A valve unit for adjusting gas flow to a two-stage combustion apparatus includes a valve member that is moved by a magnetic field generated by a coil to vary gas flow rate through the valve unit, and first and second connectors configured to receive a high-stage activation signal and a low-stage activation signal, respectively. The valve unit includes a valve controller that is configured to control the coil to establish a high-stage gas flow rate while the high-stage activation signal is present, and configured to control the coil to establish a low-stage gas flow rate while the low stage activation signal is present up to a predetermined low stage time limit. The valve controller is further configured to establish at least one gas flow rate between the low-stage and high-stage gas flow rates when the low stage activation signal is present beyond the predetermined low stage time limit.