Water temp and elements for hot water heater seem fine. Why is water heater discharging water through pressure valve usually after extended hot water use?

One possible reason for the temperature/pressure (tp) valve to start leaking may be from expansion of the water as it is being heated if there are check valves in the water system this is sometimes the culprit .In most water systems there is plenty of room for expansion in the distribution piping and service line and private well systems have a pressure tank that can absorb the expansion. You may have to add a small expansion tank to the water piping at the heater to solve this problem. hope this helps.

Bob