It is difficult, but what you do not try, is lost cause, if you have time, you can do the following,
empty hot water tank and pipe. Shopping in a wholesale supermarket 10 or 15 liters of acetic acid, (alcohol vinegar, you close all the faucets, made the acid in the hot water tank, filled with water, heat the hot water tank and leave everything hot 24 hours, the acid attacks the tartar, only, empty and change all the water a couple of times, ready the chicken.