CYA is gone after winter

[pploco]

I checked my CYA at closing in early October and it was sitting about 45-50. I SLAM'd for a couple days, winterized, and closed the cover. This year has been cold and rainy, so the pool hasn't been opened since October. I opened it on Saturday and it looked pretty bad. Lots of earthworms and debris, and the water was pretty cloudy. No problem, been here before. I measured FC and PH - 0 and 7.4. I started up the pump, put DE in, and let it run. Like a moron I didn't check CYA.

I brought the FC up to SLAM level and by the end of the afternoon, the water was clearing up. Checked again and it had dropped from 18PPM to 12PPM within about 3 hours - YIKES!. I added more, closed the cover and called it a day. Sunday I checked in the morning, and FC was down to 8PPM. I added enough to bring it back up to 18. Checked again, back to 12. That's how things have been since Saturday. It dawned on me this morning that CYA might be off. Sure enough, it won't even register on the test.

Is it possible for CYA to drop from ~45 to undetectable over the winter? If I just bring it back up, should FC hold or is there something else going on?

 

[proavia]

Yes, happens to a lot of pool owners.
If you add chlorine and it holds after 30 minutes, you can add CYA (using the sock method).

The reason for adding chlorine and seeing if it holds after 30 minutes is to see if you have ammonia in the water. If FC holds, no ammonia.

The FC drop of 6 ppm in ~3hours you experienced is do to the sun burning thru the FC do to no CYA.

 

[IceShadow]

We have a mesh cover and every year open to 0 CYA or close to it. I suspect we have that bacteria in our soil that eats CYA and converts to ammonia and it gets into our water. Then, my initial FC adds burns through the ammonia in it and then kills the bacteria.

 

[pploco]

Thanks for the replies. So if FC only drops 4PPM overnight with the cover on, would Ammonia be present? Or would Ammonia take FC down to zero?

 

[IceShadow]

Ammonia would probably take FC down to 0, though we've seen some strange cases where it doesn't. That said, you may have had ammonia but the 4ppm was able to neutralize what was left.

Take it to 10ppm, mix for 15 minutes, and test to see if the FC dropped again. Let us know

 

[pploco]

Okay. I took it up to 10ppm, mixed for 30 mins and FC stayed the same. Should I start adding CYA?

 

[IceShadow]

Yep, you're safe then. Keep in mind the test doesn't give a result if it's lower than 20-30ppm of CYA, so you could have some, just too low for the test to register.

In my case, I tested low, I added 20ppm of CYA, and then I ended up around 40, which was fine for me. So undershoot, add granular via sock method, give it a couple of days to mix into the pool and register, and test again. Adding more is simple, removing requires draining/refilling.

 

[pploco]

Thank you very much for the help!! I'll shoot for 20ppm and measure a few days later.

 

[IceShadow]

I would do another overnight test to see if it drops again (Overnight Chlorine Loss Test). If it doesn't, then you've killed off or neutralized whatever caused you to drop 4ppm last night. If it drops again, then you've got something growing in the water and you should follow the SLAM Process. I'd start with the 20ppm level of FC until you test in a couple of days and then adjust then if needed, if you do need to SLAM.

 

[pploco]

No drop on my overnight chlorine loss. The water is clear my CC is <.5. I started adding CYA back in today, and will check in a couple days to see where I'm at..

Thanks for the advice. Love this site!