Failed Pressure Reducing Valve.

The Pressure Reducing Valve (PRV) replaces the need for a header tank.

A PRV generally sits below the Hot Water Cylinder (HWC) and reduces the inlet pressure on the HWC. The pressure is controlled by a spring and diaphragm and is adjusted to allow for an element of expansion so that the vent pipe does not over flow when the HWC is at its hottest. The retaining pin in the picture had snapped. This holds the washer in place when the system has come up to pressure. As the pin had snapped the vent pipe was overflowing very rapidly. There are various manufacturers of valves and therefore different pin sizes and shapes.

If your vent pipe is overflowing onto the roof it’s probably the PRV, usually just a washer replacement then give me a call. Meeting your plumbing needs.