Hoping for some advice here to rectify a long-term problem that could get pricey to resolve.

A year ago we had a leaky T&P valve on our ~14 year old water heater. Replaced the valve with a brand new one and all was fine. About 2 months ago, it starts leaking again, but this time it was only leaking, as far as I could tell, after it had completed a heat cycle. At the same time, we noticed the water not getting very hot or not staying hot and decided it was just time to replace the entire thing.

Purchased a new Richmond gas model, got it all installed, and set the thermostat in between low and high, just as the manual advised. I let it fill and run it's first cycle, and the thing is leaking everywhere out of the T&P valve, just like before. In the days since, I have turned the temperature lower and lower, to the point that the shower water is warm but not hot, and it continues to leak after a heat cycle, albeit a little less as I turn it down.

I called Richmond and they have sent out a new T&P valve, but said if that doesn't solve it that I need to install an expansion tank. Even though the home never needed one before, and the other homes in the neighborhood on the same water line don't have/need one.

Is it really possible that I had TWO faulty T&P valves in a row, including this brand new one? Because I keep turning the thermostat down and the water temperature seems to go with it, I'm not convinced that the thermostat is faulty. Is it possible that there is a sudden, new water pressure issue in the house causing it to overpressurize, regardless of temperature setting? Our home does have a pressure regulator in the crawlspace, however, it was set years ago by the home inspector and has never been touched since. I'm just at a loss, and really tired of dealing with leaky water heaters, particularly after spending hundreds of dollars on a new water heater only to have the same issue. Help!

get a hose pressure gauge and screw it on the drain valve of the water heater. open the valve and leave it open for 24 hours. if the pressure increased a lot then you need an expansion tank.

the red hand will tell you the highest pressure it had reached.

Or a new pressure regulating valve.

Thanks for the responses. If the water pressure had changed enough to cause an issue, would I have noticed a change in pressure in the shower and faucets throughout the house?

noticeable only if it stayed high.
once something is used the pressure goes back to normal.
get a gauge and check your pressure so we can all stop guessing.

Get a brass cap and have it on hand, in case the drain will not stop dripping after you open it.