Obviously, both HVAC technicians and plumbers provide important services but they do so from completely  different mental views.  

For instance, a highly qualified AC technician will not provide standard plumbing tasks e.g. snaking drains, running cameras through sewer lines, digging trenches cutting through concrete to repair or replace broken sewer or other drain or supply pipe systems.  They will also not replace old corroded attic galvanized water supply lines.  This is not because he or she cannot or does not know how to do it but because this type of work is mentally not what normally intrigues a student who wishes to be a highly  trained HVAC technician in the first place.  Technicians of this type are intrigued by electromechanical and enthalpy more in line with physics than the physical.