the temperature pressure relief valve only releases when the temp or pressure gets to high. It has no function that controls the heat.
either
1/ the element is faulty
2/ the thermostat is faulty
Either realy needs a electrition to look at it properly.
But “one good bath” usually is about all you’ll get from anything under 140ltr tank
DISCLAIMER
All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or
will be taken. And yes, I am a licensed plumber with my own business in Brisbane Australia