Why will it no shut off flow

Indicates low water flow. possibly bad electrical connection to flow detector in dhw pipe or seized dector in pipe or blocked up heat exchanger. certainly a blockage somewhere in the pipes

Dead thermostat or element?

These unit is design to operate at a specific flow rate and cannot be ajusted if you need to have a warm shower you will need to get a bath remote where you can turn down the temp whenever you need without disrupting the flow rate

Check temperature off the water for proper functioning thermostat. Look at set points on heater's thermostat. It's made to let water out when water get's to a certain high temp. More than likely it will be the pressure relief valve going bad and not holding.

T&P releif valve can be replaced on heater. Turn water off and remove with a pipe wrench. It will come out and any hardware store will carry it. T & P pressure relief valve is 3/4 threads.

When installing new one use teflon tape on threads.

possible problems not knowing age of unit the
control valve could be sticking open allowing gas to continue to flow until high limit is reached and shuts down valve another possible problem with the newer style back flow preventer being install by water companys the rapid heating and expansion of the water over pressures the water system causing releif valve to pop open and reclose most water heater manufacters are recommending instalation of an expansion tank to allow the growth in volume of water somewhere to go is valve constantly leaking or only during heating cycle if you could supply additional information does unit only leak when heating
or is steam expelled when releif valve opens
t-stat not opening gas valve sticking partially open
bad high limit bad pressure releif valve no expansion tank to allow for thermal expansion if steam is present and unit is boiling water inside replacement of water heater for safety concerns,would be reccomended if only water and pops couple of times water temp not over 140 degrees after most hot water has been used out of tank and is reheating most of capacity probably lack of expansion tank as the releif valve is the final safety device on a water heater is of great concern
the temp control gas valve or high limit are supposed to shut unit done long before the safety limit of releif valve is reached

I have a triton opal shower.

I turn shower on low pressure and cold water. then no low pressure and piping hot water for a very short time then low pressure and cold water.

I've changed the solenoid and the little black plastic ball.

Help !

I have a problem with what you are asking,, the numbers you have are for a fan convection ROOM heater,, does the fan come on within this 3 minutes of running, if not it could be over heating and shutting down on a safety, if it does the thermostat could be bad and it is reaching the desired temp and shutting off like it shoud,, check the thermostat to make sure it is working properly,, set it at it's lowest temp turn on the unit have a themometer infront of the air flow see if the temp is the same as the setting,, how old is this unit, it may just be time to change with new,, if the controls are bad this unit sells for about 600 and to replace these parts is about 400, go to there web site,, write back and let me know how it goes,, good luck

water temp setting should be appx 120, any higher heat exchanger trips on overload and you must reset each time..

Change your air filter, if the system has to work too hard, it will over heat. If the filter is allowing air flow and the system still shuts off it is the thermostat. Keep in mind the Thermostat gauges the temperature where the thermostat is located, if its next to a running fireplace it will think your entire house is that temp.