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Reason For Changing Water In Hot Tub


Cheetiah
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I'm a fairly new spa owner (approximately two months).

The commonly held rule for spas and hot tubs seems to be to change the water every three months (or more often, depending on who you ask). When I asked my dealer the reason for this I was given a nonsensical answer, so I googled it for myself. Multiple sources have indicated that the reason for this is that the TDS level rises over time. Eventually it gets too high and chlorine is no longer effective; thus, the water must be replaced.

This answer made sense and I accepted it for the past month, but I was doing some additional reading on the forums and there were multiple comments that TDS seldom, if ever, contributes to problems with water chemistry. The most common reason necessitating changing the water is actually a buildup of CYA (and less commonly, calcium). Thus the dichlor-then-bleach method of sanitation.

This also makes sense and several reputable sources seem to concur with the claim, at least when it comes to pools. So now I'm back to wondering, why does spa water need to be changed every three months? Most of the TDS is sodium chloride anyway, which is intentionally added to SWCG pools and spas. If the water is properly maintained, balanced, and sanitized and the CYA levels don't rise above 20-50 ppm, and the TDS level isn't actually an issue (by itself), what is the reason for changing the water?

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I use the dichlor then bleach method of sanitation, I also have an ozonator, I test my water monthly and I change my water annually. As long as my water tests are in range, I don't see any reason to change out the water.

Dave

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It isn't TDS that makes chlorine less effective, but the buildup of Cyanuric Acid (CYA) from using Dichlor-only. If you use Dichlor-then-bleach, just touching up CYA with Dichlor for one day per month, then you can go at least twice as long between water changes.

The rough rule-of-thumb with no ozonator and using the Dichlor-then-bleach method is that the
Water Replacement Interval (WRI) is (2/9) x (Spa Size in Gallons) / (# of Person-Hours per Day).

When you manage the CYA level, the primary reason for needing to change the water is the buildup of unoxidized organics. Chlorine doesn't oxidize everything and not all organics are insoluble enough to get caught in the filter.

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  • 4 weeks later...
  • 2 weeks later...

+1 on the CYA build up. To avoid this, I calculate the amount of di-chlor that represents 30-40 ppm CYA in my spa. that is a fixed amount that can be mesaured. I set this aside , and use this first. When this is used up, I switch to bleach.

Another point to make ... as we're talking about neat and clean water -- is my own learnings regarding the importance of a good purge. According to my own experiments, I'm convinced that my spa came delivered with biofilms already present and growing. I had troubles with sanitizer demand even on the first fill, and didn't get things settled down until purging with "Ahh-Some". I've written about this previously, but basically nothing else (that I tested) comes close to the effectiveness of this stuff -- I now purge every fill or sometimes every other fill.

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I think the primary reason water changes are reccomended at 3-4 months because it's all in the response of the water. Dirty water (high tds) is slow to respond to bubbles. (they move slower) And with fresh water the bubbles travel faster and the water clarity is beautiful. It's crisp.....less tiny foam bubbles. Theres nothing like fresh water and that first soak after a water change. So manage your CYA and TDS once that's licked, sounds like you have it, then go 4-5 months. If that doesn't work go less, or more.

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I took some photos of 6-month old well-maintained perfectly crystal clear water compared with a fresh fill, and can corroborate the above. Water can be crystal clear but still not sparkle, and you can detect this without any agitation or bubbles, although I fully agree with your comments re: nature of the bubbles.

I can't emphasize enought that there are benefits from purging with Ahh-some every drain, which I am doing now with very few exceptions. It takes so little extra time to add a few dollars worth of Ahh-some to the water before you drain, and the reward is, well you just don't know until you see what you think isn't hiding in your pipes.

If you start out with a clean spa, manage balance, CYA, and don't let the sanitizer level drop to zero, then you won't be concerned about TDS as a measurement, and you will drain the water because it doesn't sparkle any more, even though its crystal clear.

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