Genuine Ariston TBS Plus Thermostat, 16A The thermostat is an instrument that enables you to adjust the temperature of your boiler. The thermostat isolates the engine from the radiator until it has reached a certain minimum temperature. Without a thermostat, the engine would always lose heat to the radiator and take longer to warm up. See suitability below: Ariston ST 100 PROTECH ELECTRIC WATER HEATER - JAN 2011 Ariston ST 50 PROTECH ELECTRIC WATER HEATER - JAN 2011 Ariston ST 80 PROTECH ELECTRIC WATER HEATER - JAN 2011