I had the same problem a few houses ago. I had to drain the water heater and pour lime remover into the relief valve fitting. I couldn't even get any water out of the tank drain at first. After a couple of doses of lime remover I was able to flush the water heater and get all the lime out of the bottom of the tank. After the tank was really flushed out well I reinstalled the relief valve and then did as the instructions say and drained some water out of the tank every week or two.
Hope that helps