in the bottom of the tank in the hot water heater. to try and clean it out you can try shutting off the water heater letting it cool down and then attaching a water hose to the drain faucet at the bottom of thw water heater and drainging and flushing the tank, Some DIY shows recommend diconnecting the cold water supply line and pouring vinegar into the tank and letting a sit for a while to help dissolve/dislodge some of the built up minerals....then reconnecting the line and flushing the tank clean several cycles thru the drain faucet with the water hose attached. run the water house out into the yard or into a nearby sink . good luck and hope this helps. LJ