There is a faint sound of running water coming from the water heater and the drain pipe that leads from the valve
down through the floor is hot, not just right next to the valve, but also 3 feet downstream of the valve the pipe is hot.
This tells me that there is hot water running through this pipe.

The unit is made by Ruud and has a date of 1994 on it. There is some blurry cryptic test on the button on the valve,
something about "watts regulated" and a series of lines. The only number I can see is 175, but there are other numbers on it.
Almost looks like an indicator of the pressure it is experiencing or it is an adjustable valve of some sort.

My thermostat is set at 140 - is this too hot and may be overpressurising the system? Or does the T & P
valve need to be adjusted to accomodate the pressure that the unit is experiencing? Or do I need to
replace the valve?

What steps should I take to safely replace it? I have an expansion tank on the cold water inlet.

Thanks for any advice anyone can offer.