Size is key for MVHR — CVC Direct

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“All too often MVHR [mechanical ventilation with heat recovery] units are under specified and forced to run at high capacity, and the outcome of this is a noisy and inefficient system,” CVC Direct’s Nicholas Vaisey told Passive House Plus.

“Typically we aim for 50-60% of the unit’s maximum capacity for normal ventilation, allowing for sufficient boost ventilation.”

CVC supplies Brink MVHR units with 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,” he said. “So, it is just as important to size the ducting correctly and use the correct ducting,” he continued. “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 two metres per second at normal operation, to maintain these velocities we recommend the 92 mm semi-rigid ducting, rather than the commonly used 75 mm. 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 with specifications, Vasiey said. Passive house guidelines outline sound pressure levels of less than 25 dB(A) in living and sleeping rooms, and less than 30 dB(A) in kitchens, bathrooms, WCs 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,” he said.