"I had to replace my water heater last year. When I drained the old tank, a lot of sand came out."

Our friend had a close encounter with sediment. It wasn't sand in his water heater, although the two sometimes bear a close resemblance. It was calcium carbonate, a mineral present in water which the heat causes to precipitate out, and which settles into the tank bottom, where it does all kinds of bad things.

High on the list is forming a layer of insulation between the gas burner and the water. The sediment slows heat transfer and overheats the tank bottom. Overheating weakens the steel and damages the glass lining, hastening the day when the tank will fail altogether. In the case of electrics, it can bury the lower element, causing it to burn out.

It also creates a playground for corrosive anaerobic bacteria. In addition, it lowers the energy efficiency of the tank.

Sediment can drift into recirculation lines, jam open check valves, and cause the recirc pump to stick until it burns out.

Finally, it causes noise, sometimes enough noise to annoy, or even frighten, people if the tank is inside a dwelling. The noise is caused by small amounts of water under the sediment layer turning to steam bubbles, which then collapse violently.

Water heater makers have not done their customers many favors toward controlling sediment. The cold-water inlet, called a dip tube, is straight on many water heaters made today. That means the water strikes the bottom of the tank and causes the sediment to settle evenly on it instead of making it gravitate toward the drain.

Most manufacturers boast "self-flushing" dip tubes that supposedly keep the water so stirred up that sediment doesn't have a chance to settle. We have our doubts about these and prefer to actually eject sediment from a water heater.

Also the flue that runs down the center of gas-fired tanks creates an additional obstacle to sediment removal. Most tanks, both gas and electric, have domed bottoms so that sediment falls out toward the edges and stays there.