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It sounds like your pressure relief valve was discharging on excess pressure. When the heater heat a tank full of cold water there's a lot of thermal expansion in the water which raises the pressure as the water gets hot. The fix is to install a thermal expansion tank (see attached pic) to absorb the excess pressure so the relief valve won't trip. If you open a hot water faucet then turn the water to the water heater back on the discharge should stop. If it doesn't you'll have to replace the relief valve as well.
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