Design & Planning
Planning and Coordinating MVHR Ductwork
A well-designed MVHR system starts with early coordination between architects, timber kit manufacturers, ventilation designers and installers, whose practical experience is invaluable. From ceiling void depths and structural joists to ductwork routes, insulation and unit positioning, we explore the key considerations that can help avoid costly changes during construction.
11 min read ·
Incorporating mechanical ventilation with heat recovery (MVHR) into a new home requires more than selecting a suitable unit and drawing ductwork routes onto a floor plan.
It's relatively straightforward to take a set of architectural drawings and produce an MVHR ductwork layout. However, a route that works on a drawing doesn't necessarily work when the system comes to be installed.
Ceiling void depths, structural joists, other building services and the location of the MVHR unit all need to be considered. This requires coordination between the architect, timber kit manufacturer, ventilation designer and installer, whose practical experience can be invaluable.
Although some coordination will naturally take place on site, considering MVHR during the building design stage can help avoid unnecessary changes, additional costs and compromises during construction.
Choosing the MVHR Unit Location
One of the first decisions is where the MVHR unit will be located.
Wherever possible, the unit should be positioned within the heated envelope of the building. Utility rooms, dedicated plant cupboards and other accessible internal spaces are often suitable locations.
It can be tempting to position the MVHR unit in an unheated loft, particularly where internal space is limited. However, there are several factors worth considering.
Firstly, the unit will require regular maintenance, including filter changes and occasional servicing. Consider whether the proposed location provides safe and practical access throughout the life of the system. Ceiling-mounted units may require a suitable access hatch, while wall-mounted units typically need adequate clearance in front of the unit. Always check the manufacturer's maintenance requirements.
Positioning the unit outside the heated envelope introduces additional thermal considerations. Heat loss from the unit and associated ductwork can reduce overall system efficiency and may affect the building's energy calculations.
The unit will also require a condensate drain connected to a suitable drainage point. This can be overlooked when positioning equipment away from existing plumbing services.
Consider how the homeowner will monitor and control the system. A suitably positioned controller can make operation and fault monitoring more convenient, particularly where the unit is less accessible.
Ideally, the MVHR unit should also be positioned relatively close to an external wall, allowing the outside air intake and exhaust ducts to be kept short. The Passivhaus Trust's How to Build a Passivhaus guidance recommends locating the unit adjacent to an external wall or within approximately two metres of it.
Unit location also needs to account for noise transmission, particularly where bedrooms or other noise-sensitive rooms are nearby.
Getting the unit location right at the beginning can make the remaining ductwork design considerably easier.
Coordinating the Ductwork Routes
Once the MVHR location has been established, the next stage is determining how the ductwork will reach each room.
This is where coordination with the architect, structural engineer, timber kit manufacturer and installer becomes particularly important.
An MVHR drawing may show ductwork running neatly through a ceiling void, but that doesn't necessarily mean there is enough space to accommodate it.
The available void depth needs to be confirmed, along with structural members and other services occupying the same space.
It's also important to consider where ducts need to cross, change direction or connect to ceiling valves.
For example, two 204 × 60 mm flat ducts may require a 121 mm deep drop-down fitting to allow one to cross the other. If both ducts also require 25 mm insulation, the overall depth at that crossing could increase to approximately 221 mm, depending on the fitting arrangement.
It's a small detail, but one that can easily be missed when reviewing routes on a floor plan.
Similar problems can arise where ducts transition into ceiling valve connections, where additional depth may be required to accommodate the fitting.
There is also increasing competition for space within ceiling voids as more mechanical services are incorporated into new homes. Plumbing, electrical services, heating and cooling systems may all need to share the same areas.
Identifying potential clashes during the design stage is generally easier than resolving them once construction is underway.
Choosing the Right Ductwork
Several types of ductwork are commonly used for MVHR installations, including galvanised steel, rigid PVC, thermally insulated ductwork and semi-rigid systems.
Each has advantages, and the most suitable option depends on the building layout, available space and airflow requirements.
Flat rectangular ductwork can be useful where ceiling void depths are restricted, while circular ductwork may be preferable where space allows.
Semi-rigid ductwork is also widely used in residential installations, particularly where individual duct runs can be routed through intermediate floor zones to serve different rooms.
One advantage of long-coil semi-rigid systems is that individual runs can often be installed with fewer joints. Compared with rigid ductwork assembled from multiple shorter lengths, this can reduce the number of connections that need to be sealed and therefore the potential for air leakage.
However, airtightness still depends on using compatible components and following the manufacturer's installation instructions.
The choice of ductwork shouldn't be based on available space alone.
The designer must also consider airflow requirements, duct dimensions, pressure losses and the potential for noise generation.
Reducing duct sizes simply to make a route fit can affect the overall performance of the ventilation system.
Duct Sizing and Air Velocity
Air velocity is an important consideration when sizing MVHR ductwork.
As air travels through a duct, resistance is created by the duct itself, along with bends, fittings, transitions and valves. Higher air velocities can increase pressure losses and contribute to unwanted noise.
For low-noise residential MVHR design, Passivhaus guidance favours relatively low air velocities, typically below 2 m/s within distribution ductwork, with velocities up to approximately 3 m/s potentially acceptable in larger main ducts close to the MVHR unit.
These are useful design benchmarks rather than universal statutory limits.
The designer should calculate the pressure losses across the proposed ductwork system and confirm that the selected MVHR unit can deliver the required airflow against the resulting resistance.
This is particularly important where duct runs are long or where numerous bends and transitions are required.
A ductwork layout should therefore be more than a drawing showing how ducts connect to each room. It should be supported by calculations demonstrating that the system can perform as intended.
Keeping Ductwork Within the Heated Envelope
Wherever possible, we recommend keeping the MVHR distribution ductwork within the heated envelope of the property.
Routing ducts through unheated spaces, such as cold lofts, introduces additional considerations around heat loss, insulation and condensation.
The insulation requirements depend on the function of the duct and the temperatures of the air passing through it.
For example, ductwork carrying warm extract air through an unheated loft can lose heat before the air reaches the heat recovery unit. This reduces the amount of heat available for recovery.
Similarly, ductwork carrying cold outside air through a warm internal space may require suitable vapour-resistant insulation to prevent condensation forming on the external surface of the duct.
When reviewing insulation specifications, it's important to consider both insulation thickness and thermal conductivity, normally expressed in W/(m·K).
A 25 mm insulation layer with a thermal conductivity of 0.040 W/(m·K), for example, provides less thermal resistance than a 50 mm layer of the same material.
The appropriate insulation specification should be established during design, taking account of applicable building standards, duct location, condensation risk and any additional project requirements, such as Passivhaus certification.
The Passivhaus Planning Package (PHPP) also accounts for thermal losses associated with the ventilation system, including the arrangement of external air ducts.
Insulation affects the physical space required for installation, which is another reason why ductwork coordination must consider more than nominal duct dimensions.
Semi-Rigid Ductwork and Timber-Frame Coordination
Semi-rigid ductwork is a popular option for residential MVHR installations, particularly where duct routes need to pass through intermediate floor zones.
These systems are commonly available in nominal 75 mm and 90 mm sizes and are supplied in long coils, often allowing individual runs to be installed with fewer joints.
The ductwork provides flexibility while maintaining its shape, making it useful when coordinating routes through a building.
Some manufacturers also offer products with smooth internal surfaces and antistatic or antimicrobial properties, although the specification varies between systems.
One of the most important considerations when using semi-rigid ductwork is the construction of the intermediate floor.
Understanding the Joist Layout
Before finalising the ductwork design, establish what type of floor joists will be used.
Traditional solid timber joists, engineered I-joists and metal-web joists have different requirements when it comes to routing services through the floor structure.
Some joist systems provide openings through which ductwork can be routed, while others may require appropriately positioned penetrations.
Any openings must follow the structural designer's and joist manufacturer's requirements. It should never be assumed that a joist can simply be drilled or altered to accommodate a duct.
Where a timber kit manufacturer is involved, we recommend requesting the proposed joist layout and sharing it with the MVHR designer.
Ideally, the joist arrangement should be incorporated into the ventilation drawings so duct routes can be coordinated with the actual floor structure.
This can make a significant difference during installation, particularly where several ducts need to pass through the same area.
It's also worth informing the timber kit manufacturer early that MVHR is being incorporated into the property.
This allows ventilation requirements to be considered alongside the structural design rather than being addressed after the floor system has been finalised.
Allowing Space for MVHR Manifolds and Attenuators
Where a radial semi-rigid ductwork system is used, individual room ducts normally connect back to distribution manifolds.
Typically, there are two manifolds: one serving the supply air ductwork and another serving the extract air ductwork.
On the room side of the MVHR unit, the main supply and extract connections will commonly connect through acoustic attenuators before reaching the respective manifolds.
These attenuators help reduce noise transmitted from the ventilation unit into the distribution ductwork.
The attenuators and manifolds are often positioned close to the MVHR unit, sometimes above it within a ceiling void or in an adjacent plant space.
However, it's important to allow sufficient space for the complete arrangement, including the connecting ductwork and the fittings needed to change direction.
Some attenuators can be substantial in length, so their dimensions should be checked rather than assuming they will fit within the available space.
Several semi-rigid ducts may also need to leave each manifold in different directions. Sufficient room must be available to route them without excessive bends or restrictions.
The manifold and attenuator positions should therefore be considered when selecting the MVHR unit location, rather than being left until installation.
Coordinating External Air Intake and Exhaust Terminals
An MVHR system requires connections to outside air for both the fresh air intake and the exhaust air discharge.
Their locations should be agreed during the design stage, particularly where ducts need to pass through external walls or roof structures.
The proposed positions should account for the building layout, available duct routes, external appearance and potential sources of contamination.
It is also important to consider separation between the intake and exhaust terminals to minimise the risk of discharged air being drawn back into the system.
Nearby flues, vehicle exhausts and other potential pollution sources should be considered.
The external duct routes should be kept as short and direct as practical, with suitable insulation and condensation protection where required.
Coordinating these openings early can help avoid difficult duct routes and unnecessary changes to completed external finishes.
What Should Be Confirmed Before Installation?
A coordinated MVHR design should consider how the complete system will fit within the building, not simply how air will be supplied to and extracted from each room.
Before installation begins, the following details should be established:
- MVHR unit location.
- Ductwork routes and ceiling void space.
- Timber kit and joist coordination.
- Manifold and attenuator locations.
- External intake and exhaust positions.
- Airflow requirements and pressure losses.
- Have the installer review the proposed ductwork layout.
The earlier this information is available, the easier it becomes to produce a design that can be installed without unnecessary alterations.
Final Thoughts
A successful MVHR installation starts well before the ductwork arrives on site.
Taking the time to coordinate the ventilation design with the building structure and other services can prevent many of the problems that otherwise emerge during construction.
For homeowners planning a self-build, it's worth involving an MVHR designer early enough that ventilation requirements can influence the building design where necessary.
For architects and building designers, obtaining structural information and coordinating services with the timber kit manufacturer and installer can make a considerable difference to the finished installation.
There will always be details that need to be resolved on site, and an experienced installer plays an important role in making the design work in practice.
Ultimately, the objective is straightforward: to produce an MVHR system that meets the ventilation requirements and can be installed, maintained and operated efficiently throughout the life of the building.