A client complaining about MVHR noise is not just voicing an impression, there is a figure behind it. Approved Document F advises that the average A-weighted sound pressure level of a ventilator running under normal conditions, and not at boost rates, should not exceed 30 dB LAeq,T in noise-sensitive rooms such as bedrooms and living rooms when a continuous system runs at its minimum low rate, nor 45 dB LAeq,T in less noise-sensitive rooms such as kitchens and bathrooms. Two consequences for the diagnosis. The level is read at the low rate, not on boost. And what the guidance frames is the noise in the room, not the noise of the machine, so a unit that looks faultless on its datasheet can still miss these levels once installed.
A noisy MVHR unit is diagnosed by ear first, on low speed then on boost. A low droning that rises and falls, the helicopter noise clients describe, signals a vibration. A high-pitched whistle signals air squeezed through too narrow a passage. A click or bang signals a loose part or a damper. Each family has its own fix, and a fix applied to the wrong family gains no decibels.
Noisy MVHR unit: recognising the cause from the type of noise
| Noise heard | Most common cause | Check on the survey | Fix |
|---|---|---|---|
| Low droning, helicopter-like beat | Casing in rigid contact with the roof timbers, the floor or a lightweight ceiling | Hand on the casing, then casing lifted a few centimetres | Elastic suspension, anti-vibration mounts, flexible couplings |
| Growing rumble, scraping | Worn bearing, dirty or unbalanced impeller | Impeller turned by hand, power off: play, tight spot | Clean the impeller, replace the motor or the unit |
| High-pitched whistle at one vent | Clogged or wrongly sized vent, excessive flow, blocked air inlet | Noise compared with the window shut then open, flow measured at the vent | Cleaning, vent of the right size, air inlets restored |
| Whistle in the ceiling | Crushed duct, tight elbow, reduced diameter, loose joint | Duct route followed in the loft, from vent to unit | Route reworked, diameter restored, joints sealed |
| Constant whoosh in every room | Airborne motor noise carried along the ducts | Noise louder at the vents nearest the unit | Sound trap as close to the unit as possible |
| Intermittent banging | Non-return damper or outlet flap flapping, loose duct | Listen in strong wind and when the speed changes | Suitable damper, duct secured, terminal tightened |
Helicopter noise or droning: look for the vibration
The low beat that comes and goes is a mechanical vibration amplified by a surface. The textbook case is a casing screwed to a joist, laid flat on the loft floor or pressed against plasterboard, which then acts as a sound box for the whole house. The test takes ten seconds. Lift the casing slightly or slide a soft insulation pad under it. If the noise drops, the cause is the fixing, and the fix is a suspension.
If the noise persists with the casing suspended, switch off the power and turn the impeller by hand. A tight spot, play or scraping point to a worn bearing. A layer of greasy dust on the blades is also enough to unbalance the wheel, especially on the kitchen side. Note too whether the noise only appears on boost: on a two-speed unit, a fixing that is fine on low speed can resonate at the higher one.
High-pitched whistling: air squeezed through too narrow a passage
A high-pitched whistle always comes from a spot where the air speeds up. At the vent, the causes are dirt, a self-regulating vent of a smaller size than designed or a stuck humidity module. In the home, the cause is blocked or missing air inlets: the dwelling goes into depression, air rushes under the doors and the vents whistle. The test is simple. If the whistle changes when a window opens, the home lacks fresh air, not adjustment.
If the whistle comes from the ceiling rather than the vent, follow the duct: a crushed section, a tight 90° elbow right behind the vent or a reduction in diameter produce the same sound. To decide between vent and ductwork, measure the flow at the vent with an anemometer or a flow hood (the tools for measuring ventilation flow) and compare it with the flow expected for the room.
Clicks and rattles: dampers and loose parts
An intermittent bang is almost always linked to the wind or to a change of speed. A non-return damper or outlet flap that flaps, a loose roof terminal, a duct knocking against a purlin, loose cover screws or a vent grille not clipped in properly are the usual causes. Listen in strong wind with the unit off: if the noise continues, it comes from the outlet, not the motor.
Noisy MVHR unit, what to do: the step-by-step procedure
The order matters as much as the actions. A sound trap fitted to a vibrating casing does nothing for structure-borne noise, and a vent replaced on a crushed duct still whistles.
- Listen on low speed then on boost, room by room, and note the type of noise and where it is loudest.
- Clean the vents and air inlets, and check that none is blocked or has been swapped for a model with a different flow rate.
- Switch off the power, open the casing and check the impeller, the motor play and the tightness of the cover.
- Check the installation: suspension, rigid contact with the roof timbers or the ceiling, flexible couplings between casing and ducts.
- Follow the ducts in the loft: crushed sections, tight elbows, loose joints, slack flexible duct that sags.
- Measure the flow rates at the vents and compare them with the dwelling's required rates.
- Only then treat the remaining noise: sound trap, casing enclosure or replacement.
The first four steps fit into one visit and settle a large share of complaints. The next three need access to the loft and a measuring instrument, and they are what justifies a diagnostic line on the quote.
Identify the source of the noise on your MVHR unit before stepping in
Telling a normal noise from an abnormal one (humming, vibration, whistling)
An MVHR unit often makes a light, steady hum, especially on high speed. The noise becomes suspect if it appears suddenly, varies between rooms, or comes with vibrations, rattling, or a high-pitched whistle. Also check whether the sound gets louder when a door closes. That can reveal an imbalance in the flow rates.
Locating the source: unit, ducts, vents, mounting and partitions
Start near the unit. Put your hand on the casing. A clear vibration often points to a mounting or fixing issue. Then follow the ducts. A crushed, poorly tensioned duct, or one touching the roof structure, can amplify the noise. On the vents side, whistling often comes from an air passage that's too narrow, a clogged vent, or one that's poorly adjusted.
Checking the general condition: dirt build-up, mechanical play, tightness and power supply
Cut the power then check for dirt build-up and the condition of the air inlets and fan. Mechanical play (bearing, impeller) or loose screws can make the whole unit vibrate. Also check the power supply and connections. Unstable voltage or a worn capacitor can make the motor work harder. If in doubt, call in a professional. To go further on maintenance and the checks to plan, see our article on MVHR maintenance.
Reduce noise through basic maintenance and settings
Cleaning vents, air inlets and the unit casing to limit turbulence
When an MVHR unit gets dirty, the air is forced through, and the noise rises. Dust off the vents and air inlets, then vacuum the casing and accessible grilles. Gentle cleaning, without soaking the parts, is often enough to restore a steadier flow and a quieter operation.
Adjusting flow rates and limiting over-ventilation: speed, humidity-controlled settings, balancing
Over-ventilation makes vents whistle, especially in small rooms. Check the selected speed, the state of the humidity-controlled settings, and the balancing between rooms. The goal is to get back to the planned nominal flow rate, without a permanent "boost", while keeping good air quality (see also the regulatory ventilation flow rates).
Checking dampers and accessories to avoid whistling and backdraughts
A stuck damper, a poorly fitted connector, or an unsuitable accessory can create continuous whistling. Check the vent dampers, the couplings, and any backdraught dampers. Refit parts, replace worn seals, and secure the backdraught damper to prevent air from flowing back.
Improve the acoustics of the ductwork: solutions on ducts and connections
Replacing or reworking the ducts: suitable flexible ducting, diameter, elbows and airtightness
A noisy MVHR unit often comes from ductwork that's under too much strain. Restore the right diameter along the whole run. Limit tight elbows and crushed sections, especially on flexible ducting. Take care with the connections using couplings and sealing tape to avoid whistling and air leaks (see also duct network sizing).
Approved Document F is just as explicit on the ductwork itself. Rigid ducts wherever possible, flexible ductwork kept to final connections and to a maximum of 1.5 m, pulled taut so the full internal diameter is maintained. It also asks that each air terminal keep a free area of at least 90 % of the free area of its duct, which is precisely the restriction that turns into a whistle.
Adding a sound trap or MVHR silencer: where to fit it and how to size it
A sound trap treats airborne noise, the motor noise that travels through the ducts to the vents. It does nothing against a vibration. In a house, fit it on the extract side, as close as possible to the casing, before the first branch: that is where the noise travels back to the rooms, and a single silencer on the main duct treats every vent. If the complaint comes mostly from the bedroom nearest the unit, a second silencer on its branch completes the treatment. If it comes from the outlet, heard in the loft or next door, the discharge duct gets one.
Choose a model of the same diameter as the duct, never a reduction, with enough length to attenuate the mid and high frequencies where the whoosh is heard, often between 50 cm and 1 m. Check the pressure loss on the manufacturer's sheet so the unit keeps its flow rates. Fit it straight, with no elbow right at its inlet, and insulate it if it crosses a cold loft, otherwise it condenses like the rest of the ductwork.
Treating structure-borne noise: flexible couplings, anti-vibration clips and decoupling
Vibrations travel through the mounts. Insert a flexible coupling between the casing and the ducts. Use anti-vibration clips and avoid direct contact with the roof structure or partitions. A small amount of decoupling is often worth more than a "more powerful" unit.
Limit vibrations from the casing: mounts, location and sound insulation
Fitting the casing on anti-vibration mounts and removing rigid contact points
An MVHR casing quickly transmits its vibrations to the structure. Fit it on anti-vibration mounts (pads, rubber bushings) or suspend it with dedicated fixings. Avoid screwing it directly into a joist or a slab. On the ducts side, plan for flexible couplings and leave a small amount of play. The goal is simple: zero rigid contact between the casing, the ducts and the plasterboard.
Anti-noise rings and anti-vibration mounts: choosing them well
Both treat the same thing, the structure-borne noise the casing passes into the building, through two different set-ups. An anti-vibration mount is a rubber block fitted between the casing's feet and its support. It only works if it compresses a little under the load: too stiff under a single-flow unit weighing a few kilos, it transmits everything; too soft under a heat-recovery unit weighing several tens of kilos, it lets the casing sway. Choose it from the load per mount stated by the manufacturer, and do not tighten the screw so far that it crushes the rubber.
Anti-noise rings are used for hanging. Elastic rings or straps hooked to the roof timbers let the casing hang freely, without touching the insulation or the floor. It is the simplest fix in an unconverted loft, provided the casing is held in place so it does not swing on its ducts. Some suppliers also use the name for foam seals fitted between the vent and its sleeve: they calm a vent rattling in its housing, not a flow whistle.
Choosing the right location: loft, plant room, suspended ceiling and accessibility
Location often accounts for half the quietness. In a loft, install on a stable support, away from bedrooms. In a plant room, you gain in comfort and easy access. In a suspended ceiling, watch the weight and vibrations on a lightweight frame. Always keep access to the filters, tap-off points, condensate drain and power supply.
Boosting sound insulation around the casing without hurting maintenance
To reduce airborne noise, build a small enclosure with mineral wool and cladding, without compressing the materials or blocking the motor's ventilation. Plan for a removable cover or access hatches. The insulation should stay clean, dry and inspectable. Otherwise you gain decibels but lose ease of maintenance.
Noisy MVHR unit in a flat: what the occupant can do, and what falls to the building
In a block of flats, ventilation is most often communal: a fan unit on the roof extracts air from every flat through shared vertical ducts. The occupant only acts on what is inside the flat, the vents and the air inlets. The fan unit, the ducts and their settings are common parts, maintained by the managing agent's contractor.
What the occupant can do themselves
- Clean the vents with soapy water, without touching their setting, and dust the trickle vents in the windows.
- Check that each vent is the original model: in a communal system, each vent is sized for a flow rate, and a vent of another size unbalances the riser and whistles.
- Never connect a powered cooker hood to the kitchen extract vent, which is ruled out on a communal system.
- Restore the air inlets and the gap under the doors, otherwise the flat goes into depression and its vents whistle.
- Note when the noise appears (time, wind, room) before reporting it to the managing agent.
What falls to the managing agent and their contractor
A drone heard mostly on the top floors often comes from the roof unit, whose vibration travels down through the slab. Vents whistling throughout the block betray excessive duct pressure. In both cases, the request goes to the managing agent, with the occupant's notes: the contractor can rework the unit's suspension, adjust its pressure or fit a silencer on the collector. The choice between a single duct and a branched network also shapes how noise is shared between flats.
If the flat has its own unit, with a casing in the bathroom's suspended ceiling, it is the house case again, with one more difficulty: the ceiling's lightweight frame resonates. Elastic suspension and flexible couplings then come before anything else. The Approved Document F noise levels apply to new work; in an older block, they remain the benchmark for backing up a complaint.
Replacing the MVHR unit: choosing a quieter, compliant model in 2026
Comparing noise levels: dB(A), sound pressure, spectrum and measurement points
For a quieter MVHR unit, compare the dB(A) level and, above all, the measurement point. One manufacturer may quote a level at the casing, another at the vent. Check the sound pressure, the spectrum (low frequencies are perceived more strongly) and the test conditions. In a renovation, aim for a casing fitted on rubber mounts and vents suited to the sleeping areas.
Selecting the right technology: single-flow, humidity-controlled, heat-recovery and uses
Self-regulating single-flow units are robust, but not always the most comfortable. Humidity-controlled units adjust flow rates according to moisture levels, which is useful in occupied housing. Heat-recovery units make sense if you're also addressing airtightness and insulation, since they recover heat. The right choice depends on the volumes involved, the existing ductwork and access to the loft.
Securing the job in 2026: compliance, certification, funding and good installation practice
In 2026, secure compliance for flow rates, air inlets and roof or façade outlets against Approved Document F. On funding, heat-recovery units travel inside an ECO4 package for an eligible household, as the ventilation that has to accompany a tighter envelope. In practice, take care over duct airtightness, balancing, and plan for easy filter access for maintenance. The fan unit, the ducting and the acoustic accessories are then priced in the same survey as the room-by-room flow rates.
