• Nicholas Vaisey

Correct specification is important

Whenit comes to MVHR, size does matter. All too often MVHR units are under specified and forced to run at high capacity, the outcome of this is a noisy and inefficient system. Typically we aim for 50-60% of the units maximum capacity for normal ventilation, allowing for sufficient boost ventilation.

The Brink units have constant volume fans, which operate by constantly assessing the system pressure and adjusting the fan to maintain the commissioned airflow. On this basis it is very important to maintain low system pressures, to help reduce noise and provide an efficient system.

So it is just as important to size the ducting correctly and use the correct ducting. Trying to push too much air down a small duct means high velocities, increased pressure on the system and increased noise. Unnecessary bends, sharp bends and corrugated ducting also result in higher levels of resistance in the MVHR system, which increases the noise from the unit and turbulent noise generated by the airflow. At CVC we design the air velocities to be below 2m/s at normal operation, to maintain these velocities we recommend the 92mm semi-rigid ducting, rather than the commonly used 75mm. If we supply a rigid system, we would use steel rather than a pvc system, for its increased performance characteristics.

Lack of attenuators is another common failure on specifications. The Passivhaus Guidelines outline sound pressure levels of ≤ 25 dB(A) in living and sleeping rooms, ≤ 30 dB(A) in Kitchens, bathrooms, WC’s and Utility rooms. This should be demonstrated at design stage and if can’t be achieved through attenuators, then the unit selection should be revised. If a rigid system is used, cross talk attenuators should also be specified.

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