Does it make sense to continually brush the pool walls while draining to reduce CYA? One reference I found (then lost) indicated that CYA stuck to walls as the water level dropped, so the best way to reduce CYA was to brush it back into the remaining water still to be drained. Is there anything to this?

I don't think so. However, we have had several reports of CYA remaining high after a drain that should have lowered it to a more suitable level.

It has been hypothesized (but I don't think definitively resolved) that it could be the result of CYA clinging to the hard surfaces (pipes, walls, etc) of the pool and that it is then released back into solution when the CYA is diluted.

While I suppose that could be the case, I would suggest a far more likely reason for that observation is the innaccurate test result for high CYA in the first place.

So, if your first test for CYA is a result of 100, I believe it could easily be 120 or perhaps even more because the test is not very accurate at that level. The pool owner then does a 50% drain and gets a reading of 60-70 and, believing the 100, can't figure out why he/she didn't get 50.

My personal experience (having to add CYA almost every spring through my own neglect) is that my dosage calculations come out fairly accurate and I have never had to adjust for any "residual".

Others may have had a different experience.

For the CYA to get "stuck" to the walls it would first have to drop out of solution, accomplished in the CYA test by the addition of melamine. I do infrequent partial drains on my pool and estimate the water volume fairly closely so I know how much calcium, bicarbonate, CYA and chlorine to add when I'm done refilling. The drop in CYA correlates to the amount of water I've drained. duraleigh's suggestion of testing error as a reason some folks with high CYA suspect that the CYA is 'hanging on' makes sense to me.

I concur. The only way to get a reading over 100 is by dilution, which adds a source of error. And I discovered last week that temperature makes a huge difference in the CYA test. I dilute 50:50, and have been watching my CYA drop from 220 (est) to 160/180. Until I read something here that caused me to use warm water to dilute. The test came in squarely at 200. Consistently. Whereas before it seemed to fluctuate as I poured it back and forth repeating the test. This was probably as it warmed up sitting in the sun on a white patio table.

And, yes, I know that's too high. I'm working on it. Draining enough to bring it down would use 4X as much water as I use in a month - sure to bring down the wrath of the water company this year.

I have read a few places (including here) where CYA goes low over the winter. Does CYA break down once the chlorine is gone? How does it decrease if the only way to lower it (as I have been told) is to drain/fill?

Apparently there is some soil-borne bacteria that eats CYA. The Chlorine keeps it in check. Once Chlorine disappears, it can go to work.

Wouldn't you love to get a jug of someone's bacteria-laden water to use as a starter, like sourdough yeast, to rid the pool of the CYA? Let it chowdown then shock it when it's at a good stopping point.

We can dream, can't we?

Sounds good to me (after I just did a 50% drain). Maybe that's how we make our fortune. Richard & Sven's Magic CYA Down. $59.95 / quart at Leslie's right next to the Tri-Chlor tabs.

Or maybe just hire a good R/O company to save the majority of the water, lower CH and CYA (as well as the other TDS present) and get water like you really want from the start in a quick, simple process Hmmm, somebody should look into this..........

CYA can crystalize somewhere and then redissolve later. This isn't common, but it does happen, usually after CYA levels have been astronomically high. I don't believe that it has been established exactly where CYA tends to crystalize, perhaps in the plumbing or within the surface of the plaster, but it does happen now and then. However, even if the hidden CYA is in the plaster, brushing doesn't help remove it.

...and how much would such a process cost (order of magnitude), if someone were to look into it?

If the CYA has somehow crystallized and therefore dropped out of solution, wouldn't it not appreciably affect the test results?

I liken it to dropping a trichlor puck into the pool and just letting it sit there - just becaused the puck (when dissovled) adds x amount of chlorine to the water, you wouldn't necessarily get a reading of FC+x just because you technically have the puck "in" the water.

The explanation given by duraleigh seems most likely.

Undoubtedly more than it would cost to use tap water, or trucking water in, for that matter.

Although I am only assuming and have no experience with the cost of such a service.

This recent thread discussed this:
http://www.troublefreepool.com/post151243.html#p151243

Your pool, locally, $375.00. CH to 200 ppm, CYA to 30 ppm, TA to 30 ppm, TDS to 700 ppm. Way better than comes from our tap!

Richard (chem geek) has sometimes talked about a process where CYA is slowly oxidized by chlorine, however this effect was mainly seen is spas due to the higher temps. So, midsummer pool temps into the 90s might see a slow drop in CYA from this effect as well (on the order of a few ppm per month). That doesn't explain significant winter drops, really, but I thought I'd bring it up because some ways that CYA behaves aren't necessarily well understood.
--paulr

To read about CYA destruction, start with the following thread. There's also a link to a discussion in the Deep End forum that provides additional detail.

cya-gone-t11559.html

Based on this post someone already had the same idea.

Guess I will keep my day job.