Noise from heat recovery units – causes and solutions

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Noise from heat recovery units – causes and solutions

Noise is one of the most common complaints I encounter during the installation of mechanical ventilation systems with heat recovery. A homeowner installs a unit, looks forward to clean air and low energy bills, and then – at night, when it's finally quiet – hears a buzzing or humming from the wall. Or worse: an unpleasant roar spreading through the entire hallway. This article addresses precisely such situations – systematically, from the basics to specific solutions that really work in practice.

Heat recovery ventilation should be practically inaudible – or at least not disturbing – in a properly designed and installed system. A proven threshold for residential areas is up to 25–30 dB(A) in the bedroom during standard operation. When this threshold is not met, the problem always has a specific cause – and that cause can be eliminated. Let's go through them one by one.

Where does the noise actually come from – basic sources

Before addressing specific causes, it is important to understand the physics. A heat recovery unit produces noise from three main sources:

Mechanical fan noise – rotating blades, bearings, rotor imbalance
Aerodynamic noise – airflow through ducts, bends, constrictions, grilles
Structural noise (solid-borne noise) – vibrations transmitted through mounting to the wall, ceiling or floor

In practice, it is almost always a combination of all three – with the dominant source depending on the specific installation. Decentralized units (directly in the perimeter wall) have a different noise profile than centralized units with branched ventilation systems. About the basic differences between centralized and decentralized solutions, you can also read in the article Centralized vs. decentralized heat recovery units – what is better in this Knowledge Center.

Mechanical fan noise – bearings, imbalance, wear

The fan is the heart of every heat recovery unit. With proper manufacturing and maintenance, its noise should be minimal – most modern EC (electronically commutated) motors really operate quietly. Problems arise in these situations:

Worn or dirty bearings

Fan bearings have a certain lifespan – typically 40,000–80,000 hours of operation for central units, while this value may be lower for cheaper decentralized models. A worn bearing emits a characteristic high-frequency squeak or grinding, which worsens with increasing speed. A dirty bearing (e.g., clogged with dust or condensate) may emit an intermittent rattle.

The solution is to replace the fan or the entire motor assembly. In modern units, the motor and fan are often one unit – the service technician should check the availability of replacement parts for the unit before ordering a service call.

Unbalanced rotor

Contaminants stuck on the fan blades cause imbalance, which manifests as low-frequency buzzing or vibrations that vary with speed. This problem occurs mainly in units without regular filter maintenance – dust gets through the clogged filter directly onto the blades. The solution is to clean the blades with a soft brush or compressed air, or to replace the filter (more on this in the article Maintenance and cleaning of heat recovery units – how often and how to do it properly).

Loose screws or motor assembly components

Vibrations from operation over time loosen the screws attaching the motor to the unit frame. The result is a characteristic rattling (clattering) at certain speeds – so-called resonant frequency. It is sufficient to tighten all visible mounting elements, but be careful: not too tight – excessive force can damage plastic parts or damping elements.

Aerodynamic noise – airflow as a source of problems

Aerodynamic noise is dominant in many installations – and paradoxically, it can be fought most effectively, because its causes are mostly structural and solvable without replacing the entire unit.

Too small pipe diameter

This is a classic – and I'll skip the diplomatic phrasing. An underestimated pipe diameter is an error in design or an attempt to save on materials. If the designer chooses a 63 mm diameter where it should be 90 mm or 110 mm, the air flow speed increases to 3–5 m/s or more, causing intense noise from the air flowing directly in the pipe. Golden rule: in residential areas, the air speed in the ducts should not exceed 2–2.5 m/s. How to correctly dimension the pipe diameter you will read in the article What pipe diameter do I need for heat recovery ventilation.

Sharp bend in the pipe

Every bend in the duct system is a potential source of noise. A sharp 90° elbow with a small radius (R less than 1.0 × pipe diameter D) causes significant turbulence in the air flow, which manifests as noise or hissing. The correct value is R ≥ 1.5D, ideally 2D. In practice, this means preferring extended elbows or flexible hoses (flex) while maintaining a minimum bend radius. Note: a too compressed or rolled-up flex hose is just as bad as a sharp corner – the internal corrugation of the hose significantly increases resistance.

Inappropriate placement or type of outlet/grille

Grilles and outlets are the most common aerodynamic "choke points" – if they are too small or too closed (for example, slats at a sharp angle), air passes through them at a high speed and produces a whistle or noise. An external aesthetic grille with the correct cross-section, such as external aesthetic grille for IVAR.HRC – pr. 160 mm, is designed to minimize pressure losses and aerodynamic noise while maintaining sufficient protective effect. Cheaper solutions with too dense a mesh or inappropriate slat geometry worsen this problem.

Clogged filters

A clogged filter is one of the most common and at the same time easiest to solve sources of increased noise. When the filter collects dust, its aerodynamic resistance increases – the fan has to work at higher speeds to achieve the same flow, which increases noise. At the same time, a clogged filter can cause turbulence at the inlet to the heat exchanger. The rule is simple: filters must be changed according to the manufacturer's recommendations, at least once every 6 months, every 3 months in dusty environments.

Structural and transmission noise – vibrations through walls and ceilings

This is the most insidious type of noise – it is the case when the unit itself is not loud, but vibrations are transmitted through the building structure to surfaces that then "resonate" like a speaker membrane. The result is a low-frequency buzzing that is hard to locate and often seems to come "from nowhere".

Direct rigid mounting of the unit

The most common mistake by installers: the unit is bolted directly to the wall or ceiling with metal anchors or screws without any damping. Every vibration from the motor is thus directly transferred to the massive concrete or masonry structure, which functions as a resonant plate. The solution is anti-vibration pads made of damping rubber or special foam, placed between the unit and the load-bearing structure. Certified pads have damping of 15–25 dB in the low-frequency range.

Rigid pipe connection without compensators

If the duct system is rigidly connected directly to the unit's outlet/inlet without flexible compensators (most often rubber or fabric inserts), vibrations from the unit spread further into the entire duct system. A proper installation includes a flexible connection of at least 200–300 mm directly on the unit's neck. In IVAR.PROFI-AIR CLASSIC systems, certified connecting elements such as connecting element for modular distribution box for IVAR.PROFI-AIR CLASSIC – pr. 90 mm can be used, which allow proper and tight connection without excessive mechanical stress in the joint.

Resonance of the duct channel

Thin sheet metal ducts can resonate at certain frequencies – just like a can. This is typically manifested as buzzing at specific fan speeds (and disappears at others). Solutions: damping of the duct with acoustic linings inside or outside, or changing the route or stiffness of the duct. Flexible plastic ducts are less prone to resonance, but have higher aerodynamic resistance – a compromise must be found.

Leaks in the duct system

Leaks are another underestimated source of noise. If air escapes through a small leak in the joint, it passes through a narrow gap at an increased speed – a whistle or hiss heard from an otherwise quiet room is a very typical phenomenon. Checking the sealing is therefore part of every noise diagnosis.

Quality sealing of the joints is key – for example, tongue-and-groove circular gasket pr. 63 mm for CLASSIC system ensures precise sealing of circular joints while maintaining simple installation. Seals must be checked during every major maintenance – rubber hardens and cracks over time, leading to loss of sealing and the onset of noise.

In addition to the joints, you should also check:

Sealing of the unit covers and service doors (loose latches)
Sealing of wall penetrations (air draw through the wall is noisy)
Sealing at the inlet and outlet neck of the heat exchanger

Low capacity – unit operates at the performance limit

Practical example: a customer installs a 5-room house with a recuperation unit dimensioned for a 3-room apartment – either for financial reasons or due to an incorrect design. The unit has to run at 90–100 % speed to ensure the required airflow. The result is significantly higher noise (fans are much louder at maximum speed), higher power consumption and reduced lifespan. The solution is either the installation of a second unit (zonal ventilation), or – where the layout allows – the installation of a booster for diameter 75 mm or 90 mm, which helps overcome pressure losses in long ducts and allows the main unit's speed to be reduced.

Choosing the correct unit capacity is a topic that is discussed in detail in the article How to choose a mechanical ventilation system with heat recovery for a family house.

Decentralized units – specific noise problems

Decentralized ventilators (directly mounted in the external wall) have a different noise problem character than central systems. Because they are located directly in the living or bedroom wall, they are more sensitive to external noise (traffic, wind) and to the noise of the fan itself. Modern electronic versions, such as decentralized ventilation unit HRC E – electronic version – 05 Master; pr. 160 mm, have adjustable fan speeds and several noise-optimized operating modes, including a night-time silent mode. Despite this, the following specifics apply:

Noise from the wall sleeve

If the external grille is oriented towards a noisy road or the prevailing wind direction, external noise can penetrate directly into the room. Solution: correct orientation (best away from the garden, sideways from the road), use of a grille with good sealing function and possibly installation of an internal acoustic attenuator (silencer). More about choosing the location of the exhaust can be read in the article Rooftop vs. façade exhaust for heat recovery – what to choose and when.

Noise transmission through the mounting sleeve

The mounting sleeve (sleeve) connects the external and internal parts. If not properly insulated and sealed, it serves as a noise conductor. Mineral wool or acoustic foam in the cavity around the sleeve significantly reduces this transmission.

How to diagnose the source of noise – step-by-step procedure

Before any intervention, it is important to correctly identify the source. Here is a systematic procedure that works in practice:

Record the nature of the noise: Is it a tone (whistling, buzzing), or a noise (hissing, humming)? A tone usually indicates a mechanical or resonance problem, while noise is aerodynamic.
Determine the dependence on speed: Does the noise disappear or decrease when the fan speed is reduced by 20–30 %? If yes, it is likely aerodynamic or resonant. If it remains the same, it is more likely mechanical.
Check the filters: Replace the filters and observe the change. If the noise decreases, the filters were the cause.
Tactile diagnosis: Gently touch the duct or the unit casing and feel for vibrations? If yes, the problem is in the transmission of solid-borne noise.
Sealing check: Run a wet hand (or a thin strip) over the pipe joints – do you feel or hear air flowing through the leak?
Noise measurement: A mobile app for measuring noise levels (e.g. NIOSH Sound Level Meter) is sufficient for an approximate measurement in dB(A). Measure 1 meter from the outlet in a quiet night environment.

Specific solutions according to the type of problem

If the noise is aerodynamic (hissing, humming)

Check and possibly replace the filters
Check the cross-section of grilles and outlets – increase the cross-section or replace with a larger type
Verify the actual air speed in the ducts (in practice, a piece of light fabric at the outlet is sufficient)
Repair or replace sharp bends with rounded elbows or flexible hoses with the correct radius
In the case of a permanent lack of dimensioning, consider the installation of an additional branch or a change in the route

If the noise is mechanical (tone, whistling)

Check the fan blades – clean them of deposits
Tighten loose screws
If the whistling persists, order a service intervention – replacement of bearings or motor
Check the availability of spare parts from the manufacturer

If the noise is structural (buzzing in the wall/ceiling)

Install anti-vibration pads under all mounting points
Insert flexible compensators on the unit outlets
Check for contact between the pipe and the building structure (the pipe must not lie directly on concrete – every contact is a vibration bridge)
Suspended pipe clamps must have a rubber insert – a metal clamp without damping transmits vibrations

Noise and standards – what the current legislation says

In residential areas, the requirements for mechanical ventilation are governed by STN EN ISO 16032 and related construction standards. For living rooms (living rooms, bedrooms), the maximum equivalent noise level from the ventilation system is 30 dB(A) during the day and 25 dB(A) at night. These values are measured at standard operating speeds in the center of the room, 1.2 m above the floor. In practice, most quality heat recovery systems meet these values when properly installed – problems arise due to errors in installation or sizing.

For comparison: a quiet bedroom without any ventilation is typically 20–25 dB(A). A whisper is about 30 dB(A). A mini-split air conditioning unit at minimum output reaches 35–42 dB(A). A properly installed heat recovery system can therefore be quieter than air conditioning.

Preventive maintenance as the best protection against noise

In the long term, the most effective protection against noise problems is regular preventive maintenance. Recommended schedule:

Interval	Task	What to look for
every 3–6 months	Filter replacement/cleaning	Contamination, filter damage
once a year	Check and tighten fastenings	Loose parts, cracks
once a year	Check sealing of connections	Cracks in gaskets, leaks
every 2–3 years	Cleaning of heat exchanger and ducts	Deposits, mold, dirt
every 5–7 years	Service inspection of motor and bearings	Wear, noise, vibrations

A detailed cleaning and maintenance procedure for the heat exchanger can be found in the article Maintenance and cleaning of heat recovery units – how often and how to do it correctly. If you have encountered other technical issues, it is also worth reading Common faults of heat recovery units and how to eliminate them.

Most frequently asked questions (FAQ)

Why is my heat recovery unit noisy only at night?

It is usually not because the unit operates differently at night – the problem is that at night the background is quiet and the noise, which you do not hear during the day, becomes disturbing. The background (street noise, household appliances, conversation) during the day masks the noise of the system. If this is a problem for you, the solution is to reduce the speed to night mode or to perform a proper acoustic optimization of the installation. Modern electronic units have a quiet night mode directly in the control system.

My unit makes a tapping sound when it starts – is that normal?

A short tapping sound at startup (first 5–10 seconds) can be caused by the fan starting up and pressure equalizing in the system – this is usually normal. If the tapping continues during operation, look for loose components or something that has gotten into the fan (pieces of filter, insects, small objects). Do not ignore the tapping in the long term – it can damage the blades or bearings.

Can I reduce noise by simply adding acoustic foam from the outside on the duct?

Acoustic foam on the outer surface of the duct can help reduce the radiation of airborne noise through the duct walls – but only if it is a thin metal duct. On flexible plastic ducts, the effect is minimal. This method also does not help if the problem is aerodynamic (noise spreading through the air inside the duct) or structural (vibrations through rigid fastening). A more effective solution is the installation of an internal acoustic attenuator (noise silencer) – a special insert in the duct with an absorbing material inside, length 500–1000 mm.

When is the noise from the heat recovery unit a reason for a warranty claim?

If the device exceeds the noise level declared by the manufacturer in the technical documentation during normal operating mode (50–70 % output), it is a reason for a warranty claim – provided the installation was carried out in accordance with the manufacturer's instructions. If the problem is caused by incorrect installation (rigid fastening without dampers, improperly sized ducts), the responsibility lies with the installation contractor. Therefore, always verify that the system was installed according to the design and the manufacturer's instructions before making a claim.

Will replacing the grille help in reducing noise?

Yes – but only if the grille is the source of the noise (too small cross-section, unsuitable geometry of the blades). Replacing it with a larger grille with a larger free cross-section can reduce aerodynamic noise by 3–6 dB, which is a noticeable change. If the noise is of a different nature (mechanical, structural), replacing the grille will not help. It is important to choose a grille certified for the specific system – not every part is compatible with every unit.

Can a heat recovery unit become louder after years of trouble-free operation without any visible faults?

Yes, and for several reasons: bearing wear (progressive over time), accumulation of dirt on the blades (even with regular filter changes, some dirt can pass through), aging of seals (rubber hardens and cracks), loosening of fastening elements due to long-term vibrations. After 5–7 years of operation, a service inspection focused on these aspects is appropriate. Most problems can be resolved without replacing the entire unit.

Conclusion – noise is not fate, it is a diagnosable technical problem

After years of experience in this field, one reliable rule applies: a noisy heat recovery unit is always a symptom of a specific problem, not an inevitable side effect of the technology. A properly designed, sized, and installed system operates quietly – sometimes so quietly that the customer has to touch the duct to confirm that it is even running. The key is a combination of correct design (duct cross-section, placement of outlets, unit selection), thorough installation (vibration dampers, flexible connections, tight joints), and regular maintenance. If noise appears later, systematic diagnostics according to the above-described procedure will always find the cause – and the causes are mostly solvable without the need to replace the entire system.

For those who are dealing with a specific installation from the beginning, I also recommend reading How to correctly design a ventilation system with heat recovery and Installation of a decentralized ventilation unit HRC step by step – both articles focus precisely on those steps that prevent noise problems before they even arise.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your home? Write to us – we are happy to help.