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Is the MVHR system noisy?

Is heat recovery unit noisy

Is the MVHR system noisy?

Those who consider installing residential MVHR systems tend to have concerns about the noise that ventilation devices can generate. Air exchange is a process that generates a specific sound, which can be optimised by the proper combination of system components, as well as the correct system design. Because ventilation works continuously, without any interruptions, 24 hours a day, all year round, its quiet operation makes the difference for the residents’ comfort. 

What makes heat recovery silent?

Noise levels in the kitchen and toilet in residential buildings cannot exceed 40 dB(A) in accordance with the “Building acoustics” PN-87/B-02151/02 standard. The acceptable values are lower for typical comfort rooms, living rooms or bedrooms, where the maximum noise level can be 35 dB(A) at daytime and 25 dB(A) at night. 

A heat recovery system, when well designed, will work silently. Nevertheless, several aspects have an influence here, all of which must be considered when designing and planning the heat recovery system and selecting its components. 

Type of Heat Recovery Unit

Before the HRU is installed, it is necessary to choose the right one, which best fits both the system design and our individual preferences and expectations. “Silent” operation is certainly ensured by modern and efficient equipment. When buying a new product, think about the HRU class: the higher the class, the quieter the operation. This is crucial for the comfort of its use. 

In fact, HRUs differ in their individual components, including fans. If you decide on an HRU supplied with DC fans, you can expect a quieter operation. The same applies to heat exchangers. Rotary models use more moving parts, which increase noise levels. The housing is also very important. Preferably, choose those with quality thermal and acoustic insulation. This ensures both greater energy efficiency and lower noise levels. 

However, it is worth remembering that no matter how high the quality of a heat recovery unit, it will generate noise, or "humming", in the air duct and the housing. The noise levels which we manufacturers have to indicate is the acoustic power level of the device housing. You will find it on the HRU energy label and on the product data sheet.

HRU installation method

Choosing the right, modern HRU does not yet guarantee the silent operation of the entire system. The MVHR system and its components must be properly installed. Noises in mechanical ventilation are generated by aerodynamic and mechanical factors. The issue also occurs frequently when component joints are too loose and the duct diameter is selected incorrectly. When developing and executing the design, it is vital to decide if silencers (sound attenuators) are necessary, and ensure proficient insulation of air ducts. Also, installing your HRU near the bedroom is to be avoided. Perhaps you will hear no sound during the day, but at night even the slightest and merely audible noises can disturb you. 

Air duct diameters

Air ducts should be of such diameter that the air flow rates are below 3-4 m/s. They can also be soundproofed with a damper, which effectively reduces noise. In some cases, silencers are installed in the air intake. 

Do you use attenuators at the HRU? 

In our opinion, it is good practice to install a silencer downstream of a heat recovery module. It is because no matter how modern and quiet the fan is, or what sound insulation your unit housing is installed with, the heat recovery module will emit noise into the ducts. This noise will be practically inaudible, as you would expect, at the extreme sections of the ventilation system, namely grilles, air valves, and far from the HRU. But the residents can hear the noise in a room adjacent to the HRU.

We believe that, given the low cost of silencers, installing them will be negligible for the cost of the whole system. Plus fitting them on the supply side will only increase the level of acoustic comfort for users. There are HRU installers who fit silencers on the extract (exhaust) side, so that the noise is not conveyed outside. Such practice ensures your neighbours’ comfort. To recap, we recommend installing an air damper downstream of the HRU, regardless of how quietly it works according to the manufacturer.

Sound attenuators for mechanical ventilation with heat recovery?

When selecting a silencer for your system, you can use a selection software for silencers. Our AlnorSILENT selection software is free of charge and available to download from our website. Here you can read a short guide on how to use it. SIL-type rigid silencers generally show much better attenuation than the frequently-used flexible silencers. Flexible silencers are very often mounted upstream of the HRU because they take up little space. 

Check the noise data for silencers. 

How noisy is the HRU system?

The selection of the HRU, its professional installation, as well as the correct diameter of the air ducts have influence on a properly made system, whose operation is to be quiet, or virtually silent. The unit itself undoubtedly generates noise, but it is important that it is not too high. By definition, a noise is an acoustic wave which propagates through a medium like a wave on the water surface. An acoustic wave can be deflected or penetrate or bend, as well as interfere with another wave, that is, strengthen or weaken. Ventilation units are equipped with the measurement of A-weighted emission sound pressure level Lp (based on the logarithm of pressure variations) in db(A). The A range is heard by the human ear. Importantly, according to the regulations, the LWA db(A) sound power levels generated by the housing are indicated, which are values calculated from the sound pressure measurement at a distance of 1m.

It is also possible to measure the sound pressure level in the ducts and give the LWA power value for each connection pipe. However, the test procedures do not reflect the individual conditions of each system, including the type, length or method of arranging the ducts. Therefore, complying with the Ecodesign, the manufacturers give the sound power levels for their housings for 70% of the unit performance. Most units generate sound power of ca 50-60 dB(A). Values below 45dB(A) can be considered as exceptionally quiet.

If You're looking for more tips and instructions about the MVHR (mechanical ventilation with heat recovery), check also the following articles:

  1. How can You install heat recovery unit in the ready-built house?  - the article about decentralized heat recovery systems.
  2. What is an Electrostatic Precipitator and how does it work? - the article about highest filtration efficiency solutions.
  3. 14 Things You Need to Know about PremAIR - pretty self-explanatory title about the best HRU for clients looking for "premium" solutions. 
  4. Air distribution system for the MVHR - overview of FLX-REKU system: radial, semi-rigid ducting, plenum and distribution boxes, etc.