We recently replaced our water heater. The previous owners bought it in 2007, and never flushed it. My husband went to flush it not knowing this and was sprayed head to toe with muddy sediment, and then it stopped flushing. It clogged the valve. The tank was soooo heavy and so hard to get out of the utility room. Once we did, we rolled it back and forth on the ground and it sounded like it was full of gravel. So. Yeah. Definitely flush it every year.