Citric results: after one hot cycle, the reserve tank, which was spotless to start, filled up at the bottom with obvious large flakes of scale.
I sucked those out with a Mityvac and ran it to hot again. More scale, again extracted by suction. At this rate, I think I should just power-flush the whole system and start again with clean citric. Unfortunately, this batch was the very last of the citric powder.