I have just been connected to utility water, my pressure at House is 100 psi, I am having no problems with leaks or with water heater, however I want to install a prv to protect plumbing. Since the water heater is fine now with 100 psi if I cut the psi down to 50 psi even with the closed system after adding the prv I cant see a need for adding a thermal expansion tank at hot water heater, doesnt look like it would cause the psi to go back near 100 psi. Am I right or do I still need to add expansion tank?