How do I tell if the expansion tank on my hot water tank is working. I replaced my relief valve but I still get water dripping from this overflow pipe. I was told it might be defective expansion tank. How do I tell ?

What is the actual operational problem with your water heater? Water from the relief valve indicates that the temperature is too high. If it's brand new, then it could be defective. A little more data, and I might have a perspective for you.

XenoSapien