They are safety devices which are fitted to all Biasi boilers, they are also referred to as limit stats, control stats and overheat stats. The control thermostat normally sits behind the control knob at the front of the boiler, most of the other thermostats are located inside the boiler and monitor temperature. If excessive temperatures are detected the thermostats turn off the boiler and normally have to be re-set to allow the Biasi boiler to work again.
Biasi BI1432102 Flue (NTC)

Part: BI1432102

Price: £12.45
Biasi BI1442106 Activ A Thermister

Part: BI1442106

Price: £14.82
Biasi BI1172105 Overheat Thermostat

Part: BI1172105

Price: £10.95