"N8N" wrote in message

that's your T/P valve, also known as Temperature/Pressure. It pops
off whenever the water in the heater gets too hot or the system
pressure gets too high.

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Nate covered the basics well, but one thing was not addressed. Why is it
leaking? As they get old, some do start to leak but others leak because the
thermostat is bad causing overheating, or water pressure is up for some
reason, or it blew off once and grit got in there so it won't seat properly.

Keep an eye on the new valve to see if you need to investigate more.