SWG: CYA levels

[Daf-Tekno]

Hi guys & galls,

I see the Chlorine / CYA chart references CYA levels and required chlorine levels for both non and SWG pools.

But why, when the non-SWG chart ranges from 20 - 100 ppm CYA, does the SWG chart only cover 60 - 80 ppm CYA?

What CYA levels should a SWG hot tub / spa be at?

(I've tried altering the CYA levels on the Pool Calculator, but it doesn't change its suggested bleach dose, even though you guys say it will / modify the bleach requirement).

Thanks

 

[mrcarcrazy]

Here's my .02
Non SWG chart is accounting for all sorts of chlorinating methods. (solid chlorine products and liquid chlorine)....The reason the range is so large in the chart, is the "optimal range" is at the lower end, however people that use solid chlorinating products often end up at the higher end of the range, and many can't/wont' swap out water to solve this....so the chart shows these people how much FC they need to maintain in order to prevent a swamp for their high CYA level. Also people in sunny climates often find it necessary to run a little higher CYA than those who live in less hot/sunny areas.

SWG range doesn't cover this wide range for two reasons. first, the SWG doesn't create CYA...so it all must be manually added separately...thus there's no reason to show any recommendations outside of the prescribed range. To my knowledge all SWG manufacturers recommend the same range of CYA for their cell. The CYA provides two purposes. first is to protect the chlorine from the sun, second is to protect the cell from the FC it's making.

Spa/hot tub would likely have the same suggested range for reason #2 above if for no other reason.

In the pool calculator each chemicals before/after allow the calc to show you how to get from your before to your after for that chem. The CYA and FC are not connected in that manner. You have to refer to the FC/CYA chart to determine your "goals".

 

[chem geek]

Most SWG systems are undersized and for outdoor pools need a higher CYA to be able to limit chlorine losses from sunlight so that the SWG is able to keep up. Also, minimizing the needed SWG on-time reduces the rate of pH rise that is also a problem in most SWG pools. This is why the SWG chart shows a limited and higher CYA range.

Spas are very different if they have covers. They aren't as susceptible to algae since their covers keep out most light and when the spas are run hot (towards 104ºF) algae is inhibited. In a spa, bather load is much, much higher than in a pool so most chlorine is added right after a soak to handle bather waste so the FC level generally gets quite high soon after a soak. Since most people want to soak with a low level of chlorine in a spa to minimize noticing its effects, the usual rule is to add enough chlorine after a soak so that one has 1-2 ppm FC at the start of their next soak. If one does not soak every day or two, then in between soaks one would usually target 2-4 ppm FC with 30-40 ppm CYA so the minimum most of the time would be roughly similar to the SWG minimum of an FC that is 5% of the CYA level, but this applies for manual dosing as well (again, because the spa is mostly covered, the water generally hotter, and the chlorine level often varying higher).

Because of varying bather load and the slow response from an SWG, manual dosing of chlorine after a soak is usually still required unless it's just one person soaking for a shorter time and the SWG has a boost mode or there is an ozonator.

If the spa is covered, then 30-40 ppm CYA is a good range to shoot for. This is similar to what is recommended for indoor pools though they can be 20-30 ppm CYA in order to more easily have a higher active chlorine level to help oxidize bather waste due to the lack of sunlight. In spas, the water is generally hotter and the bather loads generally have the FC be higher at times so a higher CYA level target of 30-40 ppm is usually better. We don't list spas and indoor pools in the chlorine/CYA chart because they are less common, and for spas as I wrote above it's more complicated but their maintenance is described in the Using Chlorine in a Spa "Further Reading" article in the Pool School.

 

[Daf-Tekno]

Hi Guys,

Thanks for the answers.

ChemGeek, if FC is 5% of CYA (i.e. 40 CYA = 2 FC), what levels are the Algae, Shock and Algae Shock levels for SWG operating at CYA 30 - 40?

I'm currently using the lovely afternoon (20 C!) to drain & refill the tub as we speak.
Regards,
Dave

 

[chem geek]

As noted in several other posts in other threads you've started such as this one, this one, and this one, spas are treated differently since 1) they are generally covered with little or no sunlight exposure and with low water volume (so the CYA does not need to be as high as 80 ppm; 30-40 ppm CYA is sufficient even with an SWG), 2) the higher water temperature and greater aeration require a lower target TA level (usually around 50 ppm) and this is OK because spas are not usually plaster, 3) with a low TA an additional pH buffer is recommended such as 50 ppm borates (usually by adding boric acid), 4) the higher water temperatures and likelihood of higher pH mean that one should not have the CH be too high (and specifically needs to be lower than the recommendation for pools so should be around 120-150 ppm and not higher) and the CH is only generally needed to prevent foaming since the spa is usually not made of plaster.

You should read the sticky How do I use Chlorine in my Spa? rather than only looking at the main Pool School articles that are mostly for outdoor residential pools.

 

[Daf-Tekno]

Hi Chem Geek,

Yes, indeed, I've read, printed out, condensed and reformatted the info in 'Chlorine in my Spa' sticky.
There is certainly a lot of info on this forum, and, until now, I've always assumed that, unless otherwise stated, any info from any source was generic and relates to either outdoor pool or covered spa.
A local hot tub store sells bulk chemicals intended for outdoor pools, under exactly the same logic that pools and spas are both just big vats of water so should be viewed the same.

Clearly this is not quiet the case, as you and your other learned colleagues here are now pointing out to the uninitiated likes of myself.

Thanks - D