Why do you shock the pool before closing?

[TroubleFreePat]

So I'm sure this is answered somewhere, but search isn't helping me. Basically - why do you want to shock the pool at closing? Is it to try and keep as much chlorine in the water as possible over the winter?

Reason I ask is we have an auto-cover, and we were told by the installer to NOT use the cover if we're shocking the pool as it could damage the fabric over time/shorten its life span. Sounds reasonable. He said if it's not safe to swim in, don't put the cover on the pool.

 

[Bama Rambler]

It's two fold. One is to make sure there are no organics left in the pool and two is to have it last through the winter.

You just said two different things and didn't even realize it. I agree that if it's not safe to swim in you shouldn't cover it. However, it's completely safe to swim up to SLAM level for your CYA so you're not going to hurt the cover by bringing the FC to SLAM level and closing it.

 

[chem geek]

They are correct that higher levels of chlorine will degrade the cover more quickly. However, if you are elevating the chlorine to shock level for closing, then the water is presumably cooler and that will slow down all chemical reactions significantly so even with shock levels you will not have any more cover degradation than you get over the summer. Now it is true that if you didn't have high levels of chlorine during the first part of the winter that the cover would last even longer since you'd have slower degradation than you do in the summer, but that's the trade-off you have to make if you can't more continuously dose with chlorine over the winter due to freezing temperatures.

 

[TroubleFreePat]

Dave - I didn't realize that you could swim up to the SLAM level. Interesting, thanks.

Chem Geek - guess we need to use 2 parts vodka to 1 part water to keep from freezing?