Q: What is the max CFM through a 4 in. bathroom duct? Calculations for the room show I need 200+ CFM but-highest unit I can find for 4 in. is 110.
We are remodeling our bathroom and have an old, noisy/ ineffecient bathroom fan we would like to replace. I did the calculations for a "large" room from HVI.org and I get 200 CFM. However, we only have an 4 in duct. The highest CFM unit I can find for that size of duct moves only 110 CFM. All the rest say 6 in duct and, as I understand it, you shouldn't go from 6 to 4 with a collar, etc. Should I just go with 110, or is there any other possibility besides changing the entire run? There is an equally cheap fan in the toilet closet as well, but I don't know how that might factor in. We don't run both when taking a shower. What's the best solution? Any info would be appreciated.
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