A quick definition of aquastats and their specifications.
It's no mystery that an aquastat is a control that goes with a boiler. If you don't know much more about it than that, though, you have a lot of company. There seems to be confusion about exactly what an aquastat does, and especially about finding a replacement when one fails.
An aquastat does the same thing as a thermostat. But an aquastat senses and controls water temperature, whereas a thermostat does air. In hydronic heating we use both - the aquastat to control boiler water temperature, and the thermostat to control room temperature.