Apollo Enthusiast

I have a 50 gallon 2010 apollo unit that works like a champ. After 12 years, it finally started leaking. The cheapest replacement I could find was a $1350 Bradfordd White. I'm going to go ahead and replace it; the coils are in good shape and it keeps us toasty during the mild Virginia winters. Plus, I'm not quite ready to upgrade the rest of my HVAC system. My old A/C unit still uses R-22 and it kicks in the summer. I didn't see any posts about maintenance or if there's any special considerations when replacing the heater. I'd love to know how to flush the coils, and if there's any kind of priming procedure when I cut the water back on after piping everything up. I'd hate to burn up my pump. If anyone can weigh in on these questions, that'd be super! Thanks -Al