Thanks--your timing is impeccable since we still haven't gotten around to this and I was just working on figuring out exactly which fan to get! We've decided to stick with Panasonic in spite of our bad experience because it is the easiest to fit into the space (same size) and everyone else seems to have good experiences with them, so hopefully ours was just a lemon. Do you have recommendations of specific Panasonics other than the Whisper Fit Lite, though? Our current dead fan is theoretically more than powerful enough for the space (twice what we need based on the linked site) and as far as we know the ducting is good to go (was put in when the fan was four years ago) but we never got very good venting from the first fan. Possibly whatever caused it to die was also he cause of the earlier issues, though.

Mav, I know...there is that search box up by the top with a red button that is worthless for searching for posts. It is the same with galleries. The "galleries" heading up there takes you to some really useless place, but if you look at the Switch to Gallery link at the top and bottom of many of the forums, (when you are looking at the list of posts, not when one is open) you can go to a gallery of pics that users have posted. Took me a while to figure it out, too.

It's possible to get replacement bare motors sometimes. Broan are good about having replacement bases -fan/motor, that fit into the existing box. They have a plug/socket, so you don't have to do any wiring. That being said, I just replaced one, and it turned into quite a big job because the replacement base was more than a whole new unit. I put in a new one, which meant a bit of carpentry etc because the boxes were different sizes. I had to cut the nails/screws holding the old one in, and insert the new box from below, which required a bigger hole, and careful saw cuts in addition to allow for the nailing tabs. The fan cover covers the repairs from it, in the ceiling. The insulation around the duct had no vapour/air barrier so I used a mylar bubblewrap product and foil tape for it. Since the house was old enough to not have had an vapour bag around it, I built one with the mylar stuff and sprayfoamed it in. Lastly I decided the fan should have its own switch, so I could use one of those timer/humidity sensing ones. It turned out well and I'm glad I did it, it works better and is much quieter. But in most cases, just replace the fan motor and base.

Based on the info you just provided, I continue to be of the opinion that the fan is not something you can disconnect from the box and replace. Since it's a lot smaller than typical joist spacing, the box probably has an inaccessible bracket you can't see that is on one of the sides and is screwed to a joist, or there is some other kind of inaccessible bracket across the top that's adjustable to match the joist spacing. In either case, it's going to be nearly impossible to get the assembly out without cutting into the ceiling. However, many newer fans are enclosed in boxes that are larger than 7X9, so you have some leeway even if you damage the edges of the ceiling opening. The only other thing I can suggest is that you cut through the sheetrock with a multi-tool all around the edge of the box. It looks like the four sides of the box each have a half inch flange that rests against the sheetrock; once you cut away that small amount of sheetrock, it's possible that you could exert brute force upwards and break the fan box free of the bracket. Good luck. Beyond this, I'm out of ideas ;-)