The pressure release valve on my water heater will trip predictably after a significant amount of hot water has been used (e.g. dishwasher, laundry, long shower) releasing 16 oz or less of water. It only happens if a sizable quantity of hot water is used and the water heater as to heat up cold water, if the tank is sitting idle full of hot water it won't happen.
I've noticed that after using a lot of hot water, if you wait about 10-15 minutes then turn on the hot water there's almost an explosive blast of pressure as the water heater is heating cold water. Letting a faucet drip hot water usually prevents the pressure release valve from going off.
It's an electric water heater over 6 years old with a cold water expansion vessel. The temperature is set to about 110 and I've been slowly ratcheting it down to see if it affects this pressure build up, it doesn't.
What's going on?