Losing CYA

[MattM]

~4 month old pool, tested cya in january and it was >30, tested this morning after noticing FC levels being quit volatile for the last few weeks and swg running far too much, CYA seems to be <10 although exact level is hard to tell since it is so low.

Not sure how we could have lost so much cya in such a short amount of time (yes, we do swim alot with small kids that splash out water, but certainly 20 pts of cya in just a few months is much more than natural?)

There's been quite a bit of rain too, but our pool is covered and water normally just stays on the cover until it is pumped off.

One other possibility is that we have a hydrostatic valve which is apparently going up when it rains because water table rises...I find worms/etc near the drain even with cover on. I understand this isn't unusual.

So, I'm wondering...am I overthinking this in thinking we either have:
- cya eating bacteria (don't think so, but just finished shocking pool to verify algae wasn't eating chlorine - water looks crystal clear and CC is zero)
- hydrostatic valve somehow causing leak or introducing additional water that dilutes pool cya?
- other form of leak?

Water level seems relatively stable even when autofill is turned off for several days with cover on. Haven't had any reason to think there is a leak yet...and we had a good plastering company put in pebble sheen.

Update:
- Pool builder suggested cover opening/closing might be causing water loss
- However, the salt readings on the ic60 haven't shown any drop in salt over the 90 day period...so we're probably not looking at a leak or normal splashout/evaporation.

Note that I have to keep the cya low at ~30 due to ORP sensor. Perhaps there is also an artifact from the intellichem running the SWG at 100% when needed.

 

[257WbyMag]

Your CYA test result can be +/- 10 ppm depending on the time of day, amount of light, etc. I'd check it again and if the result is still low, go ahead and add CYA.

 

[JohnT]

Do you have an auto fill?

 

[MattM]

Checked cya twice.....could still see black dot even when test vial all the way full.
Put in more cya (didn't remember that cya decreases ph, but intellichem notified me right away and I had soda ash avail so no issue)
Will probably take a week to know what new cya value is.
Yes, have an autofill...it runs 4 times/day for up to 30 minutes each time. Glad we have it as it helps tremendously with splashout due to kids swimming/jumping/etc.
Water temp -- currently 79 and that is a little low, weather has been cloudy this week and rained substantially last weekend.

 

[JasonLion]

Yes, have an autofill...it runs 4 times/day for up to 30 minutes each time.

Over time that is a lot of water getting replaced.

 

[MattM]

I should have been more clear, when the autofill is running, that just means the timer has activated it...not that it is dispensing water....we don't allow the autofil to run for more than 2hrs/day, just so that it won't disguise leaks or if it breaks flood the pool. The autofill itself when active adjusts dispense speed based on how low/high the water level is.

 

[MattM]

One week later after putting in new cya, cya is now reading 33, intellichem/swg is much happier. We'll see if cya drops again (I'll measure every 2 weeks).

 

[MattM]

CYA dropped again, all the way to ~15, so lost 50% of cya in 10 days. There also looks to be some kind of algae outbreak, but only at the top of the pool and seems to like hiding under the cover (goes away in sunlight). Salt and Borates levels have remained high, so I'm guessing I'm looking at a rare cya eating algae and not a leak? How do I fight this? Standard shock only? Do I need polyquat 60 or whatever?

fyi - tested cya three times this morning (ouch)....and the first two results showed almost zero, and the final test where I was extremely slow and careful showed 20.

update - just noticed that the cya tests have an accuracy of 15..so not sure what to think.

FC has been fine in pool, perhaps a little lower than I would like (dropped to 2 for a day, otherwise staying between 3-5)

 

[JasonLion]

The current theory is that CYA is broken down by bacteria which are not really chlorine resistant at all. It shouldn't be possible to lose CYA if you aren't losing water and the FC level remains above zero the entire time. Of course if you had algae then the FC level would have been zero where the algae was, even if it was higher somewhere else.

 

[MattM]

Well, there is something at the top of the pool that is visible for 10-15 minutes after opening up cover every day.... I'll post pictures next time I open the pool.
I suppose it's possible that if the cover is restrictring circulation on the top of the pool which might cause some spots with differing chlorine levels (FC0 seems extreme though).
I'll increase the SWG run level and shock for ~3 weeks and see what happens.