Philippe Habib provides some intelligent thoughts about hot water heaters. You're supposed to drain water heaters every month or so. That means you hook up a garden hose to the faucet on the bottom, put the end of the hose somewhere that hot water won't be a problem and drain out about 5 gallons of water. If you don't do this, the sediment builds up on the bottom and insulates the bottom of the heater from the flames. The other thing that happens is that water under the sediment layer boils since its trapped there and then the steam pressure makes it pop up. That's the big noises you hear from a WH that's gonna fail soon. Some water heaters have the inlet tube shaped to swirl the bottom of the tank with the incomming water. That prevents the sediment from building up. With electric heaters, the sediment buildup isn't a problem since the element is immersed in the water and not at the very bottom. If you've got the luxury of planning your water heater replacement rather than buying whatever the seller has in stock because you haven't had a hot shower in 2 days, you can save yourself some real money in the long run buy buying a more efficient device. About 1/4 of your gas bill goes to running your hot water heater. If you can cut that by a few bucks a month over the 10 or 15 years a good water heater will last, you've got a good deal.