Hi, I've got a funny value thing stuck on the end of a pipe that takes the heating circuit into the emmersion heating coil. There is a little bit of water running out of the the top. The top has what looks like a screw cap on it. The pipework is all 22mm but this value thingy is much smaller [below 15mm]. It doesn't match any auto vent value on the web.
Does anyone have any idea what it might be? Is the small leak normal? How can I service this without draining the system?