Chlorine demand isn’t a mystery \u2014 it’s chemistry. Your pool water contains stuff that reacts with chlorine. Until all that stuff is oxidized, you won’t hold a chlorine residual.<\/p>\n\n Pool Chlorine Demand Test: How to Find Out Why Your Chlorine Keeps Disappearing: What Causes Chlorine Demand in the First Place? Chlorine demand isn't a mystery \u2014 it's chemistry. Your pool water contains stuff that reacts with chlorine. Until all that stuff is oxidized, you won't hold a chlorine.<\/p>\n<\/section>\n\n This is the big one. Organic matter includes:<\/p>\n\n A single heavy pool party can introduce enough organic load to create demand for days.<\/p>\n\n These are sneakier because they’re often invisible:<\/p>\n\n Your pool’s surroundings matter:<\/p>\n\n Here’s the step-by-step process. Plan to start this in the late afternoon or evening so UV loss doesn’t skew your results.<\/p>\n\n Before adding anything, test and record. If you are not sure about your pool size, calculate it first with the pool volume calculator<\/a>, because every chlorine dose depends on gallons.<\/p>\n\n A reliable test kit like the Taylor K-2006 Complete Test Kit<\/a> gives you accurate readings that strips can’t match at high chlorine levels.<\/p>\n\n Use the dosing chart above, run the exact dose in the Pool Chemical Calculator app<\/a>, or calculate manually:<\/p>\n\n For liquid chlorine (10% sodium hypochlorite):<\/strong><\/p>\n For cal-hypo (68%):<\/strong><\/p>\n I recommend liquid chlorine like Champion Liquid Chlorinating Shock<\/a> for demand treatment because it doesn’t add CYA or calcium to your water.<\/p>\n\n Add your calculated dose with the pump running. Distribute it around the pool’s perimeter, not all in one spot. Keep the pump running continuously throughout this entire process \u2014 circulation is critical.<\/p>\n\n Set a timer. After 3-4 hours, test free chlorine again. Record the result.<\/p>\n\n This is your chlorine demand measurement. If you added 15 ppm worth of chlorine and only 5 ppm remains, your pool had a chlorine demand of 10 ppm.<\/p>\n\n The demand test isn’t complete until your pool holds chlorine overnight. Keep adding shock based on your retest results (use the matrix above), retesting every 4-8 hours.<\/p>\n\n You’ve “broken” the demand when:<\/p>\n This process can take anywhere from 12 hours to several days depending on contamination severity.<\/p>\n\n This is where a lot of pool owners get confused. Cyanuric acid (stabilizer) protects chlorine from UV breakdown \u2014 but it also reduces chlorine’s killing power.<\/p>\n\n During a chlorine demand situation, you need to think about CYA differently:<\/p>\n\n Your chlorine is getting destroyed by sunlight AND by contaminants. This makes demand situations worse. Consider using dichlor for part of your shock treatment to add some stabilizer while you shock.<\/p>\n\n But watch the math: every 10 ppm of chlorine from dichlor adds about 9 ppm of CYA.<\/p>\n\n High CYA means your chlorine is “bound up” and works more slowly. You’ll need higher absolute FC levels to get the same sanitizing effect.<\/p>\n\n Many pool pros recommend maintaining FC at roughly 7.5% of your CYA level. So if your CYA is 80 ppm, you’d want FC around 6 ppm for normal maintenance \u2014 and much higher during shock treatment.<\/p>\n\n For demand treatment with high CYA, aim for FC levels of 30-40% of your CYA reading. At 60 ppm CYA, that means shocking to 18-24 ppm FC. If you want the math spelled out, use the breakpoint chlorination calculator guide<\/a> alongside the app.<\/p>\n\n You’ll see these terms used interchangeably online. They’re different problems.<\/p>\n\n Chlorine demand<\/strong> means your water is consuming chlorine faster than you can add it because of contamination. The solution is adding more chlorine until you’ve oxidized all the contaminants.<\/p>\n\n Chlorine lock<\/strong> is a controversial term that some professionals say doesn’t exist. The theory is that extremely high CYA levels (150+ ppm) render chlorine ineffective no matter how much you add. Whether or not “lock” is real, very high CYA does reduce chlorine effectiveness significantly.<\/p>\n\n If you’ve been shocking aggressively for days and still can’t hold chlorine, test your CYA. If it’s over 100 ppm, you may need to drain and refill 30-50% of your water to dilute it before continuing treatment.<\/p>\n\n Once you’ve fought through a demand situation, you’ll want to avoid repeating it.<\/p>\n\n Post-party shock is mandatory. For every 10 swimmers over 2 hours, add 1 lb of shock per 10,000 gallons that evening.<\/p>\n\n Rainwater dilutes your chlorine and introduces contaminants. Test and adjust within 24 hours of any significant rainfall.<\/p>\n\n A clean filter is essential. Backwash sand filters when pressure rises 8-10 psi above clean starting pressure. Clean cartridge filters monthly during swim season. Consider adding a secondary sanitizer system<\/a> like a mineral purifier to reduce chlorine dependency.<\/p>\n\n Sometimes you do everything right and still can’t break through. Here’s what to check:<\/p>\n\n At high chlorine levels (above 10 ppm), test strips become unreliable. OTO (yellow) reagents max out around 5 ppm. You need a DPD drop-based kit for accurate high-range readings.<\/p>\n\n If your FC reads “off the scale,” dilute your water sample 50\/50 with distilled water and multiply the result by 2.<\/p>\n\n Chlorine’s effectiveness drops dramatically above pH 7.8. At pH 8.0, only about 22% of your chlorine is in the active (hypochlorous acid) form. Lower pH to 7.2-7.4 before shocking.<\/p>\n\n Check for:<\/p>\n If you’ve added 60+ ppm worth of chlorine over several days and still can’t hold a residual, it’s sometimes cheaper to drain and refill. Calculate the cost of chemicals versus water \u2014 at a certain point, fresh water wins.<\/p>\n\nQuick answer<\/h2>\n
Organic Contaminants<\/h3>\n\n
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Inorganic Contaminants<\/h3>\n\n
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Environmental Factors<\/h3>\n\n
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\n\nHow to Perform a DIY Chlorine Demand Test<\/h2>\n\n
Step 1: Test Your Current Chemistry<\/h3>\n\n
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Step 2: Calculate Your Initial Shock Dose<\/h3>\n\n
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Step 3: Add Chlorine and Run the Pump<\/h3>\n\n
Step 4: Wait 3-4 Hours and Retest<\/h3>\n\n
Step 5: Repeat Until You Hold Chlorine Overnight<\/h3>\n\n
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\n\nWhat About CYA Levels During Chlorine Demand Treatment?<\/h2>\n\n
If Your CYA Is Low (Under 30 ppm)<\/h3>\n\n
If Your CYA Is High (Over 70 ppm)<\/h3>\n\n
The CYA-Adjusted Shock Target<\/h3>\n\n
\n\nChlorine Demand vs. Chlorine Lock: They’re Not the Same<\/h2>\n\n
\n\nPreventing Future Chlorine Demand Problems<\/h2>\n\n
Weekly Prevention Tasks<\/h3>\n\n
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After Heavy Use<\/h3>\n\n
After Rain<\/h3>\n\n
Maintain Proper Equipment<\/h3>\n\n
\n\nTroubleshooting: When the Test Doesn’t Work<\/h2>\n\n
Your Test Kit May Be Wrong<\/h3>\n\n
Your pH May Be Too High<\/h3>\n\n
You May Have a Hidden Contamination Source<\/h3>\n\n
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The Water May Be Too Far Gone<\/h3>\n\n
\n\nSources<\/h2>\n\n
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