As I feared, something of course has gone wrong with the install - the big fan unit that sits where the old furnace used to sit (I forgot the official name for it) requires 240v to run the circulating fan. They knew they wouldn't be able to connect the backup heat strips today due to lack of power/box space, but everyone was assuming the main fan ran on 120 but apparently the unit that they spent the whole day installing turns out to require 240 for the fan, too, and so now we have a problem.