The 29-year-old will stay with the newly-promoted Brewers until January.
Duffy told Burton’s official website: “It’s very good. I joined in with the lads (at Friday’s training) and the pace is good.
“You can see that they have been back for a few days, they all seem very fit. I’m looking forward to it.”
Duffy made only seven appearances for Birmingham last season before joining Chesterfield on loan in February.
He signed a two-year deal with Birmingham, managed by former Burton boss Gary Rowett, in June 2014 and has had previous spells at Southport, Morecambe, Scunthorpe and Doncaster.