Turn water off at cold inlet valve above the water heater.
Shut electric off or turn gas valve to pilot.
Open T&P where the spring clip holds open.
Open boiler drain below at bottom of tank and drain about 10 gallons of water out of tank.
Unscrew drain pipe attached to T&P.
Use pipe wrench and turn valve counter clockwise to remove.
When reinstalling relief valve, use teflon tape on the threads before installing new one.
Follow the above instructions in opposite to get water heater back to functioning.
Check your water pressure. It might be causing the leak.
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