchen extract branches where the airflow requirement is above the capacity of 100mm duct at acceptable velocities. In commercial and light industrial MVHR applications, 125mm ducts serve as primary distribution sections feeding multiple 100mm branch runs.
150mm Ducting
150mm is used for main trunk sections in larger residential systems and as the primary connection between the MVHR unit and the main distribution manifold. It is also the standard diameter for kitchen extract in MVHR systems where the cooker hood extract route is integrated into the heat recovery distribution, though this requires specific unit capability and is not standard on all MVHR products.
In commercial MVHR installations serving larger floor areas, 150mm and above trunk sections are common.
How Do You Calculate the Correct Duct Size for an MVHR System?
MVHR duct sizing follows a defined calculation process based on the required airflow rate for each terminal, the target air velocity for each duct section, and the resulting pressure loss across the system. The goal is to achieve the design airflow at every terminal while keeping the total system resistance within the fan's operating capability.
Step 1: Establish the Terminal Airflow Rates
Airflow rates for each terminal are determined from the building's ventilation design, which is based on the minimum rates set out in Approved Document F Table 1.1 and the specific room volumes and occupancy levels. Typical residential terminal airflow rates are:
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Bedroom (supply): 10 to 15 l/s (36 to 54 m³/h)
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Living room (supply): 10 to 20 l/s (36 to 72 m³/h)
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Kitchen (extract): 13 l/s minimum (47 m³/h), often higher in practice
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Bathroom (extract): 8 to 15 l/s (29 to 54 m³/h)
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WC (extract): 6 l/s minimum (22 m³/h)
These are minimum values. The actual design airflow rates should be set by the MVHR designer based on the full ventilation heat loss calculation and the unit's total airflow capability.
Step 2: Select Target Air Velocities
Air velocity within the duct determines both pressure loss and noise generation. Industry guidance and CIBSE recommendations set the following velocity limits for residential MVHR:
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Terminal branches (individual room runs): 2.0 to 3.0 m/s maximum
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Main distribution sections: 3.0 to 4.0 m/s maximum
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Connections to the MVHR unit: up to 4.0 m/s
Keeping velocities at the lower end of these ranges reduces pressure loss and noise. For bedrooms where occupants are present during low-activity periods, targeting 2.0 to 2.5 m/s in the final branch run is good practice.
Step 3: Calculate the Required Duct Diameter
The required duct cross-sectional area is calculated from the airflow rate (in m³/s) divided by the target velocity (in m/s). The required area gives the minimum internal duct bore, from which the next available standard diameter is selected upward.
The formula is:
A=QVA = \frac{Q}{V}A=VQ
Where:
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AAA is the required cross-sectional area in m²
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QQQ is the design airflow rate in m³/s
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VVV is the target air velocity in m/s
For a bathroom extract terminal carrying 15 l/s (0.015 m³/s) at a target velocity of 2.5 m/s:
The next standard diameter above 87mm is 100mm, so a 100mm duct is specified for this branch.
Step 4: Calculate Pressure Loss and Check Against Fan Duty
Once all duct diameters are selected, the total system pressure loss is calculated by summing the pressure losses across every duct section, fitting, filter, heat exchanger, and terminal grille in the index circuit (the longest, highest-resistance path through the system). This total must fall within the fan's available static pressure at the design airflow rate.
If the total pressure loss exceeds the fan's capability, the designer must either increase duct diameters to reduce resistance, select a more capable fan, or reduce the length or complexity of the duct distribution. This calculation is where many residential MVHR installations run into problems when duct routing has not been planned early in the design process.
What Is the Difference Between Rigid and Flexible Ducting for MVHR?
Both rigid and flexible duct types are used in MVHR installations, and each has specific applications where it performs best. Understanding when to use each type avoids the common mistake of over-relying on flexible duct in locations where it creates avoidable pressure losses.
Rigid Ducting
Rigid circular duct, whether in PVC, galvanised steel, or aluminium, offers the lowest resistance to airflow and the most predictable pressure loss characteristics. It is the correct choice for all main distribution runs and long branch sections.
Rigid duct maintains its internal diameter along the full length of the run, whereas flexible duct can compress, kink, or sag if not properly supported, all of which increase resistance significantly beyond the theoretical calculated values.
For MVHR installations in residential new build, rigid semi-rigid systems such as the Domus Radial 75mm system or standard 100mm to 150mm rigid PVC circular duct represent best practice for the main distribution network.
Flexible Ducting
Flexible ducting hose serves a specific purpose in MVHR installations: the final connection between a rigid branch terminal and the room diffuser, and the flexible connections at the MVHR unit itself to isolate vibration from the distribution system.
These connections should be kept as short as possible, typically no more than 1.5 to 2 metres, and must be installed in a smooth arc without tight bends or compression.
Using flexible duct for extended main runs is a common site practice that creates measurable performance problems. A compressed or kinked flexible duct section increases pressure loss dramatically compared to the theoretical value for a straight rigid run.
For a system that has been calculated to tight margins, this can be enough to prevent the fan from delivering balanced airflow at commissioning.
How Does Duct Insulation Affect MVHR Performance?
Duct insulation is a critical requirement in MVHR installations and one that is frequently under-specified or omitted in cost-reduction exercises. Its purpose is to prevent heat transfer between the duct and the surrounding building fabric, maintaining the thermal efficiency of the heat recovery process.
Where Insulation Is Required
All supply duct runs that pass through unheated spaces, including loft voids, under-floor spaces, and external wall cavities, must be insulated to prevent the conditioned supply air from losing heat before it reaches the occupied room.
For extract ducts passing through heated spaces, insulation prevents moisture-laden warm extract air from condensing inside the duct before it reaches the heat exchanger.
The external connection between the MVHR unit and the outside air inlet and exhaust terminals requires insulation across any section that passes through the heated envelope, to prevent condensation and heat gain or loss at the point where outdoor air is entering the system.
Insulation Specification
Insulated flexible duct with a minimum 25mm mineral wool or foam insulation layer is standard for loft-run supply ducts. For runs passing through very cold spaces or in high-performance Passivhaus-standard installations, 50mm insulation thickness may be specified. The insulation must be continuous without gaps at joints and must be sealed at connections to prevent moisture ingress into the insulation layer itself.
Leaving supply ducts uninsulated in cold loft spaces is one of the most common causes of MVHR underperformance in new build properties. The heat recovery efficiency measured at the unit may be 85% or above, but if the supply air loses 10 to 15 degrees crossing an uninsulated loft void, the effective efficiency at the room terminal drops substantially below the headline specification.
What Are the Key Fittings Required for a Professional MVHR Duct Installation?
A complete MVHR duct installation requires a range of fittings beyond the straight duct sections. Specifying the correct fittings and accounting for their pressure loss contributions in the system design is part of professional installation practice.
Key fittings required include:
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Bends and elbows: 90-degree and 45-degree bends to route ductwork around structural elements. Long-radius bends generate significantly lower pressure losses than tight 90-degree elbows and should be used wherever the routing allows.
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Reducers and adaptors: To transition between different duct diameters at branch junctions and at the MVHR unit spigot connections.
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Tee pieces and manifolds: At branch junctions where the main trunk splits into two or more distribution branches.
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Connectors and couplings: For joining straight duct sections and making secure, airtight connections between lengths.
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Duct tape and sealant: All MVHR duct joints must be sealed to prevent air leakage. Proprietary duct sealing tape or mastic sealant applied at every joint ensures system airtightness and prevents pressure losses from leakage within the distribution network.
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Brackets and support clips: Rigid duct must be adequately supported at regular intervals to prevent sagging, which creates low points where condensate can accumulate.
Where to Source MVHR Ducting and Fittings for Professional Installations?
eFans stocks a complete range of ducting and fittings for residential and commercial ventilation systems, including MVHR installations. Our range covers PVC, aluminium ducting, and insulated ducting across all standard diameters from 100mm to 200mm, alongside flexible ducting hose and flat channel systems for space-constrained installations.
For all the fittings needed to complete a professional MVHR duct installation, the eFans range includes:
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Plastic ducting connectors for secure, airtight duct joints
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Ducting bends in 45-degree and 90-degree profiles for clean, low-resistance routing
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Ducting adaptors for transitions between different diameters
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Tee pieces for branch junctions in distribution networks
Products are available from leading brands including Domus and Systemair, and all orders come with free UK delivery. Whether you are fitting a full MVHR distribution system or sourcing additional fittings to complete an existing installation, our team can support trade orders of any scale.
Frequently Asked Questions
What happens if MVHR ducts are undersized?
Undersized MVHR ducts cause air velocity to exceed the recommended limits for the system, which increases pressure loss across the duct run. Higher pressure loss means the fan must work harder to push the design airflow through the system, increasing energy consumption and fan noise. In many cases, the fan cannot overcome the excess resistance and the system fails to deliver the balanced airflow rates required at each terminal.
The result is a system that does not perform to specification, cannot be commissioned correctly, and may fail to meet the minimum airflow requirements under Approved Document F. Undersized ducts also generate air velocity noise that transmits to occupied rooms through the terminal grilles.
Can I use flat channel ducting instead of round duct for MVHR?
Flat channel ducting can be used for MVHR branch runs where ceiling void depth restricts the use of circular duct, but it introduces higher frictional resistance per unit length compared to circular duct of equivalent cross-sectional area. This additional resistance must be accounted for in the system pressure loss calculation.
Flat channel is most commonly used for short final branch connections to terminals or for kitchen extract runs where the void is constrained. For main distribution runs, circular rigid duct is preferred because of its lower resistance, easier sealing, and better acoustic properties. When a flat channel is used in MVHR systems, all joints must be sealed as rigorously as circular duct connections to maintain system airtightness.
How long can individual MVHR duct runs be?
There is no single maximum duct run length for MVHR, as the acceptable length depends on the duct diameter, airflow rate, velocity, and the total available static pressure from the fan. In residential MVHR installations, individual branch runs are typically kept to 10 metres or less as a practical design target. Longer runs are achievable if the duct diameter is increased to keep velocity and pressure loss within acceptable limits.
What matters is the total system resistance in the index circuit, not the length of any individual branch in isolation. Very long runs in the index circuit may require the designer to upsize ducts along that route or select a higher-capability fan to maintain balanced airflow across all terminals.
Does MVHR ductwork need to be airtight?
Yes. MVHR ductwork must be sealed at every joint to prevent air leakage from the pressurised supply side of the system and air infiltration into the low-pressure extract side. Leakage on the supply side means conditioned supply air is lost into the building fabric rather than delivered to the room terminal, reducing the effective airflow at the diffuser.
Leakage on the extract side means uncontrolled air is drawn into the duct from the surrounding building void rather than from the intended room terminal, compromising the system balance and potentially introducing cold, damp, or contaminated air into the extract stream. All joints in MVHR ductwork should be sealed with proprietary duct sealing tape or mastic sealant applied before insulation is installed over the joint.
What duct diameter connects to the MVHR unit itself?
The connection diameter at the MVHR unit spigot depends on the unit model and its total airflow capacity. Most residential MVHR units in the range of 150 to 400 m³/h use 160mm, 180mm, or 200mm main spigots for the four primary connections (fresh air intake, exhaust air outlet, supply air distribution, and extract air return).
These main connections then reduce down to the distribution duct diameter using reducers and adaptors at the manifold or first branch junction. Single room heat recovery units use smaller connection diameters, typically 100mm, for both the internal room side and the external wall penetration. Always confirm the specific spigot diameters for the unit being installed before specifying the ductwork and fittings, as connections vary between manufacturers and unit sizes.