Michigan Mechanical Heating Service Practice Exam

An aquastat acts as the thermostat for the boiler’s water. It has a sensor in the boiler that measures the water temperature and compares it to a setpoint you choose. If the water gets cooler than that setpoint, the aquastat signals the burner to fire (and may also trigger the fountain of heat to move the hot water through the system). As the water reaches the desired temperature, the aquastat turns the burner off or reduces its output to hold the temperature steady. Many aquastats include safety features like a high-limit to prevent overheating and a low-limit to avoid water temperature dropping too far. This control keeps the boiler producing the right amount of heat without overheating, rather than simply measuring outdoor temperature, regulating gas pressure, or powering pumps.