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. The 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.
The house thermostat on the wall that is connected to the boiler is connected to the boiler aquastat. When the thermostat calls for heat it sends a signal to the boiler aquastat to energize the circulator pump. The circulator pumps kicks on and pulls hot water through the hot water loop to provide the house or dwelling with heat. When the temperature inside the boiler jacket falls below the set point of the boiler aquastat the boiler aquastat engages the burners of the boiler and the boiler fires the burners to heat the water inside the boiler jacket.
When the aquastat is tied in with an outdoor air temperature reset control the set point temperature of the boiler will change according to what the temperature is outside. In warmer temperatures the set point is lowered on a linear scale as the temperature outdoors rises. Using an outdoor reset control in the manner can save money on the heating bill as it requires less work from the boiler when the weather or temperatures outdoors is higher. Why maintain a 180° F temperature for the boiler set point when the outside air temperature is 60° F and the boiler can efficiently and adequately heat the house with a set point of 140º F using less energy to maintain a lower set point?