If you have a PRV, look no farther for the problem. You need a thermal expansion tank. The TP is doing its job. If you put a pressure gauge on the hot water line and watch it when the water heater begins to heat, you will be amazed at how fast the pressure rises. It will get to 150 psi very quickly and the TP wiill trip as it is supposed to do. What is actually happening is the water heats and expands. It needs to go somewhere and since the PRV has a check valve in it, the expansion can not be absorbed by the city water main. This causes the pressure to rise and trips the valve. The expansion tank goes in the supply line between the PRV and the water heater.