There are 2 reasons why a properly performing T&P valve would leak,
"T" = Temperature. If the temperature exceeds 210 Degrees F it will open.
"P" = Pressure. If the pressure exceeds 150 PSI. it will open. This may be caused by high supplied pressure in which case a pressure reducing valve needs to be installed or, replaced. And, or, an expansion tank need to be installed or, replaced.
This is a critical safety Item and it would be wise to consider the services of a licensed plumber.
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