[Trainer]

Under the burner is the tempering valve. It is a brass valve with a black knob. Remove the burner. Use pliers to gently rock the black knob back and forth till it is broken loose. Move the knob all the way one way and all the way the other way three times. Return the knob to the starting point.

Install the burner and heat the coolant up. Turn the sink hot water facuet on and check temp. If cool reach under burner and turn top of knob toward the tank. If too hot, turn top of knob away from the tank.

If that does not work, replace the guts with the innards of a new valve. Loosen the nut that holds the valve in place and remove valve. Insert new valve and tighten nut. No need to solder a new valve body.

These are Watt valves and can be ordered from Grainger.

This solves temp not getting hot enough. Does not address water being to hot and then to cool. So check at sink that does not have a cartridge like the shower does.