Abstract: An appliance controller suitable for controlling a heating appliance (e.g., gas-fired water heating apparatus) having a distributed control system. The distributed control system includes a host computer and one or more heating control units. In accordance with a preferred embodiment, each heating control unit takes the form of an ignition control for controlling operation of a gas-fired burner for a multi-stage boiler. The appliance controller also includes a fault isolation system for isolating the source of a malfunction, and providing service personnel with status information regarding components of the appliance. The appliance controller is adapted to provide variable output signals for adaptive control of some appliance components, such as a blower or a circulation pump.