Aquastat Temperature Control - The boiler aquastat controls the temperature of the water inside the boiler jacket. The boiler aquastat is a device to a boiler like a thermostat is to an air conditioner, heat pump, or gas/oil furnace. A boiler aquastat controls the temperature inside the boiler just as the thermostat controls the temperature for an air conditioner or heat pump. The boiler aquastat will turn the burners on and off and also controls the circulator pump on a hot water boiler system.
Aquastat is the gray box on the left while the circulator is the black pump on the right.