Would you elaborate on this? We have a raised spa that waterfalls into the pool. Are you saying I have to test and maintain the spa independently?
Is there a forum thread you could refer me to?
Thanks!
Happy to elaborate.
When no one is using the spa and it's in spillover mode, the water in the spa and the pool are the same. So the FC, pH, TA and CH are all the same. Once you switch to spa mode, the water in the spa gets heated and aerated very quickly. At higher temperature, the oxidative properties of chlorine are more aggressive (organics, bathing suits, hair, etc). With higher temperature and aeration there is also more off gassing of chlorine and CCs. This is why chlorine spas, even when properly maintained, can sometimes have a detectable harsh odor.
As for pH and TA, you typically want a much lower TA in a spa as the higher TA combined with the higher temperatures and aeration will lead to rapid pH rises causing the water to quickly reach the 7.8-8.0 pH range and even exceed that. Higher pH combined with the stronger oxidative strength of chlorine can lead to a harsh water quality.
Finally, scaling can be a problem in spas because the higher TA, pH and temperatures combine with the calcium hardness to produce very positive CSI values. For pools we typically don't worry about scaling unless the CSI is above +0.6 and even then it often doesn't occur at an appreciable rate until the CSI > 0.7. For hot spas, scaling can be seen in water with CSI values at +0.3. So it is not true that if your pool water is balanced, then your spa water will be too.
As for personal preference, my pool with attached spa is an SWG pool. So my FC is typically 5-6ppm and my CYA ~70ppm. I keep my TA at 60ppm and my pH usually lives between 7.6-7.8. My CH is 750ppm. If I turn on my spa and just jump right in, the high FC level makes the water too harsh. As well, the pH will quickly rise to 8.0. So what I do to "setup" my spa is to let it run for 45mins or so with the SWG turned off so that some of the FC burns of. I also add 1-1/2 fl. oz of MA to drop the pH to 7.2-7.4 and take the TA down by about 10ppm. This keeps the spa water pH low and stable for the duration of use and the added time to warm up helps lower the FC to a more comfortable level. I have even added, on occasion, 4oz of 3% hydrogen peroxide to dechlorinate the spa water by 3ppm. That is a fast dechlorination and it makes the spa water very comfortable.
Some people don't care or are not sensitive to high FC and pH in a spa. I prefer to operate mine, when I occasionally use it, in a more safe and comfortable range. If I had to do that "setup" work everyday, it would be too annoying and I would opt for a hot tub that I could manage separately.
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