This cylinder tube powers the Univent and Ventomax automatic openers with no need for electricity. It contains a mineral wax which expands with heat. This pushes a piston which opens the auto vent via a balanced linkage. As the temperature drops, the wax shrinks and a strong spring closes the vent and resets the piston. Made in Denmark.
While this automatic vent opener cylinder lasts a long time, eventually the piston rod moving through the seals will wear the seal material away, allowing a gradual seepage of wax out of the tube. This in turn reduces the pressure within the tube and eventually the opening distance achieved will be affected. This is when you need to replace the power tube cylinder.