johno1234:

 

Not an issue though. The only pencil sized gap is the floor register grilles. And Mickey ain't getting back out unless he can climb vertical smooth steel. And he can't eat through the ducting as it's steel too. I don't know when this system installed but looks pretty old, well more than a decade and no sign of rodents in it.

 

A mouse will climb vertical steel (unless it's polished), but not chew through it.