I'm assuming the installation manual comes with the furnace (ie: not
something bought seperately) ... so if I were to read it from cover to
cover would it explain things enough for someone good at handy man work
to understand it and be able to double check that the HVAC contractor
did most of his work correctly? Or in other words does the
installation manual outline the correct gas pressure, gas line size,
how to properly drain the AC coil condensation, how to know if the
existing duct work is adequate, etc.?

Thanks,
Harry