The T&P valve on your water heater stands for temperature and pressure relief.
The valve is a safety for relief of high water pressure or high temperature.
Before a new temperature and pressure relief valve is installed the reason for why it failed must be established.
Is pressure to the structure normal? between 45-70#?
Do temperatures exceed the thermostats setting? or even go beyond the setting for the T&P?
T&P valves do fail and need to be tested regularly to make sure they are working properly.
When they work properly they sometimes leak and thus an underlying problem could be the reason.
It should be a minor service call charge to have a licensed plumbing contractor to come to your home to check it out, that would be my suggestion.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com
[Edited by TheLocalPlumber on 05 January 2001]