With the main valve closed an adjustable bypass provides a minimum firing rate to the burner whenever the safety shut-off valve(s) is open. This is low fire to the heater. Since the pressure regulator maintains a constant supply pressure, the minimum firing rate is also held constant. The MR212's regulator is an integral component while a separate upstream pressure regulator is required for the M411, M511, and M611.
Note: This product should be installed by a licensed gas fitter.