Sticking out of the top of the calorifier is a pressure gauge, a red knob you turn, and a hose leading to the outside.
I think this is how it works: if the pressure of water in the calorifier exceeds a fixed limit (3 bar?) then the device allows hot water to be expelled through the hose to relieve the pressure. Is this correct?I think you're supposed to turn the knob to "reseat" the thing, so I did, making it click a few times. Did I do the right thing? Hot water must have flowed, as the hose then felt warm.
I don't seem to be any nearer finding out why the immersion heater doesn't work as it used to, unless it's something to do with the pressure. Could it be just that it's furred up? Or is it the cold weather sucking more heat out of the system?