The reason for this entry is this: lots of folks use "boiler" for any big apparatus that heats water. However, a boiler is not a water heater, nor vice-versa. Using the terms interchangeably defeats the purpose of troubleshooting.

So what is a water heater? It is a tank in which water is both heated and stored, and that last word is probably the key difference, since some water heaters approach boilers in their ability to heat water.

A boiler is essentially a box, often full of copper tubing, through which water flows to be heated very rapidly. The photo at right gives you an idea. A boiler can heat a whole complex, or a pool, or a hot tub. It also requires controls and pumps that are more complex than those for water heaters.

Very high recovery water heaters can do these things, too, but they have so many flues inside to transfer heat that they begin to resemble boilers.