I am looking for information on how to properly replace a bathroom fan which is not working well enough (probably insufficient CFM). The bathroom fan I need to replace is a 90mm fan rated for 65m3/h (about 38CFM) and is connected to a long PVC duct to the outside.

This answer nicely explains how to take into account the bathroom size and the duct length to estimate the needed CFM. I have not taken measures yet, but I think I will need a 110CFM fan (e.g. 560 cu.ft bathroom with 50feet equivalent duct length).

However, the long duct is also smaller than the fan: the fan is 90mm, while the duct I think it's 80mm (slightly more than 3 inches). The duct is connected to the fan with a duct increaser.

My question is: how to account for the smaller duct?

In this other question there is a table (IRC table M1506.2 Duct Length) that says 3 inches duct cannot be used for fans with 80 CFM or more. However, no answers were provided. Here instead, the answers say that it's ok to use a bigger duct for a smaller fan, however I am in the opposite situation (fan larger than the duct).

The new fan that I would like to purchase and install is even bigger (100mm, about 105CFM) but I do not want to waste money if the smaller duct will make the new fan useless.

Note: I cannot increase the duct size since it would require masonry work that I cannot do.

  • Can you measure the static pressure drop across the duct? Apr 6, 2021 at 22:25
  • @ThreePhaseEel I do not know how to do it :)
    – BlueCoder
    Apr 6, 2021 at 22:31
  • You may need a different style of fan which is more able to work against back pressure (since the duct size ensures that you will have more back pressure than is ideal for "standard" fans at that flow rate.)
    – Ecnerwal
    Apr 7, 2021 at 0:39
  • @Ecnerwal Good point! Browsing through a vendor catalogue, I noticed that there are centrifugal, elicoidal or axial fans: only the centrifugal fans are able to work against a "large" pressure, while the rest are recommended only for very short ducts. Now I only need to work out how big is the pressure I need to work against, considering duct length, elbows and the restricted size.
    – BlueCoder
    Apr 7, 2021 at 6:06

1 Answer 1


The small duct will not make your fan "useless". However, the fan will not perform to its published air handling or noise specifications. Further, its lifetime may (or may not) be diminished and its warranty might be voided. Buy from a seller with a liberal return policy, try it, and return if you are not satisfied with the result.

Another consideration: You could replace the duct with the appropriate size for its entire length EXCEPT for the point where it penetrates the masonry. Although not ideal, this will be much better than with the undersized duct because of the duct's length. Finally, performance will be even better if the duct is replaced with one that is oversized. This is probably good practice anyway due to its fairly extreme length and this will partially compensate for the flow restriction at the masonry penetration.

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