to Shock or not to shock

[mshumack]

My question is should I shock if I have any combined chlorine? Is there a time when a shock is needed for maintenance purposes? Is there a specifc level of cholrine needed so there will never be any CC?

I have a 20K IG plaster finish pool. I maintain around 4ppm Chlorine by way of a SWG.

Water is clear. No noticible problems. Only users are two adults. Pool is in direct sun all day (central FL). No enclosure.

TC = 4
FC = 3.5
CC = .5
ALK = 100
CA = 220
CYA = 40
PH = 7.6
TEMP = 88

Thanks.

[aylad]

MOst people can live with a CC of .5--in fact, some people never get rid of that last .5, but anything higher than that is an indication to shock. Shock isn't necessary for maintanance purposes, except when there is CC, but I do it anyway after a bad storm or after a large kiddie bather load, just to head off problems before they start.

In a non-SWG chlorine pool, you do have to maintain minimum levels of chlorine to keep the CC away--that's what the "best guess"table is in the sticky at the top of the thread. It's a guideline, not a law--but it's worked well for many of us for years. My understanding is that in a SWG chlorinated pool, you can run lower Cl levels, but they generally require CYA of 80 or so.

Janet

[ChuckD]

This is a great question that connects with something I've had on my mind lately.
In another thread (http://www.poolforum.com/pf2/showthread.php?t=4078) someone wondered when to allow Cl levels to return to normal. Their FC had dropped 1.5 ppm overnight but CC was registering <.5, and should shock be maintained until level all night. One respondent said they maintain shock levels until the water was clear. My understanding is that Cl doesn't really serve any purpose in clearing, only in killing off the baddies. Once that's done (CC<.5), it's served its purpose and the rest is up to the filter. And if that's not enough, a clarifier or other remediation should be considered.

So...

Can CC be used as THE indicator of a need for shocking, or allowing Cl to drop off to normal? It seems like it'd be more accurate than clarity or smell or whether the level sustained through the night and save on some Cl.

C.

[aylad]

I don't have a problem with the CC being a signal to allow the Cl to drop off to normal, necessarily, but it can't be used as THE indicator of a need for shocking--I've seen plenty of times when algae is growing but the CC is still zero. Rather, I think it's best to use a combination--if you've shocked to kill an algae bloom, then when CC is zero and your algae is no longer green, then let the Cl fall off. It really depends on the situation.

Janet

[ChuckD]

I don't have a problem with the CC being a signal to allow the Cl to drop off to normal, necessarily, but it can't be used as THE indicator of a need for shocking--I've seen plenty of times when algae is growing but the CC is still zero. Rather, I think it's best to use a combination--if you've shocked to kill an algae bloom, then when CC is zero and your algae is no longer green, then let the Cl fall off. It really depends on the situation.

Janet

Obviously blindly going out and sampling your water without involving your other senses is not getting to know your pool. But for those times when one's unsure about what's really happening, I'd like to be able to say, 'ah, CC's dropped to 0 so I know now I can move from the Cl to attend to what is really acting on the water'. I think anyone who's homebrewed beer can appreciate this.

But... really? Algae's growing, CC=0 and (I assume) FC is high? I'm definitely not doubting you, but it kind of goes against what I've learned here.

Chuck

[Watermom]

I, too, have seen a pool with slight algae growing, but CC of 0.

[KirstenHW]

I believe mine is doing just that now. We've had lots of rain in Orange County this week (which I tested and is it ever acidic!). As a matter of fact, I will get green starting along the seams of my liner (IG 30-36K) and I can count on my hand the number of times in the past 2-3 years my CC has been .5 - rarely happens.

[aylad]

What I said was that I've seen pools with algae growing with zero CC, so that you can't use CC only as an indicator of when your pool needs shocking. However, if you have an algae bloom and CC greater than zero, and you've shocked to kill the algae, my point is that falling CC together with visible signs of algae death can be used as an indicator to let your Cl fall off to normal. My point is that while CC is one of the best indicators, I wouldn't usre it as the sole indicator.

Janet

Janet

[ChuckD]

And now I guess I've seen for myself in my own pool.
My numbers are all in line although my Cl is a bit elevated (CYA 45, FC 9, CC 0). Yet this weekend I got in with a mask for the first time and started poking around in the corners and in the deep end and found light traces of green algae! My water's crystal clear (see my post in the IG pool gallery) and after two days in the bright sun no increase was apparent. I brushed it and won't give another thought.
I did notice that the algae seemd to be growing in areas where the currents from the jets weren't moving the water too fast.

C.

[cwstnsko]

I've had the same thing this year. FC always 3-5 ppm, CYA 25-30. Water perfectly clear for 2 years now, never even a trace of green in the water, Never any CC. Small spots of green algae growing in the corners and in a few spots on the bottom. It brushes off easily, but comes back consistently. Shocked a few weeks ago to 15 ppm for 24+hours, algae back about 2 weks later. Phosphates tested at pool store at 3000-5000 ppb, home test indicates 2500+ ppb. Shocked the pool up to 20 ppm yesterday, dropped from 18 at dusk to 16 in the morning. Brushed thouroughly. No CC. Dropped to 10 today. Poured in 2L of Phos-free into skimmer of 13,500 gallon pool with 420 ft^2 cartridge filter, no rise in pressure. Tiny bit of cloudiness within an hour or so, clear again within 12 hours. Time to get in with the goggles and see if there is any algae still hanging around. Will test again for phopates tomorrow, see if the Phos-free did any good.