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I live in a prewar apartment building. At the top of the shower, on the wall where the water comes in, is a passive vent. There is no powered fan that is installed and no nearby electrical wiring to facilitate it.

When I open a window in my apartment, air clearly flows nicely through the vent - a piece of toilet paper can remain suspended to the vent grating.

However, with the window closed, no air flow occurs.

How can I fix this? My assumption is that air flow through the vent is possible but is occurring through a very narrow opening. When a window is open, air can flow one way through the vent and be replaced by air coming from the window. When the window is closed, the air vent is too clogged and thus too narrow to permit two way air flow and so no air flow occurs.

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The air has to come from somewhere to provide a natural flow. Hot air will rise and slowly vent out being replaced by cold air through the same vent which makes it seems like there is no flow. You mention "two way air flow" so I'm hoping you understand and this is more of me repeating the same thing in different terminology. –  Jason Feb 21 '13 at 18:13
Since it's not in the kitchen you shouldn't have much of a grease problem, I'd suggest using a pipe cleaner or maybe a chimney brush if you want to get aggressive. Aside from cleaning the pipes, the main thing is to clean both the entrance and exit points, after that leaving the window open and "forcing" air through may clean it enough for you. –  Jason Feb 21 '13 at 18:17
With respect to the two way air flow, should I be able to have a piece of toilet paper suspended on the vent? I thought I saw in other units that this was possible with the window closed. –  User7391 Feb 21 '13 at 18:50
I could be wrong but my understanding is unless there is a large difference in temperature or humidity to create a noticeably airflow; it would move about the same amount as if it's suspended to a wall. How large is this air vent? I have a 48" box fan connecting my hallway to the attic and straight up to outside, unless I turn it on I don't see airflow but I know it's sucking up my power bill. –  Jason Feb 21 '13 at 18:54
It's about an 8 inch by 8 inch opening. –  User7391 Feb 21 '13 at 21:25

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