Originally Posted by Dave in Wi
I don't want to hijack your thread, but thought I'd put this out there. I aborted my bath fan replacement Saturday after I opened the box and got a look at the fan. The bottom of the housing has a flange on it, which means I'd have to cut the hole in the drywall ceiling oversized to get the fan in from above. It seems the fan is designed for new construction, to be installed before the drywall is in place. It's quite possible I'm over thinking this, but I'd rather not have to cut the hole oversized and have to seal up the perimeter with expanding foam. I'm going to stop in the stores this week and take a closer look at what they've got. And see if they have a good fan that might work better for existing construction.
Generally, yes, if the old one has the flanges, it was designed to be installed before the sheetrock goes up.
And inside, it's very modular, so if your heater craps out, or your fan is louder than a chevy, or goes tits up, or if the lens just starts to turn yellow & crappy, you just call them up for a replacement module, and install it in the existing shell.
But retrofit fans do exist with alternate means of affixing a frame. It's not so bad.