FC sun loss during SLAM?

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G

glo76

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Jun 3, 2015
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Missouri City, TX
Hi - was starting to get some algae building up on the grout on my waterline tiles, so decided to go ahead and SLAM the pool. I'm using the TF-100 kit.

Numbers before the SLAM:
FC 6
CC 0
CYA 60
TA 70
CH 325
PH 7.5

I added 1 qt of MA to lower my pH, set my target FC to 24 and added the expected amount of 10% chlorine around 7:30pm. Brushed the pool and tested at 9pm last night after sundown, FC was 26, CC <0.5. Retested again at 5am this morning before sunup, FC was 24.5, CC <0.5. As I was at work all day and not able to monitor/add chlorine, I added 1/2 gal 10% at 5am aiming to bring the FC up to 28 to allow for FC loss during the day and still be above the 24 level. Just retested at 4:30pm and my FC is down to 17.5! So I've lost ~11.5 ppm in <12 hrs.

Trying to reconcile the results from my OCLT which indicated only 1.5 ppm lost over about 8 hrs. If I scale up the OCLT test it means the chlorine load from breaking down algae/whatever is about 2.5ppm in 12 hours, and puts my sun loading at 9ppm in the same period....it's been hot and sunny all day and the pool gets full sun from about 10am, but that seems like a really high FC loss. Is it to be expected that you simply get more FC loss when the levels are high during a SLAM? I'm reasonably confident in my CYA numbers, they have typically been in the 50-60 range. I'm reasonably confident in my CYA testing, and the pool store gets similar numbers. Certainly my CYA is no lower than 50.

I didn't have a lot of visible algae buildup just small amounts on the grout in some places, I'm not dealing with a swamp. Water is clear.

Any suggestions / advice? If I don't pass the OCLT tonight, I'll have to add much more 10% tomorrow before I leave for work, to make sure I stay above shock level all day. Based on today I'll need to aim for a FC level of 36....!
 
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A few things going on, but the main ones are that the algae is more able to grow when it's getting sunshine, and secondly the "buffering" capacity (hence the sunshine protection) of the CYA is overwhelmed when you're above it's capacity to protect the chlorine. Hence why MMMaintain speeds up the SLAM so much.

- - - Updated - - -

At a higher level, loss will be even faster. If you can guess and pre-measure some for someone else to put in around lunchtime, that might be a helpful option.
 
Thanks for the response. I read a bit more and it does seem like the high FC loss is not unusual when FC is at SLAM level for the CYA buffering capacity you mentioned.

So at 5pm I added another 1.5 gal of 10% which should have raised my FC to about 27. Tested again at 9pm after brushing and got 27 & 28 with a couple of different tests. Will do the OCLT tonight again and see how it goes. Pre-measuring is a good idea, at the rate I lost FC today it'd be a gallon that would need to go in at lunch so that makes it nice and easy for my wife.
 
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Thanks - first time pool owner. Would be clueless without this forum....

Been hitting it hard since yesterday. I really want to get on top of this, did a SLAM earlier this year but the algae returned so want to make sure I get it right this time.

OCLT showed about a 1ppm loss (28 down to 27). I think I might be there - I'm not sure my FC testing is consistent enough to be confident after one night so kept it going. Added 1 gal at 5am and my wife added another 0.5 gal at 3pm. Between the two additions that should have been enough to raise FC to about 36. At 5pm, my FC was 25. By 9pm today FC was down to 23.5 so still burning about 12 ppm in a 24 hr period. We've had a couple of typical Texas summer days here, hugely hot and sunny. Maybe I need to wait for cloudy days to SLAM....

One more question. I pulled my lights out yesterday evening and brushed the niches - no visible algae but wanted to be absolutely sure. Anything else I should be looking at pulling and brushing behind? Thinking of the main drain covers in particular. I have no ladders/slides or other places for algae to hide.

Finally, we lived in Australia (Perth) for 4 years before coming back to Texas 18 months ago. Got to visit Sydney a couple of times....amazing city.
 
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Well, I've heard it said that drains are unlikely because of the good flow, but if it's easy, why not.

Here's a few to think about... behind skimmer weir, inside skimmer weir, water features and their plumbing if seldom used, stone waterfalls, under coping overhang, dead pipe circuits, inside returns, pool toys, pool cleaner, solar heating seldom used (unpainted pipes), mesh bag on pool cleaner, the list goes on. Hate algae.

Use the OCLT, carefully done. You might be there. How are CCs? Is the pool clear?

OCLT tips... add last chlorine after sun is off pool, pump is running 24/7, right?, test FC an hour later for start of OCLT. Test in the morning before sunrise, and at least before any sun could have hit the pool.

My son was in Perth for around that long too. Writing on the wall due to iron ore prices, so he bailed back to Canada and a good job there. Yeh, Sydney is a great place, especially after you get wherever you're going... traffic is interesting to say the least, haha :) Can't wait for summer!
 
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OCLT showed FC at 22, so a loss of 1.5. CC's were a little higher at 1.0, previously <0.5. Pool is/has been clear the whole time, at least what I consider clear. Maybe last night's brushing found one more algae source, anyway seems like I'm not quite done yet. It's been a long wait for what I previously thought was just a small amount of algae. Day off today so at least I'll be able to monitor and add chlorine more frequently.

I've been following those tips - pump running 24/7 throughout this time, and it's been several hours after last chlorine addition when I test FC. Testing at 9pm and 5am both in darkness.

Got into the skimmer so reasonably happy with that - looked like mildew rather than algae, same for the cover on the overflow drain, and should probably do the overhangs as well. Main drain is not easy, in 6-ft water with screwed on covers, so requires multiple dives to get down in there. Hence why I was asking if it was a likely source.
 
Was looking at maps for your location. Must be toasty and humid there. Good day to be in the pool!

That OCLT sounds not too bad, so hopefully you're close. I kinda wish someone would come along and say no way there's algae in the drain, but I just don't know. I put rocks in the pockets of cargo shorts to hold me down, but have a buddy watch over you. Shut off suction from the drains. Probably no big deal, but be safe.

Good work on seeking out the slime. Yep, top up the FC every couple of hours and you'll kill it as fast as it lets you.
 
Very hot and humid here. Pool is a very nice thing to have....except this time of year the water tends to get very warm in the afternoon, up to 34C, so the water isn't as cooling and refreshing as it you'd want it to be. Need to look into the DIY pool misters I've seen on here.

So today I added 1 gal at 5:30am, 0.25 gal at 1pm and another 0.5 gal at 5pm. Net gain of 2ppm over the day, indicating I burned up 9ppm through the day compared to 11 or 12 the past couple of days. Hopefully a sign that whatever was in the pool has stopped eating chlorine and the end is in sight.

Didn't end up doing the drain, will trust that it's ok. Did notice that my spa light had a green buildup, really stubborn and scrubbing wouldn't cut it. Had to use Barkeeper's Friend to get it off. Must admit my spa doesn't get as much cleaning attention as the pool. Wondering if that was the source - need to brush and vacuum in there more often.
 
Just when I thought I might be getting close....OCLT showed a loss of 2ppm, most I've lost in a night so far. Added 1/2 gal at 6am (+3 ppm), by 12:30 it was at 23.5 (+1.5ppm) so added another 1/2 gal.

Frustrating because I have perfectly clear water and CC<0.5. Just can't pass the darn OCLT! Hard to explain to wife/kiddos why they can't really swim today.
 

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It is safe to swim when:
You can see the bottom everywhere (for swimmer safety)
FC is between bare minimum and shock level, for a known CYA level per this chart: Pool School - Chlorine / CYA Chart
pH is between 7.2 and 7.8

(we like to keep our pools available for swimming)

You probably have this in hand, but just in case, here's a few OCLT methods tips:
- Evening, near sunset, add last chlorine for the day
- Wait 30 minutes to and hour for pump to mix the water
- Test FC after sunset: this is the start point for the OCLT
- Do not add more chlorine; turn off SWC or other chlorine dosing
- In morning (pump has been running all night, so water is mixed)
- Test FC before sunrise; this is the endpoint for the OCLT
 
Thanks, guys. Much appreciate all the help so far. Yeah the kids got in yesterday. FC was slightly above target level but they didn't swim for long and I hosed them off immediately afterwards. Water is crystal clear and my pH was fine before I started the SLAM.

Need some help understanding my most recent results. My OCLTs the last two nights have been getting worse. First three nights lost 1.5, 1, 1.5. Fourth night, lost 2. Last night, lost 4. Intuitively I'd have expected the losses to be highest in the first few days of the SLAM and get progressively less, but mine are the reverse. Thoughts?
 
Seeing the OCLT go backwards would make me think you still have some active algae hiding in there somewhere. Your pool is relatively new, but once algae starts, it will hide anywhere. Did you look behind your light if you have one (in the niche)? Do you have any steps or ladders that are in the water that could hold algae? There was lots of swimming yesterday, so what about the pool toys and clothing? Those items will transfer algae back to the water. :hammer:
 
Thanks for the replies. My pool is fairly simple, no ladders, the only 'hiding places' I can think of are the light niches, skimmers, returns, bubblers and drains. I pulled the lights out on Wednesday night, scrubbed and replaced on Friday morning. No sign of algae in the niches. There haven't been any toys in the pool since I started the SLAM and the kids only swam for about an hour on Friday night and Sat morning.

Spa/pool do share water and the same filter. There are a couple of returns in the spa that are on the pool circuit so the water has been circulating through there during the SLAM. I did run the spa circuit for a few hours yesterday to get some circulation through the main spa returns and drains. We don't use the spa that often so I'm wondering if doing that and giving it a good scrubbing last night managed to find a source. If so, I have a hard time thinking that a 750 gallon spa can contain enough algae to cause such a large FC loss, especially since there are no visible signs of algae?

At this point probably the only thing left for me to clean is the cartridge filter. Pressure has been showing as normal but maybe a preventative cleaning would help?
 
If the spa is sharing the same water it need to be a part of the SLAM. Flow water through it 24/7 during the SLAM.

You are chasing that last obstacle, the OCLT. You would be amazed at what I have seen found on this forum as the cause of failing the OCLT.

Do you want to chance it??

Dom
 
Thanks Dom - yes my commitment has wavered a bit at times, but so far staying true to completing the SLAM! To answer your question, there is water flow through the spa when the pool circuit is running, I have a couple of returns in there that flow when the pool returns are on. I don't think it provides great circulation though so I've started a couple of hours a day to run the dedicated spa circuit (more returns, and activates the spa drain) so water 'really' flows through it.

I guess I was looking for some confirmation that it's not unusual for overnight FC loss to get worse during a SLAM?

Cleaned my filter and pump baskets today - didn't see anything particularly nasty in there.
 
Good to hear that the water is always flowing through the spa.

Be sure to brush in there as well.

Stick to it, you'll beat this, and as long as you maintain your FC target, should never have to do this again.

Dom
 
It is not unusual for CC to vary and OCLT to vary. Brushing and stirring up stuff and oxidizing it will vary over the course of the SLAM. Running the spa circuit is probably the source. Or it could be the kids kicking up the algae. Algae will form a biofilm to protect it from chlorine and brushing disrupts it and exposes the algae to the chlorine to kill it.
 
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