To lower CYA --

[gemphx]

My CYA is at about 75, I believe (that test is so funky ) So, that means I have to keep chlorine fairly high, which at this point is not happening, unfortuanately. I am thinking of backwashing some water out and refilling to lower CYA - how much do you think I should start with ? Plaster pool, with 11,730 gallons.

 

[duraleigh]

How do you chlorinate?

 

[gemphx]

I use HAASA liquid. 12.5%
When I asked how much to start with, I meant how much should we look at draining?

 

[duraleigh]

I would suggest draining 20% of your pool and bringing CYA down to 60 ppm.

 

[domct203]

My CYA is at about 75, I believe (that test is so funky ) So, that means I have to keep chlorine fairly high, which at this point is not happening, unfortuanately. I am thinking of backwashing some water out and refilling to lower CYA - how much do you think I should start with ? Plaster pool, with 11,730 gallons.

What do you mean that keeping FC at the proper level for your CYA "at this point is not happening"? Once FC is raised to target, it should just be a matter of adding 2-4ppm daily, regardless of target value.

Does the calculated amount of chlorine not raise FC as expected? Does your FC drop below target very quickly?

High chlorine demand is a sign that something is being oxidized in the pool.

 

[gemphx]

Well, to be nice about it, I gave the pool care to my husband when he retired. I emphasized the need for the FC to be at the target level per CYA levels. Anyway, here in Phoenix, if you don't check daily it can go sideways really quickly. Now, it will take a lot of chlorine to get to slam levels, so thought if we at least dropped CYA, slamming would not be so difficult.
Yes, the chlorine raises as expected when using calculated amounts, and yes, there is a demand for it - visible algae because chlorine was not kept high enough.

 

[domct203]

Well, to be nice about it, I gave the pool care to my husband when he retired. I emphasized the need for the FC to be at the target level per CYA levels. Anyway, here in Phoenix, if you don't check daily it can go sideways really quickly. Now, it will take a lot of chlorine to get to slam levels, so thought if we at least dropped CYA, slamming would not be so difficult.
Yes, the chlorine raises as expected when using calculated amounts, and yes, there is a demand for it - visible algae because chlorine was not kept high enough.

Ok, I get it.

Yes, if you need to SLAM you should lower your CYA, but seeing that your pool is in the full AZ sun don't go too low, you'll actually use more chlorine as the sun burns it off every day. Draining 30-40% and refilling should bring you down around 40-50ppm.