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The outside drain pipe is connected to a relief valve.
The relief valve is actually a safety device to keep the water heater from exploding from pressure build-up. A leaking valve is a good thing, since it means that it is doing it's job. However, you do not want it to continue to leak, so you will need to correct the cause.
The valve can begin to leak water either because the valve has become defective, or the temperature/ pressure in the tank is getting to high.
Before replacing or blaming the valve, you should inspect the TPR valve by manually raising and lowering its handle, a few times to see if there is just sedimentation or mineral build-up causing it to leak; be sure to have a bucket to collect the water. Water will run out of the bottom of the valve/ tube when lever is lifted and should stop when the lever is released. If water continues to leak, then either the defective TPR valve needs to be replaced and/ or the pressure is too high. If water is still leaking after the new one is installed, your pressure may be too high and this can be a dangerous situation. You may want to hire a plumber to diagnose the reason, after testing for high pressure with the link below.
Click on this link to learn more about testing for a defective valve & testing for high pressure. Also, instructions on how to replace the valve are included.
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Best of Luck, Brian
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