The temperature and pressure relief valve on my gas hot water heater has been leaking for a while, we have a small bucket catching the water and dump it every few days. I decided to do something about it this weekend and replaced the T&P valve. New T&P leaks so I was thinking I may have a pressure problem coming from the city. The water temperature is turned to it's lowest setting so I am confident temperature is not the problem. I am going to buy a pressure tester tomorrow and check water pressure at an outside spicket and I will also use it to test pressure at the hot water tank drain valve.

I have been researching online and found new code requirements have required a check valve to be installed at the street/main line to prevent backflow into city water, several places online have suggested an expansion tank may be necessary to stop the T&P from leaking. Can anyone expand on this?

several places online have suggested an expansion tank may be necessary to stop the T&P from leaking. Can anyone expand on this Do you have a sprinkler system or a backflow preventer installed? (see image).
First you have to understand a expansion tanks function. When new codes forced us to install check valves and backflow preventers on water service lines that it could no longer expand back along the water service line and would build up pressure in the system this would kick off the T & P ,(Temperature and Pressure Relief) valve aqnd the valve would discharge hot water out the relief line. To prevent this we installed a Expansion,(Air) tank on the heaters cold water supply line for some place the water to expand to. More questions? I'm as close as a click. Tom