Boston City Retirement System is searching for an executive director to replace Donna Mueller, who left Jan. 31 to become the chief executive officer of the $14.8 billion Iowa Public Employees' Retirement System, Des Moines. The search is being conducted in-house, said a spokeswoman for the $2.9 billion Boston fund. No timetable has been set for the hiring, the spokeswoman said. Daniel Indiciani, city comptroller and interim executive director, said there are no specific qualifications for the job, though experience with a public pension fund is preferred. He said retirement system board members are handling job inquiries.