Aquastat
A temperature-sensitive controller that commands a hot water boiler's burner to a setpoint, with a high-limit function for safety.
What is an aquastat?
An aquastat is a controller sensitive to water temperature. Fitted to a hot water boiler, it measures the circuit temperature and commands the burner to hold it around a setpoint. It is the device that translates a heating setpoint into starts and stops — or into output variation — of the heat source.
Control and high limit
An aquastat often combines two roles: routine control, which holds the water at the desired temperature, and a safety high limit, which cuts firing if the temperature exceeds a maximum threshold. This second function protects the installation against overheating, independently of heating demand.
Aquastat and modulation
On a boiler with an on/off burner, the aquastat drives on/off cycles. With output modulation, the temperature measurement is instead used to continuously adjust the flame to demand, which reduces cycling. In both cases, checking the setting and the sensor is part of maintenance carried out by a qualified technician, following the manufacturer’s instructions and applicable requirements.