I am trying to determine the cause of my water heater pressure relief valve engaging. Details:

50 gallon gas Braford-White (12 years old)

House water pressure is 60 PSI after reduction by the house pressure reduction valve.

Before the water heater starts heating the pressure in the tank is 60 psi.

The heater has an expansion tank.

Once heating begins the pressure in the tank creeps up to 150 psi and the tpr engages.

About two cups of cold water is discharged.

The temperature dial is on the Hot mark with plenty of room to increase the htemperature.

Could this be a bad thermostat allowing the water to heat too much before it shuts the flame down? I've ben slowly dialing the setpoint back but still am having the problem.

Any other possibilities or things I should check?

Thank you in advance for any ideas.