The Danfoss ATC Cylinder Thermostat controls the heat supply that leaves the boiler for the hot water cylinder. The switch is turned "on" when the temperature drops below a certain level and then switches off once that temperature has been met. The ATC is an electro-mechanical surface-mounting cylinder thermostat, which is clamped to the cylinder wall by means of a spring clamping band. This thermostat is white and has the manufacturer part number 041E001000.