Pucks VS. chlorine usage conundrum

JasonLion · Jun 23, 2010

It can take a couple of weeks to a month to get CYA up to the right level with pucks.

woodyp · Jun 23, 2010

The only other thought I had about the logo image was to maybe add "Puck Free Zone" text curved and flowing on the inside circle at the top and bottom.

Jimbolo · Jun 23, 2010

Hi Richard, Yes I did inherit the pool when I bought it about 3 months ago. The pool was full and clear then and now, but I think it was kept that way by the realtor. There were pucks in the filter basket when I first started working on the pool a week or so after moving in. The people that lived in the house were not big on maintenance, so I imagine they just kept filling the puck dispenser and adding water to compensate for evap until moving out (house sat empty for several months, but the pool never got green that I saw). That would account for the calcium buildup on the pool block, the sky high calcium hardness, and sky high CYA.

The only way I can get the algae spots to fade to a very light hue is to add a gallon or two of 10% chlorine every 4 days or so, in addition to 3 pucks a week.

I think I'll start draining tomorrow night. I think I'll drain and refill in 25% increments. It'll take more fresh water to achieve the same level of dilution, but I'm too afraid to drain half of my pool. I've heard too many horror stories about pools "popping" out of the ground in this part of Florida. The extra $$ that it'll cost me in water won't come near what a pool repair will cost.

Thanks again for all of the technical expertise on this site. I learn something every time I log on.

Jim.

chem geek · Jun 23, 2010

For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So if your daily chlorine demand is around 2 ppm FC per day, then your CYA will build up at around 8.5 ppm per week or around 36 ppm per month. So it's a pretty slow rise. You can either add pure CYA or you can use Dichlor to chlorinate which will raise the CYA a bit faster. For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm. So shocking with 10 ppm FC will give you 9 ppm CYA. Most people just use the pure CYA and more quickly dissolve it by putting it in a sock or stocking hanging over a return flow (I put mine into a T-shirt in the skimmer, but I have alternate flows for the skimmer and also have floor drains -- without that, the skimmer could get clogged and crack the basket).

sh4unz0r · Jun 23, 2013

chem geek said:
For every 10 ppm Free Chlorine (FC) added by Trichlor, it also increases Cyanuric Acid (CYA) by 6 ppm. So if your daily chlorine demand is around 2 ppm FC per day, then your CYA will build up at around 8.5 ppm per week or around 36 ppm per month. So it's a pretty slow rise. You can either add pure CYA or you can use Dichlor to chlorinate which will raise the CYA a bit faster. For every 10 ppm FC added by Dichlor, it also increases CYA by 9 ppm. So shocking with 10 ppm FC will give you 9 ppm CYA. Most people just use the pure CYA and more quickly dissolve it by putting it in a sock or stocking hanging over a return flow (I put mine into a T-shirt in the skimmer, but I have alternate flows for the skimmer and also have floor drains -- without that, the skimmer could get clogged and crack the basket).

I would also assume that this would be a logarithmic curve (decelerating) of increase due to the FC demand decreasing with the addition of CYA would it not?

Richard320 · Jun 23, 2013

Benoit Perron said:
I currently have a CYA level of near zero (as per my cheap strip test but "confirmed" by tests at the pool store) and am using pucks in the hopes of raising it to an acceptable level. Since I have the blanket on very often I have to put the chlorinator on a low setting otherwise it's up to shock level in a week.

Would I be better off just putting stabilizer into the pool? How long would it take otherwise to raise the CYA using 2-3 pucks per week and periodic backwashes? (hopefully not in years!)

thanks!

You're one of the extremely rare individuals who can get away with pucks for a long time, since you need so little chlorine because the pool is covered so much. Well... maybe.

Besides adding FC and CYA, trichlor also reduces pH and as a result, lowers TA.

http://www.poolcalculator.com/ can tell you how much each 8 oz puck will add to your pool if you enter the volume at the top. For a 10,000 gallon pool, each puck will raise FC by 5.5, raise CYA by 3.3, lower pH by .29 and raise salt by 4.5. TA will likely drop about 4 ppm; that's the amount it will go down when you add enough muriatic acid to lower pH by the same amount as one puck.

Just plug in your pool size at the top and scroll down to where it says Effects of Adding Chemicals.
If you've been having to add chemicals to raise pH and/or TA, you might find bleach is actually much cheaper to use.

chem geek · Jun 23, 2013

sh4unz0r said:
I would also assume that this would be a logarithmic curve (decelerating) of increase due to the FC demand decreasing with the addition of CYA would it not?

Yes, up to a point. Even a pool with an opaque cover still loses chlorine, though more slowly. It depends on the temperature of the water. In my pool if I have it constantly covered but at 88ºF I lose around 0.7 ppm FC per day. At a cooler 80ºF, I might lose only 0.4 ppm FC per day. So while the rate of FC loss will drop as the CYA rises, you'll still lose chlorine though you are right that the rate of buildup will slow down. Using a cover or otherwise shielding from sunlight is a more dramatic effect than the higher CYA level, IF you raise your FC target in proportion to that higher CYA level (it's not logarithmic, but a small decline in loss -- see this post) . If you don't raise the FC with the rising CYA, then you'll lose less FC but risk getting algae growth.