No heater. We have a centralized hot water system in block. It split the usage to each unit, may be it's in a low efficiency because of the age. I will talk to neighbor to see their bill.
$300 per quarter? I saw unit 1/2's hot water gas meter, but not mine. It's probably in the locker room. 339 looks still "normal" to me if compare to yours. I know we take a little long time for bathing.