Mold under the ventilation unit in the bathroom

I rent a small apartment with a windowless bathroom. There is a ventilation system for the air circulation, but it constantly causes problems. In humid and cold weather, water flows out of the ventilation grille and soaks the plaster underneath. By now, water and mold stains have surely formed. I have tried to solve the problem by heating more, but that didn’t help. I don’t think water is coming in from outside (rain), it seems too little for that; it looks more like condensation water to me. Does anyone have an idea?

Condensation could be the cause; why so much condensation occurs would need to be investigated on site (this should actually be addressed by your landlord). The ventilation capacity is probably too weak, either the pipe is too long or the fan is too weak; maybe an electrician should take a look at the fan, it might be the motor.

When I was still renting, I had the same problem. The vents for windowless bathrooms are usually undersized. After showering or bathing, more moisture accumulates than a small fan can quickly extract. Most of the time, the water condenses inside the cold metal housing of the fan, and from there the condensation water flows down the wall. There isn’t much you can do about that; very few landlords will install a bigger fan for that reason. I just repainted under the fan more often with mold protection paint and accepted it.

The landlord is responsible and must have the mold removed. However, the source of the mold must first be thoroughly investigated. This can only be determined by an expert.

one of the old problems with internal bathrooms. Here the fan is usually too weakly dimensioned, so moisture from condensation accumulates between the seals. This often causes mold to form in the seals. A larger fan often helps, but the landlord must be asked here

I know exactly that too, and a bigger fan works wonders. But the landlord must have an interest in preventing mold from forming. For that, he has to invest in a bigger fan, and without that, the problem will persist.