Water is considered “hard” when it contains any appreciable amount of minerals, especially calcium and magnesium. It becomes obvious that water is hard when pot of boiling water forms white residue on the sides and bottom of the pot. A more accurate assessment can be made by laboratory water testing.
The minerals in hard water can build up on the inside of plumbing pipes, in fixtures like toilets and sinks. Hard water shortens the life of plumbing and fixtures. The coating of minerals in plumbing and fixtures is called “scale.” The formation of scale is even more pronounced when water is heated.
Scale problems are different from gas to electric water heaters. In both, it prevents efficient transfer of heat from the heating element or flame to the water to be heated. In other words, it costs more to heat water and the water heater will not last as long if the water is hard. Built up minerals in a water heater can be so bad as to take up space that should contain hot water, making the holding capacity of the heater less than it should be. In electric water heaters, scale first forms on the heating element then flakes off into the tank, building up on the bottom. When the buildup is deep enough to cover the bottom heating element, chances are good that the element will over heat and burn out.
Gas water heaters work by putting a flame under the water tank. Mineral deposits build up in the bottom of the tank causing it to over heat. A sudden boiling action causes the mineral deposits to be lifted up into the water. This makes a rumbling noise.
There are several ways to deal with problems related to hard water and the water heater. One is to install a water softener in the plumbing so that minerals are removed from the water before the water enters the water heater. If the softener is installed as a whole house system, it will stop the problems in all of the fixtures, not just the water heater. Water softeners require regular maintenance so be prepared for the added chore.
Another way to deal with hard water is to drain and flush the water heater tank on a regular basis. I have seen recommendations that it should be done as often as every two or three months, but most manufacturers suggest maintenance every six months. The procedure is, well, a pain. The water heater must be turned off. A hose is screwed onto the water outlet and an inlet is opened at the top of the tank to let in air. This allows the tank to drain. After the water is gone, the tank must be flushed out, sometime several times, until the water runs clear again. Clear water indicates that the mineral deposits have been flushed out. The tank must then be refilled, turned on again, and the air bled out of the pipes. If the tank is old or if it has not been regularly maintained, all that action may rupture the tank. A tank rupture is usually cause for buying a new water heater.
For many people, draining and flushing the hot water tank every few months is just not worth the trouble. That brings us to the third option. Some people do nothing and accept that they will need to replace the water heater more frequently than if it was maintained with regular flushing. No matter what you plan to do about it, it pays to be aware of a hard water problem. At least that way you can choose a way to deal with it instead of having it choose you.