I'm with you sbpse, I've seen/heard/read that it is 5-6 years (my neighbors experience) to 3-4 years to one year to 10 weeks before CYA becomes an issue. There doesn't seem to be any consistency that can be explained by normal water replacement due to rain/evaporation/splashout etc. Maybe there are differences in the brand of pucks used?
I will tell you this from experience - 99.99% of the time, people have no clue how high their CYA is. If you want proof, go read through the Algae threads and you will see the pattern I described. People using pucks for years thinking everything is ok (by mostly relying on pool store testing) and then, in the moment of desperation just before phoning in the backhoe to come and demolition the pool, they land here, order a test kit and are then floored at how far off all of their water chemistry levels are. As I said, it happens all the time. Also, when those people finally do clear their pools, they are AMAZED at how clear their water is and can not believe that they considered the cloudy water they had lived with for years swimmable. It's all in those threads, just go read them.
As for my analysis of trichlor, most high quality trichlor pucks are 99% trichlor, no additives. Places like Costco, Sam's Club and Walmart often sell "multipurpose" tablets that are a mixture of stuff (trichlor, baking soda, copper-based algaecides, clarifiers, etc). So, I guess using those types of tabs would extend the timeframe but they would also add lots of junk your pool does not need. If you're getting trichlor tabs from the pool store, chances are they are 99% trichlor and 7-8oz per 3" puck. My analysis is also MATH. The math does not lie. If you consistently add as much trichlor as I described, your CYA will be as I high as I predict.
So, to your points -
1. Water replacement - backwashing, at best, only causes about a 1-2% change in water volume. Unless your are backwashing every week (not recommended), then you're not changing your water volume significantly. Folks that live in areas of the country with winter climates will, as part of the winterization/closing process, typically drain 1/3-1/2 of their pool water. Those pools can use trichlor as they have enough annual water replacement to dilute the CYA from the previous season. As well, over-wintered closed pools typically develop small algae blooms and bacterial loads. The flora developed from that process can sometimes consume CYA and it is reported by those with that particular situation that CYA goes to 0ppm over winter (often leaving ammonia behind which is a much worse problem).
2. Rain - typically inconsequential. Unless the rains add several inches of pool water that you subsequently drain off, then the change in CYA by dilution is minimal. The recent deluge in Texas is not the norm for your area and, if you poll Texas folks here on TFP, you'll find their change in CYA was probably ~ 10% or so.
3. Evaporation - does nothing to change CYA. Evaporated water is replaced with fill water. Therefor there is no change in chemical levels like CYA.
4. Splashout - inconsequential. Splash out, even at it's worst, will have almost no effect on CYA levels. In a 20,000 gallon pool, 1% of the water volume is 200 gallons. That would be a phenomenal level of splash out.
Good points. I am guessing either errors reading the test or the above pride or a combo of both... Thanks for the discussion on the subject though, helpful...
