About a year ago I installed a new gas water heater in my home. It works fine, except for one problem I’m not sure how to correct (or even if I can).
If no one uses any hot water for 12 hours or more, so that the water is just sitting in the tank, the thermostat does not kick in when the temperature starts to drop in the tank. When I take a shower, the water is still “warm,” but not as hot as normal. As soon as the water starts to flow, the thermostat kicks in, and then we have hot water again, after 10 - 20 minutes.
First, check to see that the upper water temp is set to at least 120F.
There may be a problem with the limit (or differential) switch. I’ve never seen one that’s accessible, but if the lower limit is set too low then it will take an idle heater a long time to get that cold and to call for heat.
The reason it comes on when you start running water is that there is an inrush of cold water into the tank that immediately drops the temperature to the point where the burner kicks in.
Now some HVAC type will come along and make me a liar.