I am not sure what you mean by expansion relief valve. If it indeed is an expansion relief valve the unit may be doing its job. Watts makes such a device that releases water to reduce pressure built up by water expansion. This valve should not run or drip constantly. Perhaps a better solution is a thermal expansion tank. This will allow extra pressure to be absorbed by a bladder within the thermal expansion tank itself.
IT IS POSSIBLE THAT YOUR RELIEF VALVE IS OPENING DUE TO EXCESSIVE PRESSURE OR TEMPERATURE. THIS MUST BE DETERMINED PROPERLY. IF YOU ARE NOT QUALIFIED TO MAKE THIS DETERMINATION HIRE A LICENSED PLUMBING CONTRACTOR. THIS CAN SAVE YOUR LIFE AND PROPERTY. WATER HEATERS CAN BECOME SUBSTANTIAL BOMBS WITH A MALFUNCTIONING TEMP/PRESSURE VALVE.
Good luck Fred [Edited by safe-flow plumbing on 16 January 2001]