Chlorine will not hold

I've been taking care of a pool for about 5 years now and every time its opened it requires 4 pounds of shock twice per week for about a month to bring the chlorine to a stable level. I will shock it on a Monday and by the time I return on Thursday the Chlorine level will be hovering near 0, even with the addition of 6-8 chlorine tabs each visit. Here are the stats on the pool:

-25K gallons
-sand filter
-in-ground
-vinyl liner

And here are the test results from yesterday (May 17):

pH: 7.2
TA: 100 ppm
Free Cl: 0 ppm
Total Cl: .3 ppm (may have been a residue leftover from my personal pool, which has chlorine around 5 ppm)
CYA: 100 ppm

-Clarity of the water is great, it has been used every season for the past 5 years. It was in fairly good shape when it was opened April 30.

This year, the owner wanted the pool open early, so I have attempted to expedite the chlorination process by using 8 pounds of shock instead of 4 the past couple visits. I've put in nearly 40 pounds of shock since the pool opened and have been consistently adding chlorine tabs but the level will still not hold in the ideal range. Anyone have any theories as to why this is happening or how I could rectify it?

No theory. We know exactly why. Your CYA level is 100ppm which is very high.. That means you need to SHOCK the pool at about 25ppm and maintain it between 8 and 15ppm.

The tabs and powders you keep using keep raising the CYA level so you can't keep the pool clear.

It can be done--you just must follow the "Best Guess" table here on the Forum and use an FAS-DPD chlorine test kit to measure it.

Or you can dump about half of the water and refill it with fresh and use only liquid chlorine. That will put the CYA around 50 which means you shock at 15-20ppm and maintain roughly at 6ppm.

Thanks for the assistance, Carl. I greatly appreciate it.

This is exactly the same response I found on another forum, so it sounds like reducing the CYA is the only way to go.

Just to be clear, the only way to do this is to drain 50%of the pool, correct?

Thanks

Will

Not Carl, but ... yes. Unfortunately, that is the only way to reduce the cya. Even if you do that, your cya will still be 50 which is OK, but you really don't want it any higher than that. As Carl said, at that level, you won't be able to use any more trichlor pucks in this pool or the cya will soon be right back up to 100 again.

I assume, from your post, that you are in the pool industry -- servicing pools? Forum rules are that anyone in any aspect of the pool industry must identify themselves as such in each and every post. By the way, welcome to the forum!

You should be able to put that information, that you are in the pool industry, in your signature line so you don't have to think about it each time.

It doesn't matter if Lisa (WaterMom,), Jan (aylad), Al (Poconos) or Ben (PoolDoc) answers instead of me. The message will be the same.

There are others whose advice will be the same or similar, but we make up the administration of the PoolForum, with Ben as the owner, and we 4 as the moderators.

I'm not sure I would consider myself "in the industry", but I do care for two pools: my own and a friend's. I will gladly identify myself if those are the rules.

I assume this can be included in my signature, what should I say, exactly?

If that is all you do besides your own pool is care for one friend's pool, then don't worry about it. I thought that might be your job. Never mind, and again, glad to have you as part of our forum. Hope you will find a lot of good information here that can help you!

Thank you so much for the advise and the welcoming attitude.

It's nice to know there are some knowledgeable people not trying to sell me a myriad of chemicals.

Do you recommend a specific test kit for checking chemicals? There is one model that seems to keep popping up in my searches and I was wondering if it was your kit of choice as well.

Yeah, A really good one is the Taylor K-2006 or K-2006C, both from Taylortechnologies.com

Equivalent is the FAS-DPD Service Test Kit from Leslies on-line service.

Search out Chem_geek's recent post--he had a link to the Taylor at a great price. Expect to pay $50-$80 for the kit and it will pay you back 100x over! I have something like 3 versions of it in various forms.

At www.amantoind.com you can get the Taylor K2006 for $46.25, product #13633.

The 2006 is the basic kit with .75 oz size reagent bottles. The 2006C is the same kit with 2 oz size bottles. This supplier also sells Taylor refills in a variety of sizes at good prices. I've found it very cost effective to buy the basic 2006 and stock up with larger sizes of DPD powder, titrating reagent, and CYA testing solution, all of which I keep in the beer and wine refrigerator (not to be confused with the Yuengling Lager! :eek: )

Thanks for all the input you'll post here. In reference to the Taylor 2006c kit, I myself purchased a Taylor 2005C kit a few months ago , which also has the 2 oz size bottles of testing chemicals . Mine does not have any kind of powder included with the other items. Everythings in liquid form. What is DPD and should that be something I need to add to mine? I also purchased some Borate test strips to use when I added borax to my pool a couple of weeks ago.
Again thanks to all the moderators for taking time out of their lives to help guide us who are beginners at this stuff! Doug

FAS-DPD chlorine testing allows you to test higher chlorine levels than most other kits can. If you have already bought the K-2005, you can buy the K-1515. This will give you the additional test (FAS-DPD) that the K-2006 has. It comes in two sizes -- the K-1515A has 0.75 oz. reagent bottles and the 1515C has 2-oz. reagent bottles. Both kits have the same 10g. size bottle of powder.