A plumber installed new 40 gallon gas water heater only two weeks ago. Hot water temperature at all faucets and two showers is fine. Yet when using either shower, hot water begins losing temperature after two minutes. Then after turning shower temperature knob to fully hot the water temperature remains warm but does not return to desired hot.

When the showers cool off can you still get really hot water anywhere? Did you watch/see him do the install? If so did he solder the pipes onto the fittings aon top of the tank or did he solder the fitting first and then thread it onto the tank? I am guesssing he melted or broke the dip tub drectly below the fitting.

THANKS for your quick response! The shower water doesn't get cold - it just loses desired level of heat. We can still get warm water at faucets - but you raise a good point. We haven't noticed effect at faucets but that's not to say they didn't also lose a few degrees following shower use. The level of temperature loss is more noticeable in shower simply due to physical exposure.

Try draining water from the spigot at the heaters base, does that water stay hot for a long time. Do this by connecting a garden hose to waste