I have a 60s house with an open central heating system, once powered by oil, now using gas. It has a component that I have never encountered before (see attached picture). The component is essentially a right angle bend in the plumbing, lying in the vertical plane, between the pipe that goes to the lower union of the hot-water tank heat-exchanger, and the main riser from the boiler. A grey, plastic 'screw-head adjuster' pokes out horizontally from the end of a tube that otherwise would be the right -angle bend. In effect, we have a Tee-piece. On the upper side of the horizontal tube there is a window, which presumably allows one to see the condition of the water in the system. My thoughts are that this is: 1. a control for the hot water/central heating balance; 2. a sludge indicator device. However, why have the window if its 1, and why have a controller if its 2??!! The name on the device is Rauchmann, or Raukmann. Any thoughts?