Calcium and heaters

I thought I read somewhere that if you have a heater you should have a calcium reading of at least 200 in a vinyl pool and more in a plaster/gunite/concrete pool. Is that true and if so, what effect does the calcium have on a heater?

Thanks!

This was a question that Ben and I were discussing a few days ago through e-mail. He suggested that a calcium level of 60 is sufficient when you have a heater.

Is that 60 or above? Is there a limit?

You should look at your overall water balance (saturation index). Low calcium will contribute to a corrosive water condition (in conjunction with low alkalinity and/or low pH levels.) In addition, it is desired to be a little on the scaling side rather than corrosive side as this will allow some calcium to coat the inside of the heat exchanger, which may reduce some efficiency but it will offer a little protection against heat exchanger corrosion damage. I don't deal with vinyl liner pools that much but I do deal with heater issues occasionally and alot of water chemistry problems.