My old fan was nailed against a joist which means it wasn’t centered between the two joists. Can I enlarge my existing opening on three sides to accommodate this fan and use my existing opening?

Yes! You can enlarge the opening on three sides to fit the RG‑R811A. This fan comes with a Flex‑Z Fast® bracket that makes installation easy, even if your old fan wasn’t centered. Just cut back the drywall as needed, attach the bracket between the joists, and install the new fan from below, no attic access required