The AT pipe thermostat is used to regulate the medium temperature in the pipe on which the thermostat is mounted. AT senses the temperature in the pipe through its contact surface and is therefore suitable to control boiler or flow temperature in central heating systems. The AT pipe thermostat comprises a bimetallic sensor, a contact system and a setting knob. On connection to terminals CO and NC the thermostat breaks on a rise in temperature. On connection to terminals CO and NO the thermostat closes on a rise in temperature. The AT thermostat can be mounted on the pipe without emptying the plant of water.