Simple answer, shock when your combined chlorine is .5 ppm or greater. (If there is algae in your pool you WILL have combined chlorine or your chlorine will be depleted.) It might be every week, it might only be a few times a year. Depends on how you keep on top of your water maintenance! Another reason a GOOD test kit is a wise invenstment. It will help you save money on unnecessary chemicals.
I suggest shocking regularly for a couple of reasons.
1. Shocking oxidizes oils and other contaminants that remain in the pool under "normal conditions"
2. Shocking raises chlorine levels to such that some chlorine resistant bacteria and algae will killed.
On this we agree to disagree. It's standard pool store rhetoric that really isn't applicable if the FC level is kept high enough for the CYA in the pool. It does help to sell shock, however. Shock is a noun, not a verb and it means raising the FC high enough to burn off organics in the water. If there are organics in the water that have not been oxidized then there will be measurable CC. If there is no measurable CC then there is no need to shock. If the FC level is kept high enough to ensure that there is enough active FC for the level of CYA (NOT the 1 to 3 ppm level that has been pretty much superceded) then nascent algae and even resistant pathogens are killed. (crytosporidium is one exception but that has been found to be resistant to even very high chlorine levels in some studies and requires more than just shocking. It requires that shock levels either be EXTREMELY high or that contact time at normal shock levels be extended for several hours, a minimun of 8 if my memory serves me correctly, while the FC level is maintained at the shock level and not allowed to drop. Routine 'shocking' will not really have that much effect on it. Luckily, it is really only a major concern when there has been a fecal accident in the pool.)
I am new to this whole pool experiance. Having installed my 27' agp last year, and having a relatively easy time with it until I opened it up this spring.
That is another topic completely.
My question to waterbear concerning this thread is, if you keep your levels consistant for normal pool usage you should never need to shock your pool, which I can agree on.
Say you have a big party that gets a little out of control (drinks spill, lots of sunscreen, and what ever else can be released into the water) you should not have to shock the pool?
I would be very concerned the few days after the party that my normal levels can't keep up with the amount of usage that the pool had that day.
Your test results will tell the tail; if you have combined chlorine over 0.5 ppm then you need to shock. In situations like you describe though, I usually start the party with a little extra free chlorine in the water (8-9 ppm with CYA of 30 ppm) and dump in another jug (another 3.8 ppm) at the end. The next morning (or afternoon if it was a really good party), my free chlorine is usually in the 4-10 ppm range with no CC. Occasionally, especially with kids' parties and all that they add to the water (hey, has anyone been to the bathroom in the last 4 hours?), I can smell what I assume are chloramines but I have no CC by the next day.
Like waterbear said, it's really a function of how well you stay ahead of the water.
Thanks for the reply
As a new pool owner i know i have much to learn, and as in life you have to roll with the punches.
I am just trying to learn as much as I can over a short period of time.
And I dont want to hijack the OP'S thread
Thanks for the information