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The first thing we would do in a case like this is replace the relief valve. If the problem persists you need a thermal expansion tank. When a tank full of cold water is heated up the resulting exansion of the water in the tank can raise the pressure enough to cause the relief valve to discharge.
All water heater relief valves are standard temp & pressure valves. The only difference is the length of the probe that extends into the tank. Some have a longer shank which allows for it to clear thicher insulation. You just have to shut off and drain the water to just below the level of the valve. Turn thw gas valve back to pilot. Best to take the one you have with you so you get the same thing. Just because you have an expansion tank doesn't mean its proeprly charged or that it hasn't failed. The tank should be pressurized to 2 psi below the static water pressure in the house. You can only check this when there is not water pressure on the tank. If you rap the side of the tank with the handle of a screwdriver and you get a dull thud top to bottom the tank has probably failed. If you remove it and it's full of water then that's a confirmation of a failure.
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