An MVHR system can keep running even when it is no longer moving the right amount of air. The first clues are often practical: a bedroom feels stuffy by morning, bathroom mirrors stay misted for longer or a familiar supply valve seems unusually quiet.
Reduced MVHR airflow does not automatically mean the fan has failed. Restrictions, settings and maintenance issues are more common, and several can be checked safely before an engineer is called. The key is to work from the simplest cause to the least accessible one.
First, confirm that airflow has really changed
Compare several rooms rather than judging the system at one valve. A single weak outlet suggests a local obstruction or duct problem; weak supply and extract throughout the home points towards filters, controls, outside terminals or the unit itself. Also check whether the system is in holiday, away or low-speed mode. A sheet of tissue can suggest movement, but it is not a substitute for measured airflow and should not be used to alter settings.
Clogged or unsuitable filters
Why filters are the best place to start
As dust and pollen collect, air meets greater resistance. Airflow can fall and fan noise may change, even though the unit still appears to operate normally. Inspect the filters as the manual describes and replace them when indicated. For a compatible unit, select the correct komfovent filter by model, dimensions and specified grade rather than appearance alone.
Check that each filter faces the correct direction and sits squarely without gaps. A filter that is too dense, badly fitted or left in protective packaging can also restrict air. Reset the filter reminder only after the maintenance is complete, and note the date so future changes are easier to compare.
Blocked outdoor terminals
Leaves, insects, wind-blown debris, snow and ice can obstruct the intake or exhaust. Inspect accessible grilles from ground level and clear loose material without forcing it into the duct. If the terminal is damaged, high up or repeatedly icing, ask an engineer to investigate the cause.
Dirty or disturbed room valves
Clean without upsetting the balance
A ring of dust around a valve can narrow the opening. Wipe accessible surfaces gently, but do not twist the centre disc, interchange valves or close an outlet because a room feels draughty. Valve positions are normally set to deliver measured air quantities, so casual adjustment can reduce ventilation elsewhere.
Restricted, leaking or disconnected ductwork
A crushed flexible section, loose joint or poorly sealed duct can reduce one branch or the whole system. Ducts in a loft may also suffer if stored items press against them. Visible damage can be noted, but concealed ductwork should be tested by a specialist rather than opened at random. Improvised tape repairs can hide the fault without restoring an airtight connection.
Fan settings, controls and system balance
A changed schedule, sensor setting or commissioning value can make airflow appear weak. Boost mode may temporarily mask the problem, but it is not a repair. If filters and terminals are clear, an engineer can measure supply and extract rates, inspect the fans and restore the designed balance.
Allow for cold-weather protection
During very cold conditions, some units alter fan operation to protect the heat exchanger from frost. A temporary change may therefore be normal. Airflow that stays low after the weather improves, or is accompanied by alarms, ice or water leakage, needs further investigation.