HVAC is a very specialized field and involves a breadth of experience
and knowledge that spans electrical, control systems, mechanical,
thermodynamics and an endless list of codes and regulations. It's
also a field where the "devil's in the details" and jack of all
trades/master of none won't cut it.
Because of high technical burden there are lots of hacks in the trade
who don't have the apptitude to master it yet they're still in the
It would be very tough for a homewowner to master HVAC however some
homeowners can certainly master their own HVAC system repairs and
instalations and probably do a much better job then all the hacks and
maybe even some of the pros. To say a homeowner can't diagnose and
replace a contactor, blower motor, thermostat, or even top off a
refrigerant charge (with the right EPA 608 card) is simply blind
arrogance on the part of some pros.