Installed new water heater but no hot water comes out just lukewarm water what could be the problem .never had this problem before.
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Gas or electric? Also, if the inlet nipple was overheated during soldering (if soldered) the dip tube may have melted off. This would cause lack of hot water. If electric water heater, and the power was turned on before the tank was completely full of water, it would have burned at least one element out.

I agree with MGD, most luke warm problems are caused by a missing or melted dip tube.