Is there a safety concern with hot water systems that haven't been used for two weeks or more?

Yes — under certain conditions, hydrogen gas may be produced in a hot water system that has not been used for two weeks or more, and hydrogen gas is explosive. Before using the dishwasher in this situation, turn on all hot water faucets and let the water flow from each for several minutes to release any accumulated hydrogen gas, and do not smoke or use an open flame during this time. (Page 2)