+1, having similar experience in other fields can help a LOT. Understanding electrical/electronics goes a LONG way in troubleshooting today's systems. Appliance repair and HVAC are closely related, having experience doing appliances is a nice boost to learning HVAC. A lot of the knowledge "transfers over". Same for electronics, A/V techs tend to have good experience hooking things up and understanding wiring diagrams. Understanding inputs/outputs on circuit boards is essential to being a good HVAC tech IMHO.