The Circled Item is your aquastat this controls the maximum boiler temperature, If you ad all of the above components together with a thermostat and or zone valves. you have a complete system.
How does it work? In most applications the boiler or furnace (depending on what you call it) is normally off and will cool to room temperature if there is no demand for heat, In some cases, when an indirect water heater is installed, or a garage unit heater is installed on the system, the boiler will maintain water temperature. In either case the boiler or furnace runs in the following order.
1 The thermostat calls for heat, this completes a circuit
on the main control board
2 The control board power to the combustion motor, this starts the spinning of the motor.
3 Once the combustion motor reaches it's designed RPM Rounds Per Minute. The air flow sends a signal to the pressure switch.
4 Once the pressure switch senses a positive draft of air going up the vent, it sends a signal back to the control board.
5 Once the control board or module receives the signal from the pressure switch indicating there is a positive draft, the control module sends power to the HSI (hot surface igniter) for about 45 seconds.
6 After the 45 or so seconds the control module sends power to the gas valve, causing gas to go to the burners.
7 Once the burners are lit the HSI or separate flame sensor sends a signal back to the control module,
8 Once there is a positive flame signal the boiler will continue to run until the aquastat reaches the set temperature about 180 degrees Fahrenheit.
9 The circulator or pump starts to run as soon as the thermostat calls for heat.
When the aquastat reaches temperature and turns the boiler off, the pump or circulator continues to run.
when the thermostat stops calling for heat, the pump or circulator stop and the boiler shuts down, the combustion motor continues to run for a short time after to clear all the fumes.