Why Do Online Sources Consistently Suggest Much Higher Fac Levels Than Leslie's And Manufacturer's Instructions?

[TXPoolOwner82]

Leslie's recommends 2-4ppm FAC ( Question #2 under Water Balance Basics: http://www.lesliespool.com/faq.html#Water_02 ), and the label on their shock recommends that you not swim until the water is at 1.0-3.0 FAC. You might be inclined to blame this on the fact that they recommend 30-50 ppm CYA, and while that is a valid observation, online sources often suggest certain FAC levels, depending on CYA levels, and even in that range of CYA, they're still typically significantly higher (e.g. 3-6 ppm.)

Why is this the case?

[chem geek]

First, let me talk about the maximum FC you'll ever see on any chlorine product. The EPA drinking water limit for chlorine is 4 ppm and that is used as the basis for maximum chlorine levels with no understanding of CYA and its effects. All EPA-registered pesticides, including all chlorine products, are not allowed to reference higher FC levels in labels or documentation.

Note that most counties and states ignore the EPA with regard to pool chlorine maximum levels with some allowing 10 ppm FC maximum (see this link for example), but that is to allow higher FC levels in outdoor pools where CYA is being used though the regs don't usually make that distinction clear. Most jurisdictions have a 5 ppm FC maximum, but again, no distinction is made regarding CYA. The CYA limit used to be 100 ppm, but many jurisdictions now limit it to 30 ppm and a few do not allow CYA at all.

Even though the chlorine/CYA relationship has been known in the scientific community since at least 1974 as described in this paper, it is one of the most guarded, obfuscated, debated topics because acknowledgement of the FC/CYA ratio being the primary determinant to algae prevention would essentially brand Trichlor and Dichlor as having built-in obsolescence and being the source of needing algaecides, phosphate removers, clarifiers, flocculants, enzymes and other products in order to keep pools clear. The pool and spa industry has a multi-billion dollar chemical industry dependent on the sale of these high profit-margin products. Nevertheless, by obscuring the truth, the industry shot itself in the foot since the fairly rapid adoption of saltwater chlorine generator systems has partly been driven by the unnecessarily higher costs of not understanding the FC/CYA relationship.

Of course, the alternative of manually dosing every day or two with chlorinating liquid or bleach is more work than adding Trichlor tabs to a floating dispenser or inline chlorinator once a week so many people would choose convenience at higher cost anyway. There are other alternatives to help with this such as peristaltic pumps and The Liquidator, but saltwater chlorine generators also eliminate the need to buy and carry chlorine.

So you won't see any chemical manufacturer, their distributors, nor retail outlets touting the FC/CYA relationship. This is, unfortunately, something that had to come from outside the industry and was probably first discovered through observation by Ben Powell of The PoolForum and PoolSolutions. I then later contributed more by looking at the core science including that 1974 paper. This isn't the only thing the pool/spa industry gets wrong. Much more detail about the chlorine/CYA relationship and the other things the industry doesn't disclose or that they get wrong is described in my post on Certified Pool Operator (CPO) training -- What is not taught.