If you've watched your Phoenix pool go green three times in one summer despite doing everything right—testing chlorine, shocking weekly, running the pump for hours, adding algaecide—there's a very good chance you're dealing with CYA lock-up. It's the most common hidden cause of recurring green pool problems in Phoenix, and most pool owners have never heard of it.
This guide explains exactly what CYA lock-up is, why it happens so often in Phoenix, how to diagnose it with a simple test, and what to do about it when your cyanuric acid has climbed too high.
Cyanuric acid (CYA), also called pool stabilizer, protects chlorine from UV breakdown. You need some in a Phoenix pool. Too much, and your chlorine stops working. The line between "right amount" and "too much" is narrow, and most Phoenix pool owners cross it without realizing.
When CYA gets too high, you can have "normal" chlorine readings on your test kit but still grow algae in 48 hours. Shock treatments do nothing. Weekly service companies struggle to explain why the pool keeps going green. The only fix is draining water. The only prevention is understanding the problem.
Chlorine in pool water exists in two forms:
Both are called "free chlorine" on your test kit.
Ultraviolet light from the sun destroys these molecules quickly. In direct Phoenix summer sun, unprotected free chlorine has a half-life of less than one hour. That means 5 ppm free chlorine at noon drops to 2.5 ppm by 1 PM, 1.25 ppm by 2 PM, and zero by late afternoon. See our UV chlorine breakdown guide for the full science.
Cyanuric acid binds reversibly to chlorine molecules, creating a UV-resistant complex. Think of CYA as sunscreen for your chlorine. With CYA, chlorine loss drops from 50-90% per day down to 10-20% per day.
The trap: CYA-bound chlorine is less active as a sanitizer than free HOCl. That's usually fine, because at normal CYA levels there's still enough free HOCl around to kill algae and bacteria. But when CYA climbs too high, too much chlorine gets bound and not enough stays free to actually sanitize. That's lock-up.
This is the single most important ratio in pool chemistry, and hardly anyone knows it.
Your free chlorine needs to be at least 7.5% of your CYA level to maintain sanitation.
Examples:
| CYA Level | Minimum Free Chlorine |
|---|---|
| 30 ppm | 2.3 ppm |
| 40 ppm | 3.0 ppm |
| 50 ppm | 3.8 ppm |
| 60 ppm | 4.5 ppm |
| 70 ppm | 5.3 ppm |
| 80 ppm | 6.0 ppm |
| 100 ppm | 7.5 ppm |
| 120 ppm | 9.0 ppm |
| 150 ppm | 11.3 ppm |
Most pool test kits are calibrated to show normal chlorine as 1 to 3 ppm. Most pool stores tell customers their pool is fine at 3 ppm. At CYA 80+ ppm, that reading is actively misleading. Your pool says "clean" on the test but is functionally un-sanitized.
For shock/SLAM treatments, the target is 40% of CYA:
| CYA Level | Shock Target |
|---|---|
| 30 ppm | 12 ppm |
| 50 ppm | 20 ppm |
| 70 ppm | 28 ppm |
| 100 ppm | 40 ppm (impractical) |
| 120 ppm | 48 ppm (not feasible) |
Notice how shock becomes impractical above 70 ppm CYA. That's why homeowners with high CYA "shock" at standard levels (10 to 15 ppm) and nothing happens. The shock dose is simply too low for the stabilizer level.
Three main sources of CYA accumulation, all made worse by Phoenix conditions:
Trichlor is chlorinated isocyanuric acid. Each tab contains both chlorine AND cyanuric acid. Every time a tab dissolves, you add both to the pool.
The math: trichlor is 90% chlorine and 54% CYA by weight. For every 1 ppm free chlorine added, you also add about 0.6 ppm CYA.
A typical 15,000-gallon Phoenix pool running on trichlor tabs as the primary sanitizer will use roughly 8 to 12 lbs of trichlor per year during active swim season. That adds 20 to 40 ppm of CYA per year. Within 3 to 5 years of trichlor-only chlorination, CYA is in lock-up territory.
Dichlor is chlorinated isocyanurate, similar to trichlor but faster-dissolving. Same CYA addition problem. Each shock dose adds CYA on top of chlorine.
Some homeowners add granular CYA at season start to build initial stabilization, then keep adding more through trichlor use. CYA does not evaporate and does not break down naturally. Once it's in the pool, it stays until you drain.
Why Phoenix hits CYA lock-up faster than pools in other regions:
Standard pool test strips don't measure CYA accurately at high levels. You need a reagent-based test:
The CYA test uses a turbidity method: you add reagent to pool water in a small tube, then pour into a graduated vial with a black dot at the bottom. CYA causes the water to turn cloudy. You stop pouring when you can no longer see the dot. The fill level indicates your CYA.
Key notes:
Most pool stores will test CYA for free or cheap. If you're using store testing, take the water sample first thing in the morning in a clean container, and get the reading within an hour.
You probably have CYA lock-up if:
If 3+ of these match, lock-up is almost certainly the cause. Test CYA to confirm.
CYA does not break down. No chemical additive reliably reduces CYA. A few products claim to, but real-world results are unreliable.
The only dependable fix is replacing water.
The formula: The percentage of CYA reduction equals the percentage of water replaced (approximately).
Examples:
For a 15,000-gallon Phoenix pool going from CYA 120 to 40 ppm, you'd drain and refill approximately 10,000 gallons. Cost: $30 to $60 in water (depending on Phoenix provider), plus the time to drain and refill.
Before you drain:
Don't drain in peak summer heat. Plaster pool shells can crack from thermal stress when empty in 110°F+ weather. Drain in October-April, not May-September.
Check your water table. If you're in an area with high groundwater (some Phoenix neighborhoods do), a fully drained pool can pop out of the ground from hydrostatic pressure. Consult a pro before full drain.
Partial drains are often sufficient. You rarely need to drain 100%. For most CYA issues, 50 to 70% partial drain brings CYA back to range.
| Service | Typical Cost |
|---|---|
| Partial drain (30 to 50%) + rebalance | $200 to $400 plus water |
| Full drain and refill + restart | $300 to $500 plus water |
| Drain + acid wash + refill | $400 to $700 plus water |
| Drain + chlorine wash + refill | $300 to $500 plus water |
| Water cost (Phoenix municipal) | $30 to $60 per 10,000 gallons |
Most Phoenix pool owners budget $300 to $600 total for a partial drain service that fixes CYA lock-up.
Once you've fixed a high-CYA pool, don't let it happen again:
Save trichlor tabs for specific situations: vacation, short-term supplementation, or when you know CYA is on the low end and you want a small increase.
In spring, set CYA to:
Monitor monthly. CYA shouldn't increase if you're using unstabilized chlorine.
Even with perfect chlorine practices, Phoenix water concentrates due to evaporation. A partial drain every 18 to 24 months is the right cadence for most Phoenix pools, regardless of CYA.
Ask any pool service company how they handle chlorination. If the answer is "trichlor tabs in a floater" or "inline chlorinator," and they're also offering year-round weekly service, push back. That combination leads to CYA lock-up within 2 to 3 years.
Most pool owners test chlorine and pH weekly. CYA quarterly is enough to catch drift before it becomes a problem.
Some pool professionals dismiss CYA lock-up as an overblown concern. The counter-argument goes: "As long as you raise chlorine proportionally with CYA, the pool stays sanitized."
Technically true. In practice:
For a homeowner, the distinction is moot. High CYA causes problems in real-world pool management even if it's technically manageable in a perfect world.
After fixing CYA lock-up:
Most Phoenix pool owners notice the difference within 2 to 3 weeks.
We handle CYA diagnosis, partial drains, and full water replacement across the Phoenix Valley. Weekly service customers get CYA monitoring built into our regular chemistry testing. For one-time drain-and-fix service, most appointments are available within 5 to 10 days.
We don't use trichlor tabs as primary chlorination for our customers. We use liquid chlorine and/or salt systems. It costs us slightly more in materials but prevents the lock-up problem entirely.
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