Rheem heater/air conditioner. Heater works fine. When the A/C is turned
on, it blows a 3amp fuse on electronic control board inside the heater unit
and then the heater won't work. I replace the fuse and the heater now works. If I try the A/C again then the heater won't work. Help !!!!!

You need to check the low voltage wiring for a short. It can be at either inside or outside unit. If the heat and blower work ok its most likely outside. Look for any cuts in the wire insulation and make sure they are not touching any metal on your system.