Aquastat
An aquastat is a device used in hydronic heating systems for controlling water temperature. To prevent the boiler from firing too frequently, aquastats have a high limit temperature and a low limit. If the thermostat is calling for heat, the boiler will fire until the high limit is reached, then shut off. The boiler will re-fire if the boiler water temperature drops below a range around the high limit. The high limit exists for the sake of efficiency and safety. The boiler will also fire when the boiler water temperature goes below a range around the low limit, ensuring that the boiler water temperature remains above a certain point. The low limit is intended for tankless domestic hot water; it ensures that boiler water is always warm enough to heat the domestic hot water. Many aquastats also have a differential control which determines the size of the range around the low and/or high controls.