Copper does not react with cold water, hot water, or steam. However, iron reacts with steam. If the hot water tanks are made of steel (an alloy of iron), then iron would react vigorously with the steam formed from hot water.
`underset("Inron")ubrace(3Fe)+underset("steam")ubrace(4H_(2)O)to underset("Iron oxide")ubrace(Fe_(3)O_(4))+underset("Hydrogen")ubrace(4H_(2)`
That is why copper is used to make hot water tanks, and not steel.