when I lived in an old apartment building we had this neat looking old water heater that looked like a cross between something you might find in the bottom of a ship and some kind of oven, rather large with a 3x3 foot door on the side.
one day I was doing some laundry (in the same room) and a guy was there with a shovel removing an awful lot of calcium (?). and by a lot I mean enough to fill at least a few 55 gal oil drums.
he said it was a common problem and he was thankful the unit had such a large access door.
he told me you can tell when deposit build up is a problem because water heaters will start making cracking and popping sounds as they try to heat the water.

didn't get to do the wash that day, but I learned something new.