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installing new bathroom exhaust fan

  • Last Updated:
  • Feb 23rd, 2021 4:39 pm
Mar 26, 2012
5 posts
1 upvote

installing new bathroom exhaust fan

time for us to change our old 28 year old exhaust fan which is 50cfm 3" piping. the bathroom room is around 50sq ft. the old fan has a 8.5x8.5 hole in the ceiling where the new one 50cfm is only like 7.25x7.25 and runs a 3inch pipe. so the pipe works is easy because its direct size connection but then the hole is too big and i have to mess around with drywall repair to get the correct size and not have open void in the ceiling

the new 80cfm fans are a bigger hole size 9x10 which is good because i can cut the hole bigger to size and the fan will fit nicely, but the vent piping for that is 4". i can put a 4 inch to 3 reducer but i dunno how that will impact its function.
technically 80 cfm is more then enough for the size of room so any loss of air movement shouldn't effect things too much.

not sure what direction to go. not really wanting to change all the duct work up in the attic to 4 inch.
1 reply
Oct 2, 2005
421 posts
If you put in a larger fan with a 4" duct you will need to replace the attic ductwork. The 4" duct is needed for the larger fan due to the higher airflow. You should not put an adaptor which constricts the airflow to 3".

I suggest you replace it with a 50cfm fan because that is all you need. even though the hole in the ceiling is bigger than you need (8.5 x 8.5), the excess will be completed covered by the grill.

For example see this exhaust:
https://www.homedepot.ca/product/nutone ... 1001106115