How loud is an MVHR system?

How loud should an MVHR system be?

A good MVHR unit is quiet in the living space. Premium units are rated around 42 decibels at the unit, roughly the level of a quiet library, and less than that by the time the sound reaches a room. In a bedroom you should hear little more than a soft background hum, if anything. If a system is intrusive, something is wrong.

Manufacturers quote a sound level at the unit, but what matters is what you hear in the room, which depends on the ducting, silencers and where the unit is sited. A well designed system is designed to be inaudible in bedrooms at its normal background speed.

Sources: Zehnder Group UK

What causes MVHR noise?

Most MVHR noise comes from the ducting and installation, not the unit. Long or tight duct runs, cheap flexible ducting, undersized ducts and a unit sited too close to a bedroom all create noise. Airflow noise at the terminals, or a badly balanced system pushed to run fast, are common culprits. A quiet unit in poor ducting can still be noisy.

There are three sources of noise: the unit itself, airflow through the ducts and terminals, and vibration transmitted through the structure. Good design tackles all three, but the one most often neglected is the ducting, which is why noise complaints usually trace back to it rather than to the unit.

How do you make an MVHR system quiet?

Quiet comes from good design: short, rigid or semi-rigid ducts, gentle bends, correctly sized ductwork, silencers on the main runs, and the unit sited away from bedrooms, often in a loft or utility. Running the system at its designed low background rate, rather than pushed to a high speed to make up for poor ducting, keeps it quiet.

Silencers, sometimes called attenuators, on the supply and extract runs near the unit make a big difference. So does mounting the unit on anti-vibration fixings and keeping duct velocities low by not undersizing the ducts. These are design choices, which is why quiet is decided before installation, not after.

Why is my MVHR noisy?

If an MVHR system is noisy, likely causes are flexible ducting that should be rigid, ducts that are too long or kinked, a missing silencer, or a system running fast to compensate for high resistance. It is usually fixable by improving the ducting or rebalancing, rather than replacing the unit. A ventilation engineer can diagnose it.

Before assuming the unit is at fault, have the installation checked. Often the fix is replacing a stretch of flexible ducting with rigid, adding a silencer, or rebalancing the system so it does not have to run fast. Replacing the unit rarely solves a noise problem that is really a ducting problem.