cleaning pex

i have a closed loop system and i need to flush it, it is pex line that connects each baseboard and i added another four foot baseboard and noticed the line is dirty well brown coating, what to do to flush out

Heating system water is ALWAYS a dirty brown color.

well this used to be an open loop from outside furnace which was treated with anti scale treatment and we think it coated the baseboards with a film to not allow heat to come off so we made it a closed loop and need to flush the lines have you ever heard of cleaning pex

NEver heard of cleaning ANY heating piping. Possibly a boiler water treating chemical MIGHT work. It would have to be a very unusual, and thick, coating to prevent heat transfer.