MVHR Vent Positioning: Installation Mistakes and How to Avoid Them
Incorrect MVHR vent positioning is one of the most common reasons a well-specified system underperforms in practice. Where you place supply and extract valves determines airflow paths, pollutant removal effectiveness, and whether the system delivers its rated efficiency in real-world conditions.
Getting vent positioning right from the design stage saves commissioning time, reduces callbacks, and ensures the installation meets Part F requirements first time.
Why Does MVHR Vent Positioning Matter So Much?
MVHR vent positioning controls the movement of air through a building. Supply valves deliver fresh, pre-warmed air into habitable rooms, and extract valves remove stale, humid air from wet rooms. The airflow path between supply and extract points determines how effectively pollutants, moisture, and CO2 are swept from the occupied space.
Poor positioning creates short-circuits, dead zones, and stagnant air pockets that no amount of airflow adjustment can fully resolve after installation. The problem is that positioning errors are largely invisible once a building is handed over, yet their impact on indoor air quality and measured ventilation rates is significant.
The Principle of Airflow Cascade
Effective MVHR design relies on an airflow cascade, where fresh supply air enters habitable rooms such as bedrooms and living rooms, flows through circulation spaces such as hallways and landings, and is finally extracted from wet rooms including bathrooms, en-suites, utility rooms, and kitchens.
This cascade draws polluted air towards extract points naturally, using the pressure differential created by the balanced fan system.
Positioning supply and extract valves in the same room, or locating extract valves in rooms with low moisture or pollutant loads, breaks this cascade. The system continues to move air, but it no longer moves air efficiently through the building in a way that improves air quality at occupied points.
Where Should MVHR Supply Vents Be Positioned?
Supply vents should be positioned in habitable rooms only: bedrooms, living rooms, dining rooms, and home offices. These are the spaces where occupants spend the most time and where fresh air delivery has the greatest impact on comfort and health.
Within each room, position supply valves on the ceiling or high on the wall, away from external walls and windows where cold surfaces could cause discomfort from draughts. Central ceiling positions work well in rectangular rooms because they allow supply air to distribute evenly before reaching the occupied zone.
Avoid positioning supply valves directly above seating areas or beds, as direct airflow at low velocity can still cause occupant discomfort.
Minimum Distance Rules for Supply Valves
Supply valves should be positioned at least 500mm from the nearest wall to avoid Coanda effect issues that cause supply air to cling to the ceiling surface and reduce throw distance. In open-plan spaces, multiple supply points may be needed to ensure even distribution, particularly where floor area exceeds 25 to 30 square metres.
Always check the valve manufacturer's throw distance data against the room dimensions during the design stage. Our guide on MVHR system design covers valve selection and room-by-room airflow calculation in more detail.
Where Should MVHR Extract Vents Be Positioned?
Extract vents belong in rooms that generate moisture, heat, and pollutants: bathrooms, en-suites, WCs, kitchens, utility rooms, and plant rooms. These are the spaces where stale air accumulates and where the extract side of the balanced ventilation system does its most important work.
Position extract valves on the ceiling or high on the wall, as close as practical to the primary source of moisture or pollutant generation. In bathrooms, this means above or adjacent to the shower or bath enclosure rather than near the door.
In kitchens, the MVHR extract valve handles background ventilation only and is not a substitute for a recirculation or ducted cooker hood above the cooking appliance. For guidance on kitchen ventilation requirements alongside MVHR, our Part F ventilation regulations guide clarifies which extract rates apply to which room types.
Avoiding Short-Circuit Between Supply and Extract
Short-circuiting occurs when supply air travels directly to an extract valve without passing through the intended airflow path. This typically happens when a supply valve and an extract valve are located in the same room or in adjacent rooms with an open doorway between them. The result is that fresh supply air is immediately extracted before it reaches the occupied zone, reducing effective ventilation in habitable rooms.
To prevent short-circuiting, maintain clear separation between supply and extract locations. In open-plan kitchen-living spaces, position the supply valve at the living room end and the extract valve at the kitchen end, using the full room depth as the airflow path. If the project layout makes separation difficult, discuss the design with the MVHR system designer before fixing duct positions.
What Are the Most Common MVHR Vent Positioning Mistakes?
The mistakes below appear repeatedly on site and are responsible for the majority of MVHR performance complaints raised after practical completion. Most are avoidable with proper design coordination before first fix.
Supplying wet rooms: Positioning supply valves in bathrooms or kitchens creates reverse airflow pressure in these spaces, drawing moist air towards habitable rooms rather than away from them. Supply valves must never be placed in wet rooms.
Extracting from bedrooms: Extract valves in bedrooms remove air before it has had the opportunity to circulate through the dwelling. This reduces airflow to other habitable rooms and can create negative pressure in sleeping areas.
Placing valves too close to doorframes: Valves positioned within 200mm of a door opening are prone to turbulence and inconsistent airflow, particularly when the door is open or closed. Keep valves well clear of doorframe edges.
Duct drops in unheated voids: Routing supply ductwork through uninsulated roof voids or cold floor voids allows pre-warmed supply air to lose heat before it reaches the valve. This reduces the effective heat recovery benefit and can lead to condensation in the duct. Our article on MVHR efficiency explains how duct routing affects delivered thermal performance.
Valves positioned below ceiling height in high rooms: In rooms with ceilings above 2.7 metres, valves positioned at standard height may not adequately mix supply air into the occupied zone. Higher ceilings may require adjusted valve selection or additional supply points.
How Does Vent Positioning Affect MVHR Commissioning?
Commissioning an MVHR system involves balancing airflow rates across all supply and extract valves to match the design specifications. Poorly positioned valves make balancing difficult and, in some cases, impossible to achieve correctly. If a valve position creates a dead zone or short-circuit, adjusting the airflow rate at that valve will not resolve the underlying problem.
During commissioning, each valve is adjusted using a commissioning tool to match the design airflow rate in litres per second or cubic metres per hour. The total supply airflow across the building must equal the total extract airflow within a tolerance specified by the manufacturer, typically plus or minus 10%.
Systems that cannot be balanced to within this tolerance during commissioning frequently indicate a vent positioning or duct design issue that needs to be addressed before sign-off.
Documentation and Compliance Sign-Off
Part F requires that MVHR systems are commissioned and that the commissioning results are documented. The commissioning record must show measured airflow rates at each valve alongside the design targets. This documentation supports the building's compliance certification and forms part of the handover pack for the building control body.
Our article on MVHR system costs covers how commissioning is typically priced as part of the overall system installation budget.
How Does Vent Positioning Differ Between New Builds and Retrofits?
In new builds, vent positions can be coordinated at first fix before ceilings and walls are boarded, giving full flexibility to achieve optimal placement. Duct runs can be routed directly and efficiently, and valve positions can be selected purely on the basis of airflow performance.
Retrofits impose more constraints. Existing ceiling and wall constructions limit where ducts can be run without excessive disruption, and valve positions are sometimes determined by what is structurally feasible rather than what is aerodynamically ideal. In retrofit situations, single room heat recovery units offer an alternative that avoids the ductwork challenge entirely by providing localised heat recovery through an external wall without a central duct network.
For noise considerations that are particularly relevant in retrofit projects where units sit closer to occupants, our guide on MVHR system noise is worth reviewing before specifying.
Passivhaus and High Airtightness Projects
Passivhaus projects demand particular care with vent positioning because airtightness targets mean there is almost no background air movement to compensate for design errors. Every cubic metre of ventilation air must follow the intended path through the building for the system to meet the calculated heat demand.
The PHI component certification process for MVHR units used in Passivhaus buildings includes requirements that inform duct and valve design standards, so reviewing the unit's installation manual alongside the certification documentation is essential before positioning is finalised.
eFans MVHR Units for Trade Installers and Specifiers
eFans stocks a full range of heat recovery and MVHR units from Vent-Axia, Zehnder, Elta, and S&P, with heat recovery efficiency up to 93% and airflow rates from 32 m³/h to 900 m³/h. Whether you are installing a single room heat recovery unit through an external wall or specifying a whole house MVHR system across a multi-room new build, the range covers both applications with full product data for specification and compliance purposes.
Frequently Asked Questions
How many supply vents does an MVHR system need?
The number of supply vents depends on the number of habitable rooms in the dwelling and the whole-dwelling ventilation rate required under Part F. As a minimum, each bedroom and main living area should have its own supply valve. Larger open-plan rooms may require two supply points to achieve even air distribution.
The design airflow rate for each room is calculated based on floor area, occupancy, and the minimum ventilation rates set out in Approved Document F.
Can MVHR supply and extract valves be in the same room?
No. Supply and extract valves should not be in the same room. Placing both in the same space creates a short-circuit where fresh supply air is immediately extracted without passing through the building's occupied zones. Supply valves belong in habitable rooms and extract valves belong in wet rooms. The only exception is where specialist system design for large open-plan commercial spaces requires it, and in those cases the valve positions are carefully separated to maximise the airflow path length.
How high should MVHR vents be positioned on the wall?
Ceiling mounting is the preferred position for both supply and extract valves. Where ceiling mounting is not possible, high side wall mounting is acceptable, typically at least 1.8 metres above floor level. Low-level positioning is not recommended because it reduces the throw distance of supply air and places extract valves below the natural buoyancy level of warm, moist air, reducing extraction efficiency.
Does vent positioning affect MVHR noise levels?
Yes. Vents positioned close to the MVHR unit receive higher static pressure and can generate more noise at the valve face if not correctly sized and adjusted. Longer duct runs naturally attenuate noise, so rooms nearest the unit may require acoustic attenuators in the duct or lower-velocity valve selections to meet noise targets.
EN 15251 and CIBSE guidance recommend background noise levels in bedrooms should not exceed NR25 to NR30.
What happens if MVHR extract vents are blocked or covered?
Blocked extract vents reduce total extract airflow across the system, which disrupts the balance between supply and extract. An unbalanced system will either pressurise or depressurise the building, leading to either supply air failing to enter through valves correctly or extract airflow pulling in uncontrolled outside air through gaps in the fabric.
Either condition reduces indoor air quality, can cause condensation, and will result in a failed commissioning balance test. Blocked vents must be cleared and the system rebalanced before the building is occupied.
Should MVHR vents be positioned differently in bedrooms compared to living rooms?
The core positioning principles are the same, but bedrooms require additional consideration for draught sensitivity and noise. In bedrooms, avoid positioning supply valves directly above the bed or seating area, and select low-velocity valve types that distribute air gently across the room. For particularly noise-sensitive bedrooms, specify a sound attenuator on the supply duct branch serving that room and verify the valve face velocity does not exceed 0.15 m/s at the design airflow rate.