When shocking, do I need to open pool cover?

[trauh]

I have a 27K gallon in ground gunite pool with PebbleSheen finish. I've maintained it all summer with BBB, and have not shocked it once (no CCs). I need to shock it now as I have my first measurable CCs, but am not sure if I need to leave the cover open while shocking it (its an aquamatic vinyl cover). I seem to recall that the sun burns off the CCs during shocking -- is that true?

Great forum. Thanks for the help.

[tenax]

i think you'll find that the concensus is shocking with a cover on is not a good idea..i do add regular maintenance chlorine with a vinyl solar cover and have noted no problems resulting with my cover, but i would not do shocking levels with a cover that touches the water surface.

[chem geek]

There is speculation that sunlight may help further the breakdown of combined chlorines, but we do not know for sure -- this is partly based on Ben's experience between outdoor and indoor pools where the latter usually have much harder problems getting rid of combined chlorine. The other difference between the indoor and outdoor pools is air circulation and this relates to your question about keeping the cover on during shocking. It is better to keep the cover off during shocking, not only to prevent damage from the higher chlorine levels, but to allow for good air exchange since the breakdown of chlorine from ammonia and nitrogenous organics will produce nitrogen gas. If you keep the cover on, then you will build up this gas under the cover and that will somewhat slow down the breakdown process (probably not a whole lot, however, since nitrogen is the main component in air already). There are also other volatile organics that would outgas from the pool more rapidly if a cover was not present.

Richard

[tenax]

traugh, if you haven't read up on CYA here to stabilize your chlorine, do..i've got mine at 60-80ppm..my cover has been off since sunday...sunday to wed it was sunny and 28-30C...today was rainy and 15...i shocked on sunday to about 20ppm..i'm down to 15ppm after going through the weather gamut with the cover off and tomorrow night it will still be way higher than i need it (now that my algae has cleared up..i'll pretty much put money that if i had used bleach and no CYA, 3 days into that heat and sun, my chlorine would have at best been 5ppm..that is how effective CYA is to "sunscreen" your chlorine..i'm itching to try my new chlorine generator installed tuesday, but it's no point until levels are normal..sigh i expect by sunday..one week since i shocked, they will be down to what i want...