Low Cya and it won't go up

[vanman2501]

My pool is about 10 years old now and the problem I am having this year is that I can't get my CYA show a reading.
My readings
FC 0
PH 7.5 today after rain 7.2
TA 120
CYA 0
When I shock the pool I will get a FC reading between two and four. It will hold for about 4 hours then return to 0
I have added about three times the amount of CYA as the pool needs and still on reading.
I tried a new testing kit. I tried a test strip kit. I tested my neighbors pools and all their readings are good and all the kits had the same readings so my test kits work.
I thought I had a bought bad CYA so I bought more and still no reading.
I have no idea what to try next.

vanman2501

[chem geek]

Do you have a Taylor K-2006 test kit or is your CYA test at least one where you mix half pool water with half reagent and add it to a tube until a black dot no longer is visible (see this link, for example)? If not, then your test kit may be bad -- test strips in particular are notorious for mis-reading CYA. Even if it read something in other people's pools, there may be something in your pool interfering with an inferior test. The turbidity test works reliably.

You can also take a small amount of CYA and add it to a bucket of tap water to see if your reagent is good and measuring properly. 1/8th of a teaspoon of pure CYA in 2 gallons of water is around 75 ppm. Note that CYA will not register immediately as it takes some time to dissolve.

How did you add your CYA to the pool? If you added it to the skimmer to get caught in the filter, note that it can take up to a week before it all dissolves, though usually it's a few days. It is not immediate.

As for your chlorine loss, though it will drop quickly during the day if there is no CYA, dropping roughly in half every hour, you should also check the overnight (no sunlight) loss just to make sure you don't also have nascent algae growth in which case you should shock your pool by raising the FC to a higher level, probably around 10 ppm.

[vanman2501]

I have two different test kit both use the black dot, one is from the start of this year the I purchased a week ago I also purchased a test strip kit. I added the stabilizer the same as I always do, I put the pump on recirculate and dump it in the shimmer. I leave it on recirculate for 24 hours before putting back on filter. Even though I have been slowly adding stabilizer since May, plus I use trichlor pucks so I should be getting CYA from them also. I don't have any algae in the pool even though my chlorine reading is 0. I have been adding bleach to try and keep the chlorine level up enough to keep the algae from forming. I tried shocking the pool heavily for a week but that didn't help. I thought just maybe I had to much CYA in the pool so I took some of the water and started diluting it but nothing changed.

vanman2501

[chem geek]

If your chlorine level is at 0, then bacteria can be consuming the CYA and creating a high chlorine demand as well. If that is the case, then you need to add lots of chlorine to stop this process (i.e. kill off the bacteria) and it could take a lot of chlorine to do. When you first started this, the chlorine would read briefly for some hours, so that was good, but having it drop to zero is not. Just keep adding chlorinating liquid to the pool until the FC starts to hold. I wrote a timeline own experience with this problem in this post.

[madwil]

did you backwash after adding the CYA? if so, you could have washed it out of filter and it never got to pool...

[Watermom]

it wasn't in the filter. He had the filter on recirculate.

You don't want to put it on recirculate and then put the cya in the skimmer. Recirculate means you bypass the filter so the cya would just be blown through into the pool and lay on the floor of the pool.

[vanman2501]

I saw it laying in the bottom so I vacuumed the pool on recirculate until I didn't see it anymore. I have vacuumed several time since on filter but have not backwashed. My water is crystal clear. I have added a ton of chlorine this wear. I added 6 gallons a day for a week then added more CYA, but never got a CYA reading and my chlorine level fell to 0 in about 4 hours. I shocked it with pool shock 3 of 4 times. I again added 6 gallons of chlorine a day for another week added more CYA and my chlorine level is still dropping to 0 with no CYA reading. I did open the pool in May with an algae problem I was adding 3 gallons of chlorine a day and it cleared up in two weeks. I continued adding the chlorine for additional week, Then started on try to raise CYA with no luck. I think I have added. I have added approximately 150 gallons of chlorine, pool shock, 8 chlorine plucks and 10 pounds of CYA since the middle of may.

[chem geek]

If you take a 2 gallon bucket of pool water, then every 1/4 teaspoon of 6% bleach is 10 ppm FC and you can see how much it takes before the chlorine starts to hold. If you had CYA loss over the winter from bacterial conversion of it to ammonia, then for every 10 ppm CYA it can take 24-30 ppm FC so if you lost 50 ppm CYA then that's up to 150 ppm FC to clear it. Hopefully, you're close to the end, but a bucket test wil be a faster way to find out.

You could also get an inexpensive ammonia test kit from a fish/pet/aquarium store but you might have gone through much of the ammonia by now. Do you measure Combined Chlorine (CC) even though your Free Chlorine (FC) goes to 0 after some hours?

[vanman2501]

My CC has been staying between 1 and 3 since May

[chem geek]

So this does sound like you had a major conversion of CYA into ammonia and the chlorine you are adding is creating and partially destroying CC. If you look at the post I linked to earlier for the timeline of when this happened to me, you'll see that there was CC and a huge chlorine demand. The bucket test is your easiest way to determine how much more chlorine you will need. An ammonia test might also be useful, though you might not measure that much now given how much chlorine you've added.

I wouldn't add more CYA until you are able to keep the FC from disappearing so fast. With no CYA, FC should drop in half in about an hour but you shouldn't be losing much overnight. Right now your loss rate is much faster than that so you are still getting rid of ammonia, monochloramine, and partially oxidized CYA. Unless the FC is able to stay above zero, then bacteria can still survive and consume the CYA you are adding -- you're giving them a nice meal!

You can see that prevention is far easier than having to deal with this. Making sure the chlorine level never gets to zero (certainly for no more than a few hours) is key. Closing a pool as late as possible so that the water is cold and shocking with chlorine and then opening as early as possible in spring with the water is still cold will help prevent this from happening again.