City recently installed back flow. Shortly thereafter my water failed. The reset... upper thermostat was replaced. Water heater performed for about a week then failed again.

Could the additional pressure be causing the problem? Would installing expansion tank and replacing upper thermostat remedy problem?

Has anyomne checked to see exactlty how much water pressure you have now?

Water pressure has not been checked. My water heater is 20 years old. I had not had any issues until black flow was installed. I was hoping to avoid replacing water heater.

The reset is for over temperature safety device. Separate from upper thermostat. If it and upper thermostat was replaced a unit, replace lower thermostat. If over temperature device alone was replace, replace one thermostat then the other, if necessary. Don't know how to tell which one is failing if over temperature device is turning off water heater intermittently.

Water pressure not a factor. If T&P (temperature and Pressure) valve begin releasing water then installation of expansion tank will be necessary

"City recently installed back flow."...

This usually requires installation of an expansion tank as pointed out by Hkstroud. Did the "City" guys even discuss or inform people that this might be needed?