The sediment in the bottom of your water heater is any solid material that settles to the bottom; it can be sand or other debris from your well or the water mains, or it can be minerals, (calcium carbonate), that precipitate out of the water at higher temperatures.

 

In the case of water mains, the water company may periodically clear sediment from their mains by opening up a fire hydrant full blast for a while. If you happen to use your water while the sediment is flowing past your house you get to share in the bounty.

Is sediment buildup harmful?

The amount of sediment that builds up in your water heater will depend heavily on where you are located. If your city water supply is filtered, and you water is naturally soft with little mineral content, then you may have none or very little sediment.  In other areas you may have a large buildup of sediment.

A small amount of sediment at the bottom of your water heater is not serious. Only if you have a large buildup does it have a significant impact.  A deep layer of sediment displaces water and reduces the amount of hot water available for your use. It can also clog the drain valve and possibly affect the efficiency of the water heater.  Tankless Hot Water Heater
In addition, water heater sediment can slow the heat transfer and overheat the bottom of the tank. Overheating can weaken the steel and damage the lining of the tank. With electric water heaters, if the sediment covers the bottom heating element, the element can overheat and burn out. If it gets into hot water circulating systems it can cause problems with the pump and valves.

Popping and other noises

Sediment builds up on the bottom dome in gas water heaters

Sediment can cause noises like popping and other strange sounds as water between the tank bottom and the layer of sediment turns to steam bubbles.

Occasionally you hear that it is bad to drink water from the water heater.  There is no basis for such a conclusion.  Heating water with your water heater is no different than heading water in your tea pot.  If drinking water from the water heater was dangerous you would hear about it from the NSF, FDA, EPA, AMA, CDC, or someone in a position of authority.

The only difference between the water coming out of your tank and the water going in is that the water coming out is hotter and might have fewer minerals if they precipitated out of the water and formed sediment.

How frequently you might need to flush your water heater sediment depends on how much debris accumulates.  It could be none or you may need to do it once a year. If you have a real problem with sediment, and it's due to debris coming from a water main or well, then installing a whole house filtration system can reduce the sediment accumulation and extend the life of your plumbing fixtures and appliances.

Curved Dip Tubes

Some claim that by using a curved dip tube will help stir up the sediment and reduce the accumulation and make it easier to flush.  Some say that is not true. I do not know.

Original equipment drain valves are easily plugged with debris from the sediment and if you are in a position where you will need to drain your heater frequently then it would probably be smart to replace the valve with a full-bore brass ball valve and install a hose adaptor on it for quick easy draining or flushing.