To size a room air conditioner. If mounted on a boiler, it will control the temperature range that the boiler operates and the working temperature for the circulators. A thermostat transformer, relay, and circulator switch will sometimes be encased within the aquastat control on a boiler. Hydro-air systems use an aquastat to turn the fan on during heating mode. Some hot water heaters use an aquastat to regulate the water temperature.
Designs differ by application and manufacturer.
Well type aquastats are used on boilers and some hot water makers. Most Aquastats use a bulb sensor and capillary tube. A seal copper bulb about the size of a pencil is attached to a diaphragm by a thin copper tube. The bulb is kept in contact with the heat source in order to function. As the bulb warms up, the gas inside expands through the tubing and into the diaphragm.