Our apartment building is 23 years old, and the bathroom has passive ventilation: basically there's a hole in the wall just under the ceiling, where warm moist air slowly escapes. It usually has a nice cover, but I've removed it in the attached photo.

It seems that the ventilation shaft ends just below the wall hole, and the bottom of this shaft slowly fills up with condensation water. Apparently the condensation adds more water than the amount that naturally evaporates. This is a net positive so eventually the water overflows the shaft bottom and trickles down the bathroom wall. That's how I discovered this.

I've just improvised a siphon using a few drinking straws and to my horror I extracted more than two liters of old water from that shaft. This must have been accumulating over the last several years. Obviously this is a construction fault.

Question: How can I avoid this water build-up in the future? Can I add an electric fan to the hole, to blow out the moist air before too much of it condensates in the shaft?

Left: the wall. Right: the shaft with water.
Left: the wall | right: the shaft with water

  • Have you reported the issue to building management? With references to how legionnaires disease is spread? Feb 11, 2012 at 12:05
  • @TheEvilGreebo I have only just discovered it. Offices are closed because it's Saturday. My current focus is on fixing rather than reporting. Feb 11, 2012 at 12:52
  • 1
    Maybe water is entering from somewhere else? That seems like a lot of water to build up from condensation
    – Steven
    Feb 11, 2012 at 16:33
  • I would check with management. It looks like it might connect with other tenants, like a community vent, for a lack of better words. It looks like it also could be a place for mold to start showing up. I would let them do it just for the mold part of the situation.
    – lqlarry
    Feb 12, 2012 at 0:10
  • I suspect the fix for this involves structural work that you're not authorized to do as only one tenant, so temporarily reducing the water and making a huge issue about this w/ management is your best action, I think. Feb 13, 2012 at 13:22

1 Answer 1


The problem can be solved by draining the vent. Be sure to use a p trap to prevent sewer gas from entering. Yes, the problem could be condensed moisture, but it might also be rainwater that is collecting. Either way, a chimeny cap that is shaped like a cone will cause an aspiration effect (like the negative pressure under an airplane wing) which will naturally draw air upward through the vents; plus it will make sure that rain water doesn't collect in the vent.

I would also consider adding a ball valve after the p trap, because with these corrections, I wouldn't expect to find enough water (through condensation) to fill a p trap; and sewer gas could enter through the p trap if there isn't any water in it... so have the ball valve ready (to close the drain) just in case the chimney cap solved the problem.

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