Q: How often should I drain my hot water heater?
A: Proper maintenance on a hot water heater typically is to flush the bottom drain at least once a year to eliminate sediment and keep the water as clean as possible. In some localities where city officials flush the fire hydrants, it’s a good idea to flush the water heater a few days afterward to clear any disturbed sediment that ends up in your system. The problem with sediment is that sandy particles collect on the bottom of the tank and displace the water. This results in hot spots on the bottom of the tank where the heat is not evenly dissipated. When this happens the burner overheats the tank which breaks down the steel and leads to tank failure.
However, if you haven’t flushed the water heater in a very long time, and the overheating has not yet caused a complete tank failure, then flushing the tank may not be a good idea. This is because after many years without maintenance, the overheating may have created tiny holes which are “plugged” with the sediment, preventing the tank from leaking. Therefore, if you flush the tank after many years of not doing so, the sediment that has been filling the holes may be washed away and result in a leak. (Source: TipTopHouse.com)