Is there a safety risk if my hot water system hasn'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 2 weeks or more, and hydrogen gas is explosive. Before using the washing machine in this situation, turn on all hot water faucets and let water flow from each for several minutes to release any accumulated hydrogen gas. Do not smoke or use an open flame during this time, as the gas is flammable. (Page 2)