Old and New Isotherm water heater pressure valve keeps opening

[TacomaSailor]

This is a repost of a comment and question I entered into a prior thread about an over powered water heater. At the time I replied the OP had not clarified his question. It turns out my response was irrelevant to his question. However, I still have the following question.

I just replaced a 14-year old 11-gallon Isotherm water heater on which the pressure relief valve occasionally rapidly cycled open and closed. I replaced the Isotherm with an almost identical new Isotherm (SPA40) and the same problem continues to occur.

The tank would sit quietly for hours with the AC power to it turned off and no other heat source applied to the tank. Suddenly the Shurflo 3.8 GPM freshwater pump would take off and start running and the overpressure valve on the tank would open and close very rapidly while discharging luke warm water.

Sometimes the pump would stop running and the valve would stay closed. Other times the only way to stop the over pressure valve from flowing water to the bilge was to shut off the Shurflo pump.

A few seconds after shutting off the Shurflo I would turn it back on and the overpressure valve would stay closed.

I lowered the pressure on the Shurflo and the problem continued. The hot water and cold water coming from the galley sink, closest to the Shurflo, showed no unusual pressure or pressure fluctuations.

Opening the galley hot water tap sometimes will cause the over pressure valve to close and stay closed, other times nothing will cause the over pressure valve to stop cycling open and closed, except for shutting off the Shurflo.

I installed a NEW 11-gallon SPA 40 Isotherm water heater earlier this week. The same problem occurred with the new heater six times in the last four days : with no power or heat to the water heater - the overpressure valve starts surging open and closed and I have to turn off the Shurflo to stop the discharge.

Each time the over pressure valve starting cycling open/closed I checked the water temp coming from the hot water tap and found it to be only slightly warm.

I have tried adjusting the the pressure switch on the Shurflow and it is now one full turn down from the factory setting of what I think is 45 PSI.

The Isotherm manual says the cold water feed to the tank should be no more than 42 PSI.

Most of the time the Isotherm pressure relief valve cycles open and closed when a hot water tap is opened. But, occasionally the valve also cycles when the Shurflo has not run for hours.

I do have a filter in the freshwater line to the Shurflo and am pretty sure there is nothing but really clean city water is being pumped into the Isotherm. The freshwater system is not connected to the city water system and the only source of pressurization is the Shurflow 3.8 GPM pump with an 1 quart accumulator set at 40 PSI (I just measured it).

The problem did not occur during the first 12 years of the original Isotherms life but occurred ever more frequently in the last two years with the old water heater now occurs with the new Isotherm with a new valve.

While writing this question I did notice that the problem seemed to begin shortly after I installed a new Shurflo 3.8GPM pump to replace it's identical predecessor which died of old age.

Any suggestions?