Calcium and Magnesium ...

Calcium and Magnesium exhibit inverse solubility with relation to temperature. As the temperature increases, the solubility decreases significantly. In most open cooling water systems, a decent rule of thumb for maximum allowable skin temperature on a heat exchanger is 130F. Above this, the likelihood of calcium and magnesium precipitations increases substantially. 

Again this is also dependent on concentration of the calcium and magnesium species in the bulk solution.