wrote in message .

See, I think you could be right about the inspector. I'm sure he's
quite knowledgable, but if he's seen this problem often, I figure hvac
units are installed by certified hvac techs.. who would be more likely
to be the expert?

Unfortunately, in my experience in many cases it is the inspector. Many
"certified" HVAC techs take shortcuts if they can get away with it. After
the #1 shortcut (inadequate evacuation prior to system charging), failing to
downsize a breaker when installing a more efficient system is probably the
#2 shortcut taken. Having said this a reduction is breaker size from 40A to
15A seems a bit extreme unless there was a significant change in the system
(for example, the elimination of resistive heat strips in the air handler).

This is a new unit, installed a few months ago, installer seemed quite
competent and knowledgable.

See comment above. Did the installation require a permit in your
municipality? Was one pulled? Did the installation pass the municipality's
final inspection? If no permit was pulled and one is required, the HVAC
installer was probably a hack.

Finally, are we talking about the breaker for the air handler or the
condenser? Does the air handler have heat strips (aka emergency heat)?
What is the make and model of the air handler and condenser, and what are
the specifications for the electrical supply for each (note: should be on
manufacturer's nameplate)?