JoeSpareBedroom wrote:

There's ALMOST no such thing as too big a fan, especially if noise is not an
issue. How many people take showers, and in what period of time?

I think there are some possible issues with the backflow of combustion
gases from furances or hot water heaters with an overly aggressive exhaust
fan.

My guess is you need the right other ingredients (very tight house, minimal
or no fresh air intake, gas hot water heater & furnace, strong fan and
limited loss exhaust duct) for this to be a significant issue, but I've
heard it mentioned more than once.

My personal experience has been that most insulated flexible exhaust
ducting of any length is so grossly inefficient that you almost need a high
CFM fan for it to be effective at all.