I am amazed at your lack of ability to diagnose a problem when all the needed information is before you.
He needed to change his T&P valve FIRST, before all of the typical backyard mechanics "lets change and add parts and see if we fix the problem" but with the added stupidity of not starting with the actual LEAKING part.
"Hey! the T&P drips after 10 years - lets add a expansion tank to fix it!!"
Calling an average plumber is akin to heading to a root canal without novacaine. And the odds of a true repair are about like slot machine win rates.
I'll give Terry an exception.
Someone might have mentioned that his valve piping also need to be re-routed to a drain or outdoors, and not to his wooden floor.
This guy got it right. NOT a plumber.The valve should only open under three situations: you manually open it, the pressure is too high, the temperature is too high. ANY other reason means it should be replaced. I'd double-check the expansion tank, it could be shot, allowing the pressure to rise. The tank should essentially be full of air. If it thumps like it is full of water, it is probably shot.
Get a drop of wisdom and read the entire thread before shooting from the hip.