Hi
T&P valve leaking.

Backflow & pressure reduction (50 psi) on supply, expansion tank has, 40 psi per label. Direct vent water tank is < 6 years old.
Replaced original Watts 100xl valve with same and it opens at 80 psi, as old one did.

Shut off supply and opened faucet and measured expansion tank air pressure, 40 (tire gauge).

Opened supply, and measured incomming pressure, 50 psi.

No noticeable increase in temperature.

I reduced water temp but would like to fix problem correctly, any ideas?

Thank You in advance

Coffee Spew

If the new valve that you installed is leaking I would assume that after all of your checking that it is possible that your old valve went bad from normal usage, they do sometimes leak from just corrosion build up in valve. And the new valve could be a dud, they are really a good piece of equipment most of the time but sometimes things just go wrong.

T and P valve means Temperature and Pressure valve. These valves are factory preset to release excess of pressure over 150 PSI and / or temperature over 212 F.

So, your pressure is too high or your water heater thermostat is not functioning properly heating water way too much. Also, your pressure regulator may be malfunctioning gradually pressurizing your house plumbing.

From what you say your temps and pressure are well within specs. Before I say to replace the valve open it and allow it to flush for a few moments. If it still leaks after that then shut it down, drain off some water and replace the valve, keep your receipt.