Shocked to find so much mould on my MVHR extract fan

[mouldman]

Just a quick preface that I am very happy with my MVHR unit and would install one again in a heart beat, please don't let this post put you off getting MVHR!

 

My unit has been installed for 9 years roughly and during that time I have regularly changed the filters, annually or bi-annually cleaned the heat exchanger and once or twice reached in and tried to clean what I can reach of the fan blades.

 

Approaching a decade of use, I thought I should open up the unit properly and take a look at the fan blades and give them a thorough clean and was shocked by what I saw:

 

 

If you're unaware, that fan blade should be black and the inside of the unit should be white / grey, like on the exterior. 

 

I nearly fainted when I saw this as I've two kids and monitor interior and exterior air quality very carefully to try ensure they're breathing clean air. Then I see this and panic it's been blowing mould spores over us all. I've realised now that this is the fan venting to the outside, so hopefully won't have been affecting the indoor air.

 

Anyway, apologies for the rambling on but I know some other people have experienced mould and I wondered what the consensus is on preventing it, or is it just a case of checking fans more regularly than every decade.

[Nickfromwales]

Something has to get covered in stale, humid, smelly air from bathrooms and kitchens, and that's what you're seeing tbh.

 

I very much doubt there is any cause for concern, and would also ask what was the maintenance schedule that the manufacturer mandated, if any?

 

Not too bad for 10+ years of service tbf. 

[ProDave]

I would question if the flow rates are adequate or is the MVHR unit in a cold place like a loft?

 

Only 5 years uses but ours is still looking clean.

[Nickfromwales]

I'd question the quality of the unit, and how robustly the design manages condensation etc.

[jfb]

Is the mould on both fans or just the extract? Presumably just the extract.

not ideal but at least that is going in the right direction!

[MikeGrahamT21]

Mines 5 years this year and had a couple of mould blobs on the fan blades of the extract, but overall it looks the same as it did when i installed it. I clean the dust from the blades once a year and blast it out with a high powered fan.

[mouldman]

The other fan only has a small amount of dust on it. I am going to clean it all up and then monitor it a lot more closely. I have horizontal inlet / outlet and could have easily removed one of the top panels to inspect it years ago.

 

Maybe condensation is coming down the outlet to the house exterior and dripping down into the extract area of the unit. The drainage pipe is also quite dirty, so maybe that organic matter increased potential for mould.

 

ProDave: It may be worth looking behind the fan rather than just at the blades (if like me that's all you did), to check there's no nasties beginning to develop. Is your unit in a warm space? Maybe that is helping yours stay clean.

 

I contacted the installer and they just aid that "ALL" units would do this when used for nearly a decade. Which of course they would say.

 

The unit is in the attic. I've never monitored the temperature range up there but it is a 'warm-roof' and there's not been any sign of condensation or mould up there really.

[ProDave]

  On 11/03/2025 at 09:12, mouldman said:

The unit is in the attic. I've never monitored the temperature range up there but it is a 'warm-roof' and there's not been any sign of condensation or mould up there really.

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The air being expelled will contain moisture, and will have been cooled somewhat by the heat exchanger, but it is still likely to be warmer than the air in the loft.  so unless the pipe runs in the loft are impecably and well insulated there is scope for the moisture in the air in them to condense.

 

Mine is all contained within a warm space.  I get the opposite problem, in spite of being insulated, in very cold weather, the incoming air is very cold and occasionally I get condensation on the outside of the supply pipe passing through the warm space.

[mouldman]

Thank you everyone for your replies.

 

I did some cleaning out of the mould today, very unpleasant but nice to see it gone. I made a discovery that at the bottom of that area of the unit, is a drain hole. For god knows how long that has probably not been draining, or not draining very well, probably dramatically increasing the mould-growth.

 

I wanted to mention this, as a reminder to everyone to take a deep-dive on their units every couple of years. If I'd kept more on top of that area being clean, then the drain probably wouldn't have been blocked and it may not have got so bad. That is assuming the drain blockage was the main cause.

[Nickfromwales]

  On 11/03/2025 at 19:30, mouldman said:

Thank you everyone for your replies.

 

I did some cleaning out of the mould today, very unpleasant but nice to see it gone. I made a discovery that at the bottom of that area of the unit, is a drain hole. For god knows how long that has probably not been draining, or not draining very well, probably dramatically increasing the mould-growth.

 

I wanted to mention this, as a reminder to everyone to take a deep-dive on their units every couple of years. If I'd kept more on top of that area being clean, then the drain probably wouldn't have been blocked and it may not have got so bad. That is assuming the drain blockage was the main cause.

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Happy days, and good to hear this may just have been a routine maintenance thing, so hopefully the unblocked drain hole will now improve the situation going forward.