Why is my MVHR not working properly?

Start with the filters, which are the most common cause of lost performance. If new filters do not fix it, look at the ducting and whether the system was properly commissioned.

Why is my MVHR blowing cold air?

Cool supply air is often normal in deep winter, since MVHR is not a heating system. Genuinely cold air suggests the heat exchanger is being bypassed, for example a summer bypass stuck open or a frost protection issue.

Why is there condensation on my MVHR ducts?

Usually because ducts in cold spaces such as a loft, or the intake and exhaust ducts, were not insulated, so warm moist air condenses inside them. Insulating those runs fixes it.

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MVHR troubleshooting

Most MVHR problems come down to a handful of causes, usually filters, ducting or commissioning, and many are fixable without replacing anything. This guide covers the common complaints, low airflow, noise, condensation and cold air, and what to check first.

By VentRight Editorial · Last updated 2026-07-08 · Impartial · Sourced

Why is my MVHR airflow weak?

The most common cause of weak airflow is clogged filters, so check and change them first. Beyond that, blocked or crushed ducting, a system that was never properly commissioned, or dirty extract valves can all reduce airflow. Start with the filters, then look at the ducting and whether the system was balanced.

Weak airflow is the classic sign of a system that has not been maintained, and filters are almost always the first thing to rule out. If new filters do not fix it, the problem is more likely in the ducting or the original commissioning, which is where a ventilation engineer with airflow measuring equipment can help.

Why is my MVHR noisy?

Noise almost always comes from the ducting or installation: flexible ducting, long or kinked runs, a missing silencer, or a system running fast to overcome resistance. Occasionally a failing fan bearing is the cause. It is usually fixable by improving the ducting or rebalancing rather than replacing the unit.

If a system that was once quiet becomes noisy, a clogged filter making the fans work harder is a common and simple cause, so check that first. Persistent noise from the start usually points to the ducting design or a missing silencer, which is an installation issue rather than a fault in the unit.

Why is there condensation around my MVHR?

Condensation on or around the unit or ducts usually means ducts in cold spaces, such as a loft, are not insulated, so warm moist air cools and condenses inside them. Insulating those runs fixes it. Condensation elsewhere in the home may mean the system is off, undersized or not keeping up, so check it is running and commissioned.

The intake and exhaust ducts to outside, and any runs through a cold loft, are the usual places condensation forms if they were not insulated. This is a common installation shortcut that shows up later. Elsewhere in the home, condensation is a sign the ventilation is not keeping humidity down, so confirm the system is actually working.

Why does my MVHR blow cold air?

Incoming air feeling cool is often normal in deep winter, because the heat exchanger recovers most but not all of the heat, so the supply arrives close to room temperature rather than warm. If it is genuinely cold, the heat exchanger may be bypassed, in frost protection, or not working, or the summer bypass may be stuck open. Check the settings and, if needed, have it serviced.

MVHR is not a heating system, so the supply air is meant to be around room temperature, not warm, which surprises some people. Genuinely cold air, though, suggests the exchanger is being bypassed when it should not be, most often a summer bypass stuck open or a frost protection issue. Both are fixable.

When should I call a professional for MVHR?

Call a professional if the basics, filters, settings and obvious blockages, do not fix the problem, or if you suspect a failed fan, a stuck bypass, or a commissioning issue. A ventilation engineer can measure the airflow and diagnose faults you cannot see. Routine maintenance is DIY; persistent faults are worth expert help.

The dividing line is roughly between things you can see and reach, filters, settings, a visibly crushed duct, and things you cannot, the internal fans, the exchanger, the measured airflow balance. Once you have ruled out the simple causes, an engineer with the right equipment will find the problem far faster than trial and error.

Questions

Why is my MVHR not working properly?: Start with the filters, which are the most common cause of lost performance. If new filters do not fix it, look at the ducting and whether the system was properly commissioned.
Why is my MVHR blowing cold air?: Cool supply air is often normal in deep winter, since MVHR is not a heating system. Genuinely cold air suggests the heat exchanger is being bypassed, for example a summer bypass stuck open or a frost protection issue.
Why is there condensation on my MVHR ducts?: Usually because ducts in cold spaces such as a loft, or the intake and exhaust ducts, were not insulated, so warm moist air condenses inside them. Insulating those runs fixes it.
Should I call someone for MVHR problems?: If the basics, filters, settings and obvious blockages, do not fix it, yes. A ventilation engineer can measure the airflow and diagnose internal faults such as a failed fan or a commissioning issue.