I live in an old flat on the 4th floor of a housing block. The bathroom extractor fan is old and weak, and there is some distance between the bathroom and the outside, which means the bathroom often gets damp: here is a diagram


I would like to replace the extractor fan, however, the current extractor fan is quite difficult to access, with only a small port to the fan in the side of the ducting. I also can't see any obvious fixings which attach the old extractor fan, so am not sure how I would remove it.

If I am not able to remove the extractor fan, would fitting a second extractor fan at some point in the ducting be sensible, and are there any considerations I should be aware of before attempting this?

Picture of vent

Picture of fan

  • What are we looking at here? Is this a fan venting through a framed in wood box?
    – KMJ
    Commented Jan 23, 2023 at 21:44
  • 2
    I would not fit a second fan without removing the first. You may not get adequate airflow. Low airflow could cause even more issues, especially as the intake area does not appeared to be sealed in anyway. HVAC returns often used ductboard which is a fiberglass board but this should not be used for a bathroom vent. For a bathroom vent, you should use insulated flex duct or rigid ductwork. Is there a soffit in the bedroom or is this internal to the wall? If there is no solid duct on the other side of that fan, I would be worried about mold. Commented Jan 23, 2023 at 22:18
  • 1
    As stated, the fan itself would block airflow, but worse yet, it would trap dust and block the duct entirely. Commented Jan 24, 2023 at 0:55
  • 1
    Yes, theres a soffit in the bedroom. Unfortunately mold is a problem in the flat, it was built in the 1950s. I suppose I could run flex duct through the duct to join the bathroom vent directly to the external fan. Thanks for your advice. Commented Jan 24, 2023 at 13:23
  • 1
    If you can take that picture, you can smash the cr!p out of the old one. Getting the new one in might be a challenge. Also, be cognizant of the possibility of asbestos. (If concerned, you can get samples tested.) Commented Jan 24, 2023 at 14:42

1 Answer 1


Definitely have to take that old one out, and I'd very seriously be considering fitting a flexible aluminium ducting (or plastic in a pinch). It will protect the fabric of the building from all that damp air, and massively improve the efficiency of the fan. It must be a nightmare pushing air through that rough boxing.

  • Just make sure to stretch the flex taut if you do this! Commented Jan 25, 2023 at 3:25
  • @ThreePhaseEel, can you explain further please?
    – handyman
    Commented Jan 25, 2023 at 11:03
  • 1
    flex that isn't stretched taut has significant air resistance (friction loss) of its own Commented Jan 25, 2023 at 12:27
  • @ThreePhaseEel, yes that was my point, that the air moving through the original space is hindered all the way, hence I'd recommend sliding a flexible aluminium pipe in there to give the air free passage out...
    – handyman
    Commented Jan 26, 2023 at 14:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.