Hot water is dirty (rust color)

Need some help again...

My hot water is dirty with a rust color. I have an indirect hot water heater off its own zone on a hydrostatic system. The water is dirty. I thought maybe the coil in the indirect heater broke and the boiler water was entering the system so I did a test. I drained all the domestic water out of the water heater and checked if the boiler water was filling it. I did not hear any water or see any pressure building on a gauge I put on. So I wonder maybe it is just a small leak from the coil or something else. I ask your opinion.

Because the domestic water supply has a higher pressure than what's in your boiler, if there were a leak, it would tend to overpressurize the boiler, not leak water into the tank. But, the tank itself could be ready to die and the rust you are seeing is from the liner. Or, there may be a steel nipple on the supply somewhere. Is the hot water rusty everywhere in the house, or only at a single or a few places? What are your supply lines - if galvanized, you may be ready for a repipe! The hot tends to go first.