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Our bathroom fan filter was worn out and very dirty with cigarette smoke from the previous tenant¹, so we removed it. We could replace it, but why does a bathroom fan need a filter in the first place? What harm would be done if we continue using our bathroom fan without a filter?

The bathroom is a windowless room on the inside of an apartment in a 1990s apartment block in Germany with 12 units in the building in total. I suspect the ducts lead to a shared pipe system through which the air goes outside. The fan is a maico, but I don't know the exact model.

I found some discussion in German on focus, urbia, and vermieter-forum, but no clear conclusion, nor does this English language usenet discussion. DIY stores do not appear to have bathroom fan filters widely available, although they do exist for sale, for example at lowes, directly from maico, or on amazon. The product description on amazon claims:

Das Gerät niemals ohne Filter betreiben. Die Verwendung der Filter garantiert:

  • Optimale Filterung der Luft
  • Lange Lebensdauer Ihres Ventilators
  • Perfekte Passform und somit maximale Dichtheit ohne Leckluft filtAIRS
  • Luft ist Leben

which means:

Never use the device without a filter. The filter guarantees:

  • Optimal filtering of the air
  • Long lifetime of your fan
  • Perfectly fitting and therefore maximal closure without leaking air filtAIRS
  • Air is life

Only the second bullet point addresses the need. Does the filter increase the lifetime of the fan?

I recall reading that some people have a problem of bad smells entering their home from the neighbours through the ducts, but I don't know if the filter would be aimed for or effective at stopping this, and it is not a problem we (currently) experience.

¹In Germany, according to vermieter-forum, this would count as a small fix that is the tenants responsibility.

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    Are you sure that the vent fan actually vents to the outside or is one of the newer energy savers that vents back into the room through a charcoal filter? – d.george Mar 28 at 11:50
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    @d.george I think so; the building has pipes visibly going outside and there isn't anything else I can think of that would go there (building is heated by district heating). – gerrit Mar 28 at 16:50
  • @d.george If it i sfull of cigarette smoke particles (and presumably on the room-side surface of the filter), the guess that it is only outgoing seems justified. – Hagen von Eitzen Mar 28 at 18:18
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It's similar to the reasoning behind using a filter for your AC units. You want to filter the air to prevent clogging up the coils and dirtying up the ducts. There is a lot of moisture in a bathroom and if you mix that up with dust and lint from towels you've got a combination of particles that will stick to the blades of the fan and also clog up the shaft which will reduce or even stop the spinning of the fan and also settle in the duct work before reaching the outside. The filter will stop this from happening. Do yourself a favor and replace the filter. If the fan returns air back into the bathroom, a filter will clean the air for better breathing.

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