Chlorine keeps disappearing

Hi,
We have a 15', 5,000 gallon above-ground pool with Hayward Xstream cartridge pump which we got last summer. After we opened it this season, we cannot keep the chlorine level up, it's constantly at 0. After we first opened it the water was a bit cloudy, I took a sample to the pool store and they gave me a list of chemicals, including 1 lb of shock to add. I did all they told me to do, bu the chlorine kept coming up at 0. I added another pound of shock, no difference. I went to the pool store again yesterday and their analysis also showed no chlorine and low alkalinity. Last night I added 3 lbs of alkalinity increaser, stabilizer to the skimmer and another pound of shock and left the pump running all night. This morning -agian 0 chlorine, but alkalinity was up and pH was down, as intended. Can this have anything to do with the filter? We have only used it for 1 season and it was clean when my husband put it in. I did not see him put it in, but is there a way to put it in wrong so it will affect the chlorine level? Any help will be greatly appreciated. Thank you!

We really need a set of current water testing results taken with a drops-based kit. What kind of "shock" have you been using -- meaning ingredients. How much stabilizer did you add, how did you add it, and when? Do you have any idea what your stabilizer (CYA) level was when you closed the pool? What did you use for chlorine last year?

You'll need a good test kit if you don't already have one. The one we recommend is the Taylor K-2006 or 2006C (same kit, larger bottle of some reagents). If you buy it through the Amazon link in my signature, the Pool Forum makes a little money on the sale which helps us keep this form online. Only buy if the seller is Amato Industries, however. Some other sellers are substituting the K-2005 which you do NOT want.

Repost with requested info and somebody here can try and help you. Welcome to the Pool Forum!

Thank you for the reply! Here are results of the test done in the store. I have confidence in the store since it is owned by my daughter's best friend's family so they are trying to help me the best they can. If anything, they undersold some of the stuff :)
TDS: 900
CYA: 30
Tot. Chlor.: 0
Free Chlor: 0
pH: 8
Tot. Alkal.: 210
Adj. tot. Alakal.: 201
Tot. Hardness: 258
Copper: 0
Iron: 0
I added about 3 pounds of Alk. Up, about 8 oz of stabilizer (in powder, thru skimmer) and 1 lb of Multi-Performance shock (45% Sodium dichloro-e-triazinetrione).
After I posted here I added another bag of shock, tested in 30 min. and chlorine was high, but the water has not cleared up yet. I'll test again in a few minutes.

Thank you!

Sorry, in the previous post I had wrong readings. Here are correct ones:
CYS 25
pH 7.3
Tot. Alk 96
Adj. Tot. Alk. 88
The rest are the same. I also just checked the chlorine again and it's about 1 (really dropped)

In a 5000 gallon pool, 8-oz.of stabilizer should add about 13ppm of CYA to the water. So, that plus your current reading of 25 should take it up to 38. (It will take up to a week for the CYA to dissolve in the filter and show up on the test so don't retest for it or backwash for about a week.) The dichlor shock you are using also adds CYA. So, if you continue to use that, your CYA may get way too high before long. Go ahead and use another bag or two, but when you re-check the CYA in about a week, post the CYA reading. At that time, depending on your CYA level, you may need to stop using dichlor and switch to something else.

Your pH and alk are fine. Calcium is not needed in a vinyl pool, but some is ok. When it gets too high, it can cause cloudy water issues. It really would be best to stick with liquid pool store chlorine or just bleach.

Some pools open to a high chlorine demand that eventually goes away. Your pool may be one of them. It is caused by a breakdown of the CYA to ammonia and it takes a lot of chlorine to rid the pool of the ammonia. (Do you know what the CYA level was upon closing last fall?) Test the pool a few times each day, minimum of twice and more often than that is even better and will get you through this faster. Each time, shock the pool up to about 15ppm. When you get to the point where you can test it in the evening and not lose more than 1ppm of chlorine overnight from sundown to sunup the next day, then you should keep the high chlorine level for one additional day and then let it drift down. Once you report your CYA level in a week, we'll let you know what level that will need to be. (Chlorine levels are dependent on CYA levels. See the Best Guess table in my signature below.) To help you know how much bleach to add each time you test, in your pool, each quart of 6% bleach will add about 3ppm of chlorine.

Keep an eye on your pH. You are ok at 7.3 but don't let it drop below 7.0.

Hope this helps!

thanks a lot for the detailed explanation! I don't know the CYA level at closing the pool, unfortunately, it was our first season and we did not really know much. We'll be sure to keep that in mind next season. So, basically, the plan is to keep shocking the water at least twice a day? I work from home two days a week, so I could shock it more often then, but other days we are all gone until about 5-6 pm. If I keep using dichlor to shock, how much should I add each time - 1 lb to get 15 ppm? Sorry, I am very new at this and chemistry was never my strong subject.
Thanks so much!

I don't know the dosing for the dichlor. It probably tells us the label how much to add to get to a certain level for a particular volume.

Keep us posted how things are going!

OK, thanks!
They were also suggesting at the pool store if all this fails to try and hose the filter down in a couple of days in case the filter is clogged up or something. I did find out that my husband did not clean the cartrige before putting it into the pump. It was cleaned before storing after last season and was stored in the garage without a cover and may have collected dust. Would this have mattered for chlorine consumption?

No. That wouldn't have anything to do with it. Follow the advice I wrote above and you'll get it straightened out. We'll be right here to help with any questions along the way!

Yes, listen to 'Mom. Your problem is that your water has something that's causing your chlorine to be eaten up. Probably something biological or ammonia.
I THINK a pound of Di-Chlor add about the same as 1.25 gallons of 6% bleach--ball-parking here, but I know it's a little more than a gallon of bleach. One gallon of 6% will add 12ppm of FC to your pool, so the pound of "shock" was probably good but it would have added something like 8-12ppm of CYA to your pool as well. 3 pounds of Di-Chlor shock would probably put your CYA in the 30-40ppm range (when starting from 0 CYA).
Still, go back to basics. Use bleach to raise your FC to 15 and check and adjust it 3x a day until it stops "vanishing".
Since 1 gallon of 6% will add 12ppm of FC, 1 quart will add 3 ppm. You don't need smaller increments than that.

Carl