I just intalled the GE tankless water heater. The hot water line will have about 10 seconds rust like water if it it not used for a day. So every day morning, there is about 10 seconds rust like water. The rust color will be more dark if not use hot water for a longer time. The plumber checked and checked again and said it is the tankless water heater unit problem. Called the GE tankless water technician support, they said there is no any possibility the "unit" can produce the rust because it is make by copper in the heat exchanger. So no one can find what this rust color water come from. See the attached illustration drawing for details. Hope I can get the answer here.