A ventilation system that keeps occupants awake is a ventilation system that gets switched off. Acoustic performance is not a secondary consideration in professional ventilation design—it is a primary one. Noise is the single most common reason that installed ventilation systems are disabled by residents, and a disabled ventilation system delivers none of its health, moisture-control, or regulatory compliance benefits.

This guide explains the sources of ventilation noise, how NICEIC-certified engineers design around them, and what to look for when specifying a system that will run silently for decades.

Understanding the Sources of Ventilation Noise

Ventilation noise reaches occupants through several distinct pathways, each requiring a different engineering response.

Airborne Fan Noise

The fan motor and impeller generate broadband noise that propagates both through the ductwork (duct-borne transmission) and structurally through the plant mounting into the building fabric. High-quality MVHR units from manufacturers such as Zehnder, Paul, and Brink incorporate EC (electronically commutated) brushless motors that operate significantly more quietly than AC motor equivalents, particularly at part-load speeds. Flexivent specifies EC-motor units as standard for residential installations.

Flow-Generated Noise

Even with a quiet fan, excessive duct velocity generates its own noise through turbulence at bends, junctions, and terminal grilles. Duct velocity noise is a function of airspeed: at 3 m/s through a circular duct, noise generation is minimal; at 5 m/s, audible turbulence becomes likely in quiet bedrooms. Correctly sized ductwork—with properly calculated equivalent lengths and pressure drops—is the primary design control.

Structural Transmission (Flanking)

Vibration from a fan motor transmits through mounting points into the building structure and radiates as low-frequency sound from walls, floors, and ceilings. Anti-vibration mounts on the air-handling unit and flexible duct connections at the unit inlet and outlet break the structural transmission path. This detail is frequently omitted in non-specialist installations with predictable acoustic consequences.

Cross-Talk Between Rooms

In balanced MVHR systems, sound from a noisy room—a television, conversation, or musical instrument—can transmit through shared ductwork to adjacent rooms. Purpose-designed acoustic attenuators (silencers) fitted in the duct runs between rooms interrupt this transmission path. Their use is mandatory in professionally designed systems serving bedrooms adjacent to living spaces.

Key Design Controls for Acoustic Performance

Technical Tip: The World Health Organisation guideline value for continuous background noise in sleeping environments is 30 dB(A) LAeq. A well-designed MVHR system operating at normal ventilation rates should contribute no more than 25 dB(A) at the terminal grille in a bedroom—well within this threshold. Achieving this requires duct sizing for maximum 2.5 m/s velocity in bedroom supply branches, EC-motor fan selection, anti-vibration isolation, and acoustic attenuators in the supply distribution network. Flexivent engineers calculate predicted acoustic performance at the design stage, not after installation.

Duct Sizing and Layout

Flexivent engineers perform full pressure-drop calculations for every duct run, sizing circular or flat-oval ductwork to maintain velocities below 3 m/s in occupied areas and below 4 m/s in roof-void distribution. Semi-rigid ducting with smooth inner bores is specified in preference to flexible corrugated duct, which generates additional turbulence noise and has significantly higher pressure loss per metre.

Silencer Specification

Rectangular attenuator silencers—packed with mineral wool acoustic infill within a perforated inner liner—are installed in the main supply and extract branches leaving the air-handling unit. Additional in-line circular silencers are positioned upstream of bedroom grilles in layouts where cross-talk is a risk. Attenuation performance is selected from manufacturer acoustic data to achieve the target room noise level.

Unit Location

Where possible, Flexivent locates air-handling units in loft spaces or plant cupboards remote from occupied rooms. Party-wall loft spaces shared with neighbouring properties in terraced houses require additional consideration, as sound transmission through the wall is a legal noise nuisance risk.

Commissioning for Acoustic Performance

Acoustic performance should be verified during commissioning. Flexivent engineers carry out a subjective assessment of all terminal grilles at design airflow rates and adjust balancing dampers to equalise pressure distribution, which prevents individual grilles from operating above target velocity.

The Consequence of Getting It Wrong

Poorly specified or installed ventilation systems generate noise complaints, tenancy disputes in rental properties, and—critically—occupant behaviour change. A noisy system is switched off. A switched-off system cannot comply with Part F Building Regulations, cannot control moisture, and cannot protect the health of occupants. The acoustic specification is not a luxury finish: it is what determines whether your investment actually functions.

Commission a Silent System

Flexivent's engineers design every system to perform within WHO and Building Regulation acoustic guidance from the outset. We provide acoustic performance calculations as part of our design documentation, so you have confidence before installation begins.

Contact Flexivent today for a bespoke ventilation design consultation. A system you cannot hear is a system that never gets turned off—and that is where the real performance is delivered.

Flexivent: Engineering quiet, compliant ventilation for UK residential properties. Our acoustic design approach ensures your system performs silently and reliably for the long term.

The Silent System: Understanding Acoustics in Ventilation and How Professional Design Eliminates Fan Noise