← Back to Pool Toolkit

Drain / Refill to Lower CYA

Calculate how much water to replace to reach your target stabilizer (CYA).

Google AdSense Ad #1 Goes Here
Most fill water is 0 ppm CYA (unless you’re topping off with pool water).
Recommended water replacement:
Practical tips:
  • If replacing more than ~50%, consider doing it in multiple partial drains to protect liners/plaster and avoid floating issues.
  • Retest CYA after refilling and circulating for a few hours.
  • If you’re on well water or have metals, consider testing iron/copper before large refills.
Google AdSense Ad #2 Goes Here
Safety note: High-drain situations can risk liner shifting/floating, hydrostatic pressure issues, and structural damage. If you’re unsure, consult a pool professional.

How the Drain and Refill Calculator Works

This calculator estimates how much pool water must be replaced to lower Cyanuric Acid from the current reading to a selected target. It uses the pool volume, current CYA, target CYA, and the CYA level of the replacement water.

Most fresh fill water contains no CYA, so the fill-water value is usually zero. The calculator then estimates the percentage and amount of water that must be replaced.

Why CYA Gets Too High

CYA protects chlorine from sunlight, but it can build up when trichlor tablets, dichlor shock, or stabilizer are used repeatedly. Chlorine gets consumed, while the CYA remains in the water.

As CYA rises, the pool needs more Free Chlorine to stay properly sanitized. If CYA becomes too high, normal chlorine levels may no longer be enough.

When Water Replacement Is Needed

  • CYA is above the practical range for the pool’s sanitation method.
  • The required Free Chlorine level has become difficult to maintain.
  • A SLAM would require an impractically high chlorine level.
  • Tablet use has caused steady stabilizer buildup.
  • The pool also needs to lower salt, calcium, or other dissolved material.

Common Drain and Refill Mistakes

  • Draining too much from a vinyl-liner pool.
  • Ignoring high groundwater or hydrostatic pressure.
  • Testing CYA before the refill water is fully mixed.
  • Continuing heavy tablet use immediately after lowering CYA.
  • Assuming evaporation lowers CYA.

Frequently Asked Questions

Can CYA be lowered without draining?

In most residential pools, partial water replacement is the dependable method. Specialty products may make claims, but results can vary.

Does evaporation lower CYA?

No. Water evaporates, but CYA stays behind. Topping off after evaporation does not lower the concentration.

Should a large replacement be split into smaller drains?

Often, yes. Multiple partial drains may reduce risk for vinyl liners, high groundwater, and structurally sensitive pools.

When should I retest CYA?

Circulate the refill water thoroughly before testing. Large partial drains may need several hours of mixing.

Pool Gal Pro Tip 💦

Lowering CYA only works if you stop rebuilding it. After the refill, track every tablet and every dose of stabilized chlorine.