Thanks. I’ve read the “How do I use chlorine” sticky, and I notice that the recommended levels for that article are different than PoolMath (example: as you and the sticky say 3-5ppm FC, but Pool Math says 1-3ppm. Also, the sticky recommends CH at 130-150, but pool math recommends 250-650)
At zero cya poolmath is likely using the minimum ever recommended fc level .
Once you use dichlor you add some cya so that very low reccomendation immediately changes so take it into account before hand. For every 1 ppm of fc dichlor adds it also adds approximately 1ppm of cya.
It is also advisable to always have more than 1ppm fc even if you currently have zero cya as fc gets depleted rapidly as soon as bathers are introduced into a spa so you want to shore your bets so falling below minimum never occurs before you can replenish. Theres also standby fc demand to account for.
There are notes about recommended residual fc amounts in the original chart here
Chlorine/CYA Chart
Having 20-30 ppm of cya affords you the ability to have high enough starting fc levels to remain above minimum for an average soak without being too harsh on bathers & equipment/surfaces.
Starting with 0 cya & 2ppm fc for instance could put you in the no no zone & allow nasties to proliferate in as little as a 30 minute 1 person soak- leaving you to try & play catch up & trying to eliminate elevated cc’s (not advised).
With no cya involved fc levels above 5ppm are kinda harsh so you can see the quandry this presents for long relaxing soaks.
I personally initially raise fc to 10ppm with dichlor (taking into account its effect on ph) then by the time the tub is warmed up the following day the fc has lowered a dab, i can have a soak then raise it back to 10ppm again- at this point i now have approximately 15ppm or so cya in the tub. I do this a couple more times then let my swg take over & supplement with bleach as needed. These acidic dichlor additions assist in lowering my ph & subsequently my TA then I do the rest of the work with Muriatic acid when dichlor is no longer needed.
As for ch - use vinyl as your pool type in Poolmath.
Low to no ch is fine, if foaming is an issue you can increase in increments of 50ppm until it goes away up to around 150ppm usually, no need for adding more than that.
Too much ch can present a scaling condition if ph is too high also which can happen frequently in a spa.
For reference My ch is 25 - it is fine. I don’t add ch.
To Recap :
*Don’t try to skirt minimum fc levels, nasties will proliferate & things will turn sour quickly. You want to be proactive with fc not reactive.
* Do take into consideration all the effects of each dichlor addition BEFOREHAND
* ch in acrylic spas is generally a do not add situation unless there’s foaming then up to 150ppm is generally sufficient, if ch is naturally higher than 400 or so consider using softened water instead.
). It wasn't "tuned" for hot tubs, at least not then.
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