How much chlorine required?

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B

brkaus

Active member
May 28, 2022
25
Austin, TX
I know there are broad ranges, so there are no exact answers, but how much chlorine (10.5% liquid bleach) should be required for a pool over 7 days?

Pool Stats -
- 125k gallons (give or take, it's a 6 lane 25 yard pool with a bump for stairs)
- Not very heavily used. Mostly empty, a few lap swimmers. Maybe a family or two on the weekend.
- CYA - 25
- PH 7.5
- Chlorine 8.0
- ORP 698
- TDS 5000 (or more, mostly from the high bleach use I believe, almost tastes salty)
- Water temp 85f
- outdoor temp 95-100 daytime.

ORP & PH targets are by automation.

Mostly direct sunlight. Pool has been looking clear all season.

This past week it used about 110 gallons of 10.5% bleach. Is this in the expected range or crazy?
 
110 gallons of 10% LC would raise your pool’s FC by 88ppm. Over a 7 day period that’s 12.5ppm per day. A clean and clear swimming pool should lose 2-5ppm per day with use. Your pool is using 2.5 to 6X that amount. Something is not right.

A CYA of 25ppm is very low for an outdoor pool so the sun is likely burning off a good portion of that chlorine.

Is this a commercial/public pool?

What goals are you trying to achieve?
 
Also, ORP is simply a proxy for FC and only loosely correlates. It can easily be off and if your chemical dosing system is using that then it is highly likely that bad ORP data is driving excessive chlorine use.

You need to accurately measure FC and CC to know if your water is balanced. You should be taking daily measurements of FC (sometimes hourly measurements are require by health codes for public pools) and using that info to fine tune your chemical dosing system. ORP alone can not be trusted.
 
Agree ORP is only a proxy. Currently free chlorine is 8.0 and CC is 1.2 (which seems problematic).

It is a community HOA pool.

My goal is to understand if the chlorine use is as expected. It’s managed by a pool company and I’m the resident liaison.

Pool company says it’s fine, but we just did a complete drain / refill this spring and people are asking me if it’s saltwater.
 
Adding 12.5ppm FC per day is a huge chlorine demand for a lightly used HOA pool even with low CYA. I’m not sure why your service company thinks this is normal (well, I do, but I’m trying to be less disparaging to the pool service workers as many are simply just uniformed and poorly trained …).

Not sure what your roll is a “resident liaison” or if that confers any authority or obligations on you, but the HOA Board should have its own means for verifying the status of the pool water chemistry. I get it that they naively believe that the pool tech is a professional and ought to know what’s best but, as good old President Ronnie Ray-gun used to say, “Trust … but verify 😉”.
 
JoyfulNoise said:
Adding 12.5ppm FC per day is a huge chlorine demand for a lightly used HOA pool even with low CYA. I’m not sure why your service company thinks this is normal (well, I do, but I’m trying to be less disparaging to the pool service workers as many are simply just uniformed and poorly trained …).

Not sure what your roll is a “resident liaison” or if that confers any authority or obligations on you, but the HOA Board should have its own means for verifying the status of the pool water chemistry. I get it that they naively believe that the pool tech is a professional and ought to know what’s best but, as good old President Ronnie Ray-gun used to say, “Trust … but verify 😉”.
Click to expand...
Yup. I’m currently on the HOA board (I just hate to admit it). I do have my trusty tf-100 from tftestkits and regularly double check the pool companies work. 100% agree on trust but verify. I’ve flagged the chlorine use to the pool company several times and they say the readings are fine/safe/up to code. Individually there are some smart and hard working folks at their company, but all the pieces don’t always fit.

It really leaves me with 1) do nothing, they say it’s fine 2) find a new company or 3) do additional steps on our own.

The pool is clear/looks good so I don’t have much evidence to say something is wrong. The leaves get scooped several times a week and the chem system keeps the basics in check.

Now we are up to 120 gallons 10.5% chlorine in 156 hours (filled Monday early, now Sunday PM).

Does it need shocked? If so, to what level? They don’t seem to ever shock the pool. But I have seen 10ppm occasionally from the tf-100 which is SLAM level for CYA of 25. (Also of note, CYA was below 5 until recently).

Sorry for the bit of a rant, just not sure if I should worry about this stuff or not.
 
Are you able to shut off the chlorine side of the chemical feeding system?

What you want to do is an overnight chlorine loss test (OCLT). If a pool is truly clean and free of any algae growth then once the sun goes down there should be no chlorine loss over night. TFP considers an OCLT of less than 1ppm FC to be ok. You have to carefully measure the FC at sundown (no sunlight on the water) and then again the next morning before the sun comes up.

See here -> Perform the Overnight FC Loss Test (OCLT)

Edit - Don’t worry about the part of the instructions that say to adjust FC levels first before doing the OCLT. Just measure the FC/CC when the sun goes down and make a note of whatever that is. Then measure again in the morning.
 
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Thank you.

It’s my lucky day, the controller tripped offline due to over feed error (happens frequently). Sun just went down.

FC - 9.2 (filled to 25ml, so 0.2 per drop. Should be pretty accurate)
CC - 0.2
PH - 7.45
Orp - 680

I’ll check again early in the morning.

But - with the pool lights and less sunlight I can see patches of light green on the walls. They don’t brush off. So I expect that’s the problem (with cya 25, cl 9.2 is that expected?)

Thanks again!
 

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brkaus said:
Yup. I’m currently on the HOA board (I just hate to admit it). I do have my trusty tf-100 from tftestkits and regularly double check the pool companies work. 100% agree on trust but verify. I’ve flagged the chlorine use to the pool company several times and they say the readings are fine/safe/up to code. Individually there are some smart and hard working folks at their company, but all the pieces don’t always fit.

It really leaves me with 1) do nothing, they say it’s fine 2) find a new company or 3) do additional steps on our own.

The pool is clear/looks good so I don’t have much evidence to say something is wrong. The leaves get scooped several times a week and the chem system keeps the basics in check.

Now we are up to 120 gallons 10.5% chlorine in 156 hours (filled Monday early, now Sunday PM).

Does it need shocked? If so, to what level? They don’t seem to ever shock the pool. But I have seen 10ppm occasionally from the tf-100 which is SLAM level for CYA of 25. (Also of note, CYA was below 5 until recently).

Sorry for the bit of a rant, just not sure if I should worry about this stuff or not.
Click to expand...
b,

I feel for you. Been on HOA's and President and various other roles. It's a terrible role but somebody has to do it and it's a truly thankless volunteer role. Easy to make enemies of friends for no good reason other than you're trying to give back a little and your critics aren't willing to do anything but criticize. The size of your pool would seem to need commercial level care with deliveries of higher strength liquid chlorine or a gas system or multiple swg's and a salt pool. Also, with HOA's there a liability issues and local politics that makes it hard to inject common sense solutions. If it were me, I'd try to rebid and see if you can short-list commercial companies that truly understand pool chemistry with a pre-qual step to short-list bidders that understand pool chemistry in addition to having the requisite licenses and insurance. A short questionnaire about how their sanitizing chemistry works and what they do to monitor/maintain the pool should reveal what they really know. This will not likely be the cheapest service but may well be the most cost effective in the long run.

I hope this is helpful.

Chris
 
setsailsoon said:
b,

I feel for you. Been on HOA's and President and various other roles. It's a terrible role but somebody has to do it and it's a truly thankless volunteer role. Easy to make enemies of friends for no good reason other than you're trying to give back a little and your critics aren't willing to do anything but criticize. The size of your pool would seem to need commercial level care with deliveries of higher strength liquid chlorine or a gas system or multiple swg's and a salt pool. Also, with HOA's there a liability issues and local politics that makes it hard to inject common sense solutions. If it were me, I'd try to rebid and see if you can short-list commercial companies that truly understand pool chemistry with a pre-qual step to short-list bidders that understand pool chemistry in addition to having the requisite licenses and insurance. A short questionnaire about how their sanitizing chemistry works and what they do to monitor/maintain the pool should reveal what they really know. This will not likely be the cheapest service but may well be the most cost effective in the long run.

I hope this is helpful.

Chris
Click to expand...
This is so true. From HOA comments to use of commercial systems on pools.
In very similar situation but a much smaller public pool - currently using "residential" processes for sanitation but working with other companies to put in a more commercial dosing system.
 
Chris - so very true. I ended up on the board because I was the only one that was willing to deal with the pool. We have had this company for many years. It was one of the few that had been around a long time and would work on commercial pools. We have 4" plumbing, 7.5hp 3 phase pump with power drive unit, giant sand filter (5' tall, 8-ish' long cylinder), chem feed system with 300 gal chlorine tank & sulfuric acid. The chem system is included along with the service (sub-leased from another company). None of our neighboring communities are excited with their pool companies either, but they have owned cal hypo based chem systems so they can change w/o major interruptions.

In general, things have been pretty good. It's just recently this chlorine usage problem has happened and I constantly see the chem system in over feed timeout. If I don't watch it and reset it things go south real fast. We don't pay for the chlorine directly and the pool looks good, so could just close my eyes and click "esc" on the controller every few days.
 
Last edited:
brkaus said:
Overslept a bit. A little light out but still before sunrise.

FC- 6.0
CC - 0.2
pH - 7.51
Orp - 682

Chlorine feed system was still in timeout mode, so no chlorine added overnight.
Click to expand...

A 3ppm loss overnight tells me you have algae in the pool. A pool doesn’t have to be a green swamp for algae to start because it’s mostly planktonic (free floating) in its early growth phase. It’s too small to see so the water looks clear even though the organisms are growing with a doubling rate of 4-6 hours depending on the particular species. Even with chlorine in place, you’re basically fighting a battle against biology and Mother Nature - once the colony number gets to a certain size, the chlorine kill rate falls below the reproduction rate and your pool will eventually develop green patches or turn hazy.

Either way, TFP would recommend a SLAM process to restore the water. That’s an extended shocking process that requires careful attention and lots of chlorine. See here -

www.troublefreepool.com

SLAM - Shock Level and Maintain

We’ve all had to deal with a green pool. Clear algae from your pool with a SLAM! Read here to use shock level and maintain to clear your pool.
www.troublefreepool.com www.troublefreepool.com

However, with a service company in place, they are not going to agree to following TFP processes. They will have their own suggestion which will likely involve dropping the pH to 7.0 or less and then shocking the pool with a very high level of chlorine either using cal-hypo or just dosing a very large amount of liquid chlorine. They will then tell you to brush the pool and keep kit closed until levels come back down.
 
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By the way - I was HOA President for 2 years and I will NEVER do that again. The most thankless and tedious job I ever VOLUNTARILY agreed to do … I’d rather clean toilets at the county prison than work on an HOA board again.
 
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My sentence is up in October. My only fear is we get someone on that doesn't care about the pool at all and doesn’t do the verify part of “trust but verify” the pool company. It was Crud before I took over.

I agree with the algae speculation. But the Poolmath app is telling me the slam level is 10ppm and we have been at that for a long time. They only recently lowered the orp target (1 week back) to where the pool sites around 8ppm. So it seems more than the suggested slam level is required??

I did confirm the CYA level twice (the kit minimum is 30, it’s showing just slightly less) and at the local pool store.
 
Assume 30ppm CYA. Then the SLAM target is 12ppm but you could go up to 16-20ppm. A SLAM is only effective when you remember to do the “M” part — MAINTAIN! The more consistently you can maintain a shock level of AT LEAST 40% of CYA, the faster is goes. Also, a SLAM is not a one & done thing - you just keep up the SLAM until ALL THREE EXIT CRITERIA are met. The three criteria are - clear water, CC’s 0.5ppm or less, AND an OCLT < 1ppm. Your pool fails 2 of the three exit criteria. Therefore you must maintain the SLAM. Your ORP dosing system is probably allowing the FC to fluctuate enough that it gives the algae a break. You said yourself that the overdosing cutoff error happens a lot. If it does, then the FC is going up and down. A SLAM is only a SLAM when it is consistently performed. You also probably need to backwash that honking-huge sand filter as it can hold on to a lot of crud. That can also help to speed up a SLAM and get your CCs lower.
 
Thanks. Yup. I'm familiar with the value of achieving a level and holding it. I have a small pool at my house too. I have total control over that one, so it's much easier.

In this case, I need to convince the pool company to do something or I try and they get mad if they notice.
 
Well, you can let them know the results of your testing and explain to them that a 3ppm loss of chlorine overnight with no sunlight on the pool and higher than acceptable CCs indicates that there is a problem with the pool - either a biological problem like algae or a bather waste problem. You can then correlate that to the significant increase in liquid chlorine consumption which is costing your HOA a lot of additional money. You can then ask them to take corrective action by performing an extended shock of the pool until the CCs come down and a deep clean of the sand filter. They’ll probably say that they will charge extra for all that so be prepared to bring that to the board.

I had a similar experience when I was on my HOA board. The treasurer reported that our water bill had tripled and it was clear that there must be a leak somewhere. The landscaping company spent months BS’ing us about how it’s normal for water bills to increase, it’s hot outside, it’s just more demand, blah, blah, blah. Basically they were lazy and didn’t want to do anything more than blow the leaves and dust off the rocks and collect their check. Well, I was finally outside one early morning and, sure enough, there was a geyser of water flooding a portion of my wall that I couldn’t really see from my normal comings and goings. Turns out that a portion of the irrigation tubing actually ran along the common wall of my property and into my yard for a short stretch. That section developed a leak that would essentially dry up every day. It wasn’t until I was outside at that unusual early hour that I saw what was going on. All the landscapers had to do was turn on the system and let it run and they would have seen it too. But they were more interested in making excuses than servicing their customer. Suffice it to say that after they were told to fix the leak (as irrigation fixes were part of their service contract), they were fired the next month and we found a new landscaper that was actually much better.

Sometimes you just have to be demanding. If they want to walk away from a steady paying customer, that’s on them.
 
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