About 6 months ago I rebuilt both toilets in my house including supply lines. I turned off the water supply outside the house and drained the system during the work. Since that time I have been plagued with periodic discharges from my water heater. The water heater was replaced one year ago, but no problems with discharge until after the toilet repairs were made. Since then I have replaced the pressure relief valve on the water heater and the pressure regulator on the incoming supply. The new pressure regulator solved the problem temporarily, but it's back. I live at the bottom of a hill and incoming pressure is high, but I have the regulator set down to where I notice low pressure in the house and still a discharge when the water heater is operating.

You probably have an issue with the thermostat on the water heater elements. it can be hanging up and allowing the water to boil in the heater, this will cause the relief valbe to pop off once in a while. most heaters have replacable thermostats and I don't think they are too expensive.

You seem to have a closed system. Ask a plumber to check and see if there isn't a backflow prevention or pressure reducing device on you water supply system.

Never adjust your Temperature Pressure relief valve.

Good Luck

The pressure reducing valve will prevent any pressure generated by your water heater from re-entering the water service in the street. What you will have to do is install a expantion tank at your water heater. This will take up any expantion coused by the water heater (that is why it only happens near the end of the heating cycle of your water heater) in the closed water loop in your house. You also noted that you had done some plumbing repairs in your house, some of the old fixtures may have been leaking and that kept the expantion from letting the T&P valve from blowing off.