CYA goes to 0 over the winter

[cdods]

We've had the pool now for 2 years and following the TFP techniques is working perfectly.

However I have one questions. Being in Canada, we close the pool for about 6 months of the year. When I open the pool, the CYA has dropped to 0. I would expect it to drop some, as we get some overflow from snow melt and spring rain, but it doesn't make sense to me that it goes to 0. When I close it in the fall it would about 70.

Last year, I replaced the chemicals we use to check the CYA, but the new chemicals gave the same reading.

This year, we are back to 0 after the winter again. Is this normal?

 

[mknauss]

Several processes can lower the CYA in your pool water over winter. One you speak of is you drain but then get enough snow melt and rain to refill and even overflow. There might still be some CYA in the water but might not be enough to test (30 ppm is generally the limit to test).
Another is in winterized pools, some have the CYA degraded via a bacteria that gets in the pool water after the FC has dropped to 0. The first stage of the process creates ammonia in your water. If it completes the cycle before you open the pool, the ammonia transforms to nitrates/nitrites, and your pool now has 0 CYA and does not have the large chlorine demand that the ammonia would have.

 

[cdods]

Several processes can lower the CYA in your pool water over winter. One you speak of is you drain but then get enough snow melt and rain to refill and even overflow. There might still be some CYA in the water but might not be enough to test (30 ppm is generally the limit to test).
Another is in winterized pools, some have the CYA degraded via a bacteria that gets in the pool water after the FC has dropped to 0. The first stage of the process creates ammonia in your water. If it completes the cycle before you open the pool, the ammonia transforms to nitrates/nitrites, and your pool now has 0 CYA and does not have the large chlorine demand that the ammonia would have.

Thanks. Good to know I'm not going crazy. The way I handled it last year was just to use chlorine to bring the FC up, then start raising the FC and CYA until they are at the level needed to start the SWG. It's still too cold to swim & water is still too cold to turn on the SWG anyways, so my main concern to not have anything start growing as the water warms up.

Would you suggest doing anything differently?

 

[mguzzy]

Sounds good to me. There is a time frame there where the water is cold but the sun is getting higher in the sky when the algae can start to grow. That is the window you need to have your FC ready for. Carry on!

 

[mknauss]

Would you suggest doing anything differently?

Only thing I would suggest, when you open and test the pool, if your CYA is zero, I would do the ammonia presence test before adding any CYA.

1. Your CYA will be 0
2. Add chlorine for 10 ppm and test your free chlorine after 30-minutes. You likely have ammonia in your water if you lost 80%+ of FC in 30 minutes
3. Your CC tests > 0.5
4. Optional: An ammonia test indicates the presence of ammonia