tpr valve leaking

my tpr valve only leaks a small amount of water when the hot water heater is heating up . i have replced the tpr valve and it still leaks. i have checked the water pressure at an ouside faucet and the pressure is between 70- 80 psi unless the water heater is heating and then it has gone as high as 150 . i do not have a expansion tank and have been told my numerous plumbers that they dont use them . the water heater is only 3 years old . i have been told maybe i need a pressure reducing valve installed at the water meter . please help.

classic symptom of the reason an expansion tank is recommended. You would probably notice that you get more water out the valve if someone takes a long shower or otherwise empties the tank so that it has to heat a large volume of water. 70-80 pounds static is at the max recommended. An expansion tank will solve this annoyance. What you have is to be expected under the current conditions. My unprofessional opinion.

Install an expansion tank and I do believe that will take care of the problem.