MVHR noise and running costs; what to expect once it's on

The specification sheet for an MVHR unit will quote a sound-power figure in decibels, usually measured at the casing at a stated fan speed. That number is useful for comparison between units, but the number that matters to the household is the sound-pressure level at the nearest terminal in a habitable room.

Approved Document F sets a maximum of thirty decibels at the terminal in a bedroom and thirty-five in a living space, both measured at the design flow rate. A well-designed and well-commissioned domestic system typically lands well under those limits; anything approaching them at background boost is a signal that either the ducting or the commissioning needs revisiting.

Fan noise from the unit itself is usually not the culprit in a retrofitted system. The more common causes are turbulent airflow through a bend that is too tight, a terminal restricted by a decorative grille the installer did not expect, or a duct run that was shortened on site and now runs at higher air velocity than the design assumed.

The fix is almost always duct-side rather than unit-side; adding an attenuator on the offending run, swapping a rigid ninety-degree bend for a swept equivalent, or opening up a restricted terminal. Chasing the noise by turning the unit down usually just under-ventilates the affected room without solving the underlying issue.

A domestic MVHR unit sized for a typical three-bedroom house draws between twenty and sixty watts continuously at its background flow rate, rising briefly to one hundred to two hundred watts during boost. Over a year that comes to roughly two hundred to four hundred kilowatt-hours of electricity, or between sixty and one hundred and twenty pounds at 2026 tariffs.

The heat recovered by the unit through the winter months typically offsets a meaningful part of that electricity cost; a well-commissioned system in a reasonably airtight home usually shows a net benefit against equivalent trickle-vent ventilation once heating fuel is included in the sum. In a leaky home the maths turns and the fan power stops paying for itself, which is why the retrofit guidance keeps insisting on the envelope work first.

Filter replacement is the single biggest lever on both noise and running cost through the year. A clogged filter forces the fan to work harder to maintain the design flow, which raises both the wattage draw and the noise at the terminal. Twice-yearly filter changes are the honest baseline in a UK urban environment; annual is enough in a rural one.

If the electricity draw at background creeps upward over a season, or the noise at the nearest terminal starts to become noticeable, treat that as the system asking for a filter change and a heat-exchanger clean before treating it as a fault. Most calls to a commissioning engineer end at exactly that.