An aquastat is a evice used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too often, aquastats have a high limit temperature and a low limit. When the water is heated to the high limit, the aquastat will shut the boiler off, regardless of the demand for hot water. Likewise, when the water cools past the low limit, the boiler will be turned on, even if no hot water is being used.