Boilers arrived whilst people were still heating their houses with stoves and open fire places .
But these were ships’ boilers and railway steam engine boilers and and these definitely heated water to boiling point - to produce the steam.
I guess a hot water/central heating ‘boiler’ is the same shape and the name stuck.
I was thinking of the same thing…a boiler means exactly this to me too.
Thanks Phil_MCR for expanding my horizon. 
I never payed attention to this word’s etymology tbh.
AAGGHH! Trying to get a teenager coming flying in from school whose first action is to clamp one of his unwashed paws over the loaf of bread whilst hacking off a few slices is one of my bug-bears.
He puts the lid of the kettle down though, so there’s that… 