Hi Everybody, Just bought my first house with a 20x40 vinyl liner inground pool (~31k gallons -per the guy at the pool store) and they tell me I have "Chlorine Demand." Their advice was that I buy 30-40 lbs of shock to fix it.

Free cl: 0.0
Total cl: 1.5
Total Alkilinity: 105
Total Adjusted Alkalinity: 96
pH: 7.2
CH: 236
CYA: 30
No copper/iron
no TDS
Water Temp 80f

I was trying to go BBB, after reading pool school, and several other articles on this site (including this gem: turning-your-green-swamp-back-into-a-sparkling-oasis-t4147.html). I thought that to break the chlorine demand I needed to "Shock" 10x the combined chlorine. I added 10 182 oz jugs of 6% clorox last night to try and accomplish this (per the pool calculator, this should've been able to service a CC of 2.5 ppm...), but this morning my FAC was back to zero... I've been skimming a lot of gross looking junk off the top though, and my sand filter seems to get blocked up after running for about 90 minutes (so I backwash until the sightglass is clearish)... Am I heading in the right direction, or do I need to do something differnt? Thanks for your help.

Best Regards,
Nick

31k gallon inground vinyl liner; 1.5 HP Hayward SP; 24 inch american pool products steel sand filter; taylor k-2006 test kit; one frustrated wife

Added a picture for good measure

You don't have chlorine demand (that's silly, we all have some chlorine demand. It's not a pool disease)... you have an algae bloom. Why not just tell you what it is instead of giving it some stupid name that simply means your pool is eating chlorine rapidly because it's attempting to kill the algae but can't accomplish that goal because it's way too low.

In other words, yes you need to shock the pool and that means continuing to add chlorine so that it maintains shock level in relation to your CYA level (for CYA of 30, shock level is 13ppm).

The pool will continue to loose chlorine rapidly till the algae dies completely. This is why you have to maintain the shock level. You could loose all your chlorine in an hour when the algae is in full bloom (rare) so expect high chlorine losses at first. It WILL slow down once the algae stops winning the battle but even then, keep maintaining shock level.

Pool store didn't get you to buy anything did they?

Edit: Sorry, forgot to tell you that any debris in the pool will increase the chlorine loss so removing it all will really help you move forward.

To add to what frog said, read this if you haven't yet: http://www.troublefreepool.com/pool-school/shocking_your_pool

okay, that makes some sense, I just need to keep on attacking, testing, attacking, testing until I deflower the algae bloom? Thanks for the tip on removing the debris on the water - I keep skimming it, and it keeps coming back as fast as I can clear it. I spent about 6 hours dredging my pool skimmer on the pole across the bottom of the pool pulling out basket after basket full of leaves and sludge... I couldn't believe how much **** was down there!

Do you have a pool leaf rake? (looks like a butterfly net, may have a dust pan lip on one side) Use that to scoop any debris that is on the bottom. There is probably a lot that you cannot see. Push the rake across the pool and up the other side to scoop up whatever is down there. Chlorine will obliterate that junk down there but it takes a lot to destroy leaves and twigs, better to scoop them out.

Yup, I've been raking like there is no tomorrow I think I actually caught the "swamp thing" in one of the hundreds of scoops of junk that come from the bottom!

I went to the new house and upped the ante on the bleach. I remeasured when I got there and got this:
Free cl: 4.0
Total cl: 2.0
Total Alkilinity: 105
pH: 7.2
CYA: 44

So I shocked it up to 20ppm FAC, waited an hour and a half, and tested again and it read 12ppm FAC 2.0 CC. So I brought it back up to 20ppm with more clorox using http://www.poolcalculator.com/ all the while using the leaf rake to continuosly pull stuff off the top of the water that looked like gross lily pads. When I dredge the bottom with the leaf rake now, I only get a few leaves.
After letting it sit 8 hours over night, I went back and checked this morning and FAC was at 15ppm, with CC at 2.0 still, so I shocked it up to 20 again.
When I first get the pump going after backwashing, PSI is about 14. Within an hour it has consistently gone up to ~25, and the flow into the skimmers and back into the pool is noticebly slower, so I try to backwash as much as I can with that situation. It feels like I'm finally heading in the right direction, but the 2.0 CC is concerning.

Thanks!
Nick

Is listing of "Total cl" actually CC?

That's the impression I'm getting. TC would be 6ppm then of course.

When you're loosing that much chlorine while shocking, expect that 2ppm CC result. It means you're doing things right and a lot of algae is being murdered effectively. It'll go down soon if you keep maintaining the FC at shock level. POP for you sir/madam :~}

Keep monitoring the filter pressure. That you're needing to backwash frequently to keep it low enough is also a good sign. Lotta dead algae getting filtered out at the moment, rejoice in it's evacuation from your pool :~}

Are you testing yourself, or is the pool store? Exactly how are you getting those numbers? You really need a good test kit to go through the shock process, such as the TF-100 from tftestkits.net

I'm testing with the Taylor 2006 fas dpd, and I did mean combined chlorine up above. I just tested the water again after work, and low and behold, fac is still at 12 ppm with cc down to ~.5 the water looks INFINITELY better now! This forum is great, and thanks frog for the great insight!
Insee the light at the end of the tunnel, hopefully I'll be able to enjoy a nice labor day weekend of swimming in our new pool!

Water is better, still pretty cloudy, all test are in the good range except alkalinity. I've tested it several times and it reads in the 220-230 range now. PH is 7.4 - when the pool calculator says to lower the PH with muriatic acid to 7.0-7.2 and "aerate" what does "aerate" Mean? Do I need to get after it with a whisk or something?

Aerating is introducing air into the water via bubbles or splashes. You can aerate the pool by using your water features if you have them such as a spa overflow waterfall into the pool or even your return jets if you can angle them so the water jet breaks the water surface and makes bubbles. If you can't do any of those options, some people build a fountain from PVC pipe that is temporarily plumbed into a pool return.

okay great! I can adjust the jets and make it rain! Thanks Optica!