Is there a safety concern with hot water if the washer hasn't been used for two weeks?

Yes — hydrogen gas may build up in a water heater that has not been used for two weeks or more, and this gas can be explosive. Before using the washer, turn on all hot water faucets and let them run for several minutes to allow any built-up hydrogen gas to escape, and do not smoke or use an open flame during this process. (Page 3)