Two thermostats in one housing: Tamperproof (Pre-set) Thermostat NC (normally closed): Thermostat opens on temperature rise (red module housing) – for regulating heaters or for switching signal devices when the temperature has fallen below the minimum value. Tamperproof (Pre-set) Thermostat NO (normally open): Thermostat closes on temperature rise (blue module housing) – for regulating filter fans, heat exchangers, cooling devices, or for switching signal devices when temperature limit has been exceeded.