Upgrading to larger diameter duct

I was feeling all proud of myself installing a new fan in the upstairs bathroom until I got to the final step - hooking it up to the duct. The new fan has a 4-inch outlet whereas my existing ductwork up in the attic is only 3-inch. Research on the internets shows it's a very bad idea to go from 4-inch fan outlet down to 3-inch duct (noisy, inefficent, wears out the fan motor).

The 3-inch duct pipes from each of the bathrooms all run up to a single large box with four holes in it to accept the pipes and vent it through the roof. It would seem easy enough to buy a new 4-inch duct pipe, but how do I enlarge the holes in that box? Do I need special tools to do this? How do I make the hole perfectly round? Do I have to replace the entire box? Will I have to call in a pro? Might there be local codes that require inspection?

:shrug:

probably cheaper and easier in the long run. hopefully you can return the fan you installed

It'll probably give me the cfm I need to get all the moisture out of the windowless bathroom. Mold, mildew, peeling wallpaper and god knows what else....

but that list of ick is pretty scary

I'll draw a four inch circle over the 3-inch hole and cut 8 or 16 cuts into it with my tin snips. Hopefully the foil duct tape will cover any mistakes. The adventurous part will be standing up on a stepladder in the unfinished attic. The duct box is way up at the apex of the roof just slightly out of reach. I'll have to bring up the stepladder through the access hole behind me. Wish me luck!

and good luck!

OH and btw, Happy New Year too!

:hi: