This photo shows the boiler cabinet open exposing controls and basic components of the boiler. The module on the left is the ignition control module which controls the sequence of operation for ignition and burner controls of the boiler. When the boiler is enabled to run usually by a thermostat or controls the ignition control module begins the process of firing the burners. The process typically begins by having the ignition module check the safety circuit to ensure all safeties are normal and not in alarm. This includes energizing the inducer or forced draft fan motor and any limit switches and a flow switch to ensure the boiler has water flow. The inducer or forced draft fan motor causes some pressure switches to close proving that combustion air is available and that the combustion gases will be exhausted when the burners fire. After the safety circuit has been proven to be okay the ignition control opens the gas valve while simultaneously activating the spark ignition to ignite the gas now being supplied to the burner(s). Once the burners ignite the fire is proven, usually by a state of the art flame sensor, the ignition control module deenergizes the spark ignition. As the water heats up it is sense by an aquastat or temperature sensor. When the temperature of the water reaches the set point of main boiler controller the ignition control closes the gas valve and the main burners shut down. The inducer or forced draft fan motor continues to run for period of time so that all the combustion gases in the system are purged up the boiler flue or boiler stack.
The wires on the right are the wires for the DDC system to be integrated into the boiler. The DDC system simply monitors the boiler for boiler run status and temperatures of the water entering and leaving the boiler. If the boiler were to fail the DDC system would send an alarm to main monitoring station where an operator could alert maintenance staff that there is a problem with the boiler or boilers.