MVHR unit noisy or blowing cold: filters, fan or bypass

Short answer: if your MVHR unit is blowing cold air into the house, the bypass valve is usually open. In summer that is intended, in winter it is not: the outside air then bypasses the heat exchanger and is not warmed up. Check the bypass temperature in the menu (after a power cut it is sometimes reset to the factory value) and look out for codes such as Brink 103 or Zehnder A5. If the bypass is closed, the heat exchanger is often dirty. If the unit is mainly noisy, start with the filters; after that, worn fan bearings or dirt on the impeller are the usual cause.

Filters first, always

This is not a cliché: in practice, an increased noise level is due to filters that have been in too long in nine out of ten cases. The unit measures the airflow and compensates for the resistance by running faster. That costs more electricity, makes more noise and ultimately results in less ventilation.

Which filter set you need depends on the make, model and year of manufacture. Our guide on replacing MVHR filters lists the correct set with article number for each unit.

The fans

An MVHR unit has two: one for supply and one for extract. Worn bearings produce a humming or squeaking noise that often stays at one pitch. Dirt on the impeller, usually on the extract side because greasy kitchen air passes there, causes imbalance and vibration instead. Those vibrations travel through the ducts, which can make the noise in a bedroom seem louder than at the unit itself.

Modern units report a fan fault themselves. Brink, for example, reports 104 as a locking fan fault, Zehnder distinguishes E1 for the extract fan and E2 for the supply fan. All codes are listed under fault codes for MVHR units; replacement fans under fans.

The bypass: cold air in summer and winter

An MVHR unit has a bypass valve that bypasses the heat exchanger. In summer this is desirable: at night you want to bring in cool outside air without warming it with the extracted indoor air. If that valve stays open in winter, the unit blows cold air into the house and you think the heat exchanger is broken.

Brink reports a bypass problem as 103, Zehnder as A5 when the valve does not open. Also check the settings: many units have a bypass temperature you can set yourself, and after a power cut it is sometimes reset to the factory value.

The heat exchanger

The heart of the unit is a block with thin channels where supply and extract air flow past each other without mixing. If that block gets dirty, the efficiency drops and the supply air becomes noticeably colder. With most units you can pull the block out and carefully rinse it with lukewarm water; check in the manual whether this is allowed for your model.

If the block is damaged or leaking, it needs replacing. That is the most expensive part of the unit: a Brink heat exchanger (manufacturer number 532754) costs €607.95. With a unit that is fifteen years old or more, that is the moment to consider replacing the whole appliance, especially as newer units ventilate more efficiently.

Noise that does not come from the unit

Whooshing valves in the bedroom, a whistling noise in the toilet or a draughty feeling at the supply grille are usually not a defect but a balancing issue. The airflow per room should be set with a measuring instrument; if a valve is closed too far, the air rushes through it and it whistles. Have the system re-measured if this started after a renovation.

Also check the ducts themselves: a flexible hose that is kinked or full of dust causes exactly the same complaints as a unit running too hard. General information on healthy ventilation can be found under Milieu Centraal.

Frequently asked questions

Why is my MVHR unit blowing cold air into the house?

Almost always because the bypass valve is open: the outside air then bypasses the heat exchanger and is not warmed up. This can be caused by a bypass temperature set too low in the menu (for example reset to the factory value after a power cut) or a valve that is stuck. If the bypass is closed, a dirty heat exchanger is the next suspect.

What do I do if the bypass of my MVHR stays open?

First check the set bypass temperature and reset it to the value in the manual. See whether the unit shows a code: Brink reports a bypass problem as 103, Zehnder as A5 when the valve does not open. If the valve stays open after a restart while it is cold outside, the bypass motor or its control is faulty and needs replacing.

Can I set the ventilation to the lowest setting at night because of the noise?

Preferably not permanently. Bedrooms are precisely where most CO₂ and moisture build up at night. Solve the noise at the source: filters, balancing or vibration damping, rather than ventilating less.

How often should the heat exchanger be cleaned?

Depending on the type, every one to three years, provided the filters are replaced properly. If the filters have been left in for a long time, the block usually needs cleaning too.

My unit is suddenly running much harder without me changing anything.

Check whether there is a filter warning, whether the bypass is open and whether anything outside is blocking the wall grilles or roof outlet. A unit that cannot reach the set airflow compensates by speeding up.

Can I replace the fan myself?

Electrically it is straightforward and no gas or refrigerant is involved. Do note: the new fan must be the same type with the same control, and after fitting, the airflow rates must be checked again.