Although he'd spent time working with his father as a teenager, Dirk Roper, 55, had little interest in the family heating and air-conditioning business. Instead, he built a career as a financial planner, putting his computer expertise to work in helping to design an investing system for Ameritrade.But when his parents came to him in 2005, he had a change of heart.After nearly 20 years of running the business they started in 1978, Fred and Theda Roper had sold it to retire. It wasn't long, however, until it went bankrupt under the new owner. So in 1999, they started over. …