Are you talking about the T&P(temp and overpressure) valve on the hot water heater??? If yes, ifrst check the actual temp of the water at the closest faucet, it should be about 120deg. If a lot higher, find out why and correct it. Thermostat set too high, malfunction, etc.
If the temp is about right, there could be some grit in the valve. Into a bucket or something(careful, it is HOT), cycle the valve open/closed quickly. If it still leaks, repalce it. If the heater is older than 10 yrs, it might be time to consider a replacement.
You should also check your water pressure because high water pressure, or occasional spikes in water pressure, can cause the T&P valve to open and once opened they frequently will not reseat completely--particularly if a few years old.
If the pressure and temperature check out ok and the expansion tank, if there is one, is ok then replace the valve.
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