Keeping a cooling tower or a boiler within parameters. CHELANT TOO LOW The purpose of the chelant (typically EDTA) in a system is to combine with metal ions to keep them soluble so they won’t from scale in a boiler. Any metal ions that are in a boiler such as calcium, magnesium, iron etc. make up the demand. The free chelant residual you wish to carry is the residual. If anything happens to increase the chelant demand such as hardness leaking past a softener or corrosion byproducts coming back in the condensate, then the chelant residual is going to drop accordingly. PHOSPHATE TOO LOW Phosphate, in a low-pressure boiler system is used to precipitate hardness into a loose sludge that can easily be blown down. If more hardness is introduced to the system than usual due too malfunctioning pretreatment equipment or hard water leakage into the condensate system, then the phosphate demand increases and less may show up in the phosphate test. I say may show up because the phosphate test can indicate both the combined and uncombined phosphate, and the result will only b lower if appreciable amounts of phosphate sludge are blown down from the boiler. Running filtered and unfiltered phosphate tests on the boiler water will have a better understanding of what is happening.