Waterbear, I changed your italic bold, because I couldn't read it; I need new glasses. Hope you don't mind.

Regarding partially shared spa/pool water, it can be done, but if it's not, there's no point in discussing it. (You learn a lot, having to deal with old commercial vacuum DE systems that are BELOW water level! It's not fun, climbing that learning curve, however.)

I see I need to add another category, so it is:
1. Non-concrete standalone spa. (low SI or low CA is OK; hi SI could scale heater.)
2. Concrete standalone spa (need balance SI to protect spa.)
3. Concrete attached; equipment share only.
4. Concrete attached with spillover or other water share.

But, something you said seemed to suggest that there may actually be a 5th category: non-concrete spa attached to a non-concrete pool. Are there such beasts?

The hydrogen bubble thing you mention is something I want to leave alone for the moment. Also, you describe buffer action in ways I wouldn't, but again, I'd like to leave that for later.

A point of curiousity: who IS poolspaforum.com? I was surprised to find that they've elected to use anonymous domain registration, which is unusual for businesses on the Internet.

You wrote
Interesting idea but then you have to get over the hurdle of convincing users that the high FC/CYA combination is not going to kill them of make them sick and there is the real concern that it could void the warranty on their spa.
Ok by me -- this is POOLForum, not PoolSpaForum. There are many more pool and more spa owners out there than PF + TFP + Havuz + PoolSpaF can serve, all together. (The shortage is in "answerers", not "askers"!) I'm willing to add spa help here, but I have ZERO desire to be a primary destination for spa owners.

So, if HiCYA + HiCl works, that is, that it can produce -- for the average spa owner -- water quality that is generally better and safer than otherwise, that's what I want to say here at PF. If they aren't convinced, that's fine by me. If HiC2 risks their warranty, my tendency would be to (a) warn them, (b) say, "your health and safety matters more than the warranty", and (c) you don't have to tell the company you're doing HiC2 -- it won't actually affect anything that matters from a warranty POV.

Also, reading through your post leaves me convinced that it's STILL not possible to provide people a "SpaSolution" that parallels, in quality, the "PoolSolutions" I developed. Since there are still unmet needs in the pool arena, that's what I'd prefer to focus on.

Thanks for the clarification on spa/pool arrangements. The shared water arrangement is the reason I have to change my attitude. It's seems to me that this configuration is, essentially, a pool with extra aeration, which you note.

There is one sanitation aspect to this layout: with low CYA/low Cl, the total mass of FC in the spa is not necessarily enough to oxidize the total mass of swimmer goo (if say, 4+ people get in, and the water is hot). This would result in a failure to sanitize, since the chlorine present would be reduced by the goo, leaving insufficient FC to rapidly kill bacteria and viruses shed by one or more spa users.

The very high turnover rate would ameliorate this issue to a degree, since fomite associated bacteria and viruses would end up on the filter, leaving only planktonic pathogens present in the water.

Are you aware of any downsides to using DE in this setup? Doing so would improve sanitation, I think.

...........................

So far, having read some of Richard's stuff at TFP and elsewhere, and having read through your posts, I'm still inclined to say the best approach to take at PF, with respect to spas is:

1. Hi CYA + Hi Cl
2. Relatively low TA, composed of borates, cyanurates, and some carbonates, with VERY low CA.
3. Optimal pH at 7.8 - 8.2
4. Calcium adjusted to avoid scaling in the heater, but to bring the SI high enough to avoid plaster damage. However, on non-concrete setups, calcium can simply be left low.
5. Preference given to recommending DE filtration (unless you know a reason not to do so)
6. Caution given that HiC2 might void warranty, BUT they won't know unless spa owners tells, and that there's no reason to suspect that HiC2 damages anything.
7. Finally, when all is said and done, recognizing that spas cannot generally be as stable or sanitary as pools.
8. And, if "you don't want HiC2, go talk to Waterbear at PSF, and he'll explain other methods, there".

I feel like we need to offer pool owners the BEST method of sanitizing and managing their pool, not the one most acceptable to the Powers That Be. That's a call I made a long, long time ago when I started PoolSolutions, and not one I see any reason to change.

Of course, if there's some reason why HiC2 is NOT the best way, I need to understand that.