Water Heater is a gas-fired AO Smith Power Shot, 40 gallons, made in 2006. I have not changed the temp settings or any settings in years. I have cleaned the corrosion in the ignitor area a few times, and it's worked fine. In the past few weeks, the hot water in the morning is gone in like 10 minutes, where prior to this, it lasted quite a while. I'm thinking it needs to be drained (need to look up instructions). Suggestions? Other thoughts?