Gas, elec.?
Why did you think it need to be drained?
What is the setting on the temperature control. It should be slightly on the hot side and there is the possibility that it was turned to it's lowest setting while being drained, and they didn't stick around thinking that you would reset it to you comfort level.
Is there a recirculation line and pump on your hot water line?
Also, some water heaters only have a shutoff valve on the incoming cold line, others also have one on the outgoing hot line.
Make sure these are both fully open.
When you open a hot water faucet all the way, do you soon hear the main gas burner ignite?
You might be only getting a tiny amount of heating from the pilot flame, if yours has a pilot.
Post some pics of the gas valve.
You would never adjust the gas pressure on the valve.
Meanwhile, I requested a pic of the gas valve, still waiting.
I can’t see it clearly in your pic.
I see a valve on the right hand insulated pipe on top of the heater.
Is that the incoming cold?
Your pics don’t show all of the left hand pipe.
Is there a valve on that left hand pipe or not?
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