The Dip Tube
"The dip what?" you might say. In every water heater, there is a plastic tube parked in the cold port that brings cold water to the bottom of the tank to be heated -- and preventing it from mixing with already-heated water in the top of the tank.
If that falls into the tank or splits or breaks, then cold mixes with hot and suddenly your water heater doesn't work nearly as well as before.
There's a ton of Americans out there who know all about this because of a debacle a few years ago. The company that makes most of the dip tubes in this country changed its formula for the plastic. The new plastic was prone to disintegrate in certain conditions. The operative time period was August 1993 to March 1996. Aside from gradually or suddenly having much less hot water, the telltale was finding bits of plastic clogging faucet aerators.
There was a scandal, class-action suit, and settlement. But the terms of that expired several years ago, so if you find you have the problem, all you can do is try to deal with it. Even after replacing the dip tube and flushing the tank, you might still find plastic bits for awhile. But at least the water will again be hot.
Debacles aside, you should also be aware that even good plastic will eventually become brittle with age and hot water. We've found dip tubes in old water heaters that were split and broken. It's entirely possible for a water heater to outlive its dip tube. Simply replacing this inexpensive part can restore your tank's function to new and save you the cost of a new heater. So think about that before you run out and buy a new one if you experience this problem.
You may also want to flush out any built sediment. Check this out.