In the pool closing sticky it states the following:
"Two or three days before closing, I bring the pool up to shock level and hold it there until FC holds overnight (which it generally does right off on the first night). Then I let the chlorine level fall about half way back to normal levels, typically another day and a half."
There seems to be a contradiction in this statement because I don't believe it will drop that fast in water below 60 degrees. Currently my water temperature is 66 degrees. I maintain my chlorine in the 4-9 range and it took 3 days to drop from 9 to 4. I'm waiting to close my pool and my TFP calculator suggested shock level is 22. The problem is that finding a day this time of year when the level has dropped to about 11 and the weather is conducive to closing the pool becomes a low probability gamble, i.e. the level could be too high, most likely, or too low less likely.
I recall several years ago that it was late November and my CL levels were still close to shock values so I closed it anyway because the weather forecast for the next few weeks was miserable. When I uncovered the pool the bottom of the cover was bleached out. The liner was as well but it didn't have much if any color left in it anyway. However, I have a new liner that I donít want to get bleached.
The weather has been fairly warm so far and has had few nights below 50 and many days over 70 with a lot of overcast rainy days but that can change in a heartbeat. 3 or 4 nights in the 30s and highs in the 50s an that water temperature will drop rapidly but the CL level wonít.
1. I have had no algae all year and no signs of any so is it really necessary to shock the pool or is just closing it at normal levels a fairly safe option?
2. Is 66 degrees that big of a risk for algae growth compared to 60?
3. Would closing the pool at or near shock level bleach my pool liner?
Thanks for any responses.