I have a question about airflow with a bathroom fan, in this case a Ventaxia VASF100T with a front cover, duct through the wall and a gravity grille on the outside.

It seems quite inefficient at clearing steam, and I am wondering if the exterior vents are just stopping air from moving outside? What I am referring to is called a "gravity grille" I would have expected the flaps to open when the fan is on but I have checked and they do not move at all. How can air be pulled outside if there is something blocking it? Maybe the air is just recirculating..

  • Have you looked at the fan blades in the last five years? They're probably so caked with lint that they're essentially drinking straw-shaped. – isherwood Jul 11 '19 at 0:37
  • Also, it is common for people to underestimate the effect that bends in ducting have on airflow... – Jimmy Fix-it Jul 11 '19 at 4:32
  • Sometimes the pivots on the flap get sticky, did you try to open the flap and see if it moves ? – Alaska Man Jul 11 '19 at 16:48
  • This is a brand new installation. One of the flaps was jammed and has been unstuck now. They actually do open when the fan is on, but only a couple of mm The fan has its own backdraft spring-loaded shutter, so hopefully that is opening fully also. – optimyst Jul 13 '19 at 9:17

The outside grille should definitely open when the fan is running. However, it isn't clear whether this is a cause or an effect of your airflow problem:

  • The grille may be stuck, in which case you need to lubricate/free/unbind it
  • There may be some other issue with the airflow, in which case nothing's pushing the grille open

First stop: check the grill manually so that you can see if they're working (they should open easily). If not, you've found the problem. If so, then you'll need to check other components (the ducting, the fan, etc).

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