A customer reports their chlorine was at 3 ppm on Saturday and zero by Monday. They blame the service tech. The tech blames the customer. Both are usually wrong. Pool chlorine that disappears between visits has three common causes and one rare one, and the diagnosis takes 5 minutes of testing. This is the operator playbook for chlorine loss complaints, the chemistry behind why it happens, and the fix for each.
TL;DR
- The 3 common causes: low cyanuric acid (UV burns off chlorine), high organic load (bather waste or rain runoff), or a malfunctioning chlorinator
- Test CYA first. Below 30 ppm, UV destroys 75 to 90% of chlorine in a sunny day
- Test combined chlorine (CC). Above 0.5 ppm means organic load is consuming chlorine
- Salt cells stop producing when salt level drops below 2,700 ppm or the cell needs cleaning
- Rare cause: copper or iron in the water catalyzes chlorine breakdown; check stains
- A pool losing more than 2 ppm of FC overnight in shade has a real problem, not bad luck
How chlorine is supposed to behave
A balanced pool with normal bather load and adequate cyanuric acid (CYA) loses 1 to 2 ppm of free chlorine per day in normal sun, slightly more in heavy heat or direct sunlight. The pool starts the day at, say, 3 ppm and drops to 1.5 ppm by sundown. That is the chlorinator (or a service visit) replenishing it the next day.
When a pool loses 3+ ppm of free chlorine overnight in shade or 4+ in sun, something is wrong. The diagnosis runs through three causes in sequence.
Cause 1: low cyanuric acid (the dominant cause)
Cyanuric acid bonds reversibly with free chlorine and protects it from UV degradation. Without CYA, sunlight destroys 75 to 90% of free chlorine in a single sunny afternoon. With CYA at 30 to 50 ppm (the chlorine-pool target), the loss drops to 30 to 40% per day. With CYA at 70 to 80 ppm (the saltwater target), the loss drops further.
A pool that "burns through chlorine" almost always has CYA below 20 ppm. The customer added chlorine but skipped stabilizer; or they did a partial drain and refill and never re-added stabilizer; or they have been on liquid chlorine for years and never added CYA.
Fix: test CYA. If below 30 ppm, add granular stabilizer to hit the target (use the dosage calculator; roughly 1 lb of CYA per 10,000 gallons raises CYA by 12 ppm). Re-evaluate chlorine loss after 7 days of stable CYA.
“Low CYA is the #1 cause of "my chlorine keeps disappearing." Most operators never test for it. Test it first.”
Cause 2: high organic load
Combined chlorine (CC, sometimes called chloramine) is the chlorine that has already reacted with organic contaminants in the water. A CC reading above 0.5 ppm means there are organics consuming chlorine continuously. Common sources: heavy bather use, recent rain (washes in lawn fertilizer, leaves, dust), dead leaves at the bottom, dog swimming.
CC also means the chlorine is less effective; the visible smell of "chlorine" most people complain about is actually CC, not free chlorine. The remedy is shock at SLAM levels to break the chloramine bond.
Fix: if CC is above 0.5 ppm, shock the pool to break the chloramines. Use 12.5% liquid chlorine at 4x normal dose. Filter 24 hours. Re-test the next day; CC should drop below 0.5 ppm and FC should hold longer between additions.
Cause 3: malfunctioning chlorinator
For pools with a salt chlorine generator (SCG), chlorine drop is sometimes the cell itself. Three common SCG failures:
- Salt level out of range (below 2,700 or above 4,000 ppm depending on cell manufacturer). Cell shuts down or runs inefficient
- Calcium buildup on the cell plates. Cell electrolysis efficiency drops; output halves
- Cell at end of life (3 to 7 years typical). Plates degraded, replacement needed
Fix: check the SCG control panel. Most panels show salt reading, cell amperage/voltage, and a fault code if something is wrong. If salt is in range and no fault is flagged, pull the cell and inspect for visible calcium buildup. Clean the cell with a 4:1 water/muriatic acid solution if buildup is heavy. Replace the cell if it is 5+ years old and chlorine output remains low after cleaning.
Rare cause: metals in the water
Iron and copper in the water catalyze chlorine breakdown. The visible signs are colored stains on plaster (red/orange for iron, green/blue for copper) and sometimes a faint discoloration of the water itself. Source can be old galvanized plumbing, well-water fill, or copper algaecide overuse.
Fix: add a metal sequestrant (the standard product is a phosphonate-based chelator like "Stain & Scale Preventer"). Re-test chlorine consumption after a week. If it improves, metals were the cause. Long-term fix is reducing the metal source (don't fill from well water, switch from copper algaecide).
What to test first when a customer complains
- CYA (the most likely answer, and the test most operators skip)
- Free chlorine and combined chlorine (CC > 0.5 = organic load problem)
- pH (above 7.8 = chlorine is half effective regardless of FC reading)
- For salt pools: salt level + cell inspection
The customer conversation
When a customer complains about chlorine loss, the worst response is "you must be adding too many people to the pool" without test data. The right response is "let me test CYA, CC, and pH; that tells us the cause in 5 minutes."
Most customers do not know CYA exists. Explain it once in plain language: "stabilizer protects the chlorine from sunlight. Without it, the chlorine you put in disappears within hours." Customers who get this explanation become 30% less likely to complain about chlorine again because they understand the mechanism.
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