First off. Are you using a kit specifically designed to test FW PO4
Second, is there any kind of polishing filter (carbon filter) in line AFTER the water goes through the DI
? If so, this could be your source of PO4
Third, is the DI
resin still good? I know you said the unit is new but I just thought I would ask. If you have access to a TDS
meter that will give you an indication on whether the filter is performing properly. Also, some DI
resin is meant to change colors when it expires. Good DI
resin is brown while exhausted resin turns green.