Pool partial drain setup with submersible pump on a daylight deck

How to Partially Drain a Pool for Water Replacement

Partially draining a pool is necessary to lower high cyanuric acid, reduce total dissolved solids (TDS), or fix high calcium hardness. Done wrong, it lifts a vinyl liner, cracks plaster from hydrostatic pressure, or collapses the pool walls. Here’s how to drain partially without damaging anything.

When you need to partially drain

  • Cyanuric acid above 80 ppm (chlorine effectiveness suppressed)
  • TDS above 1,500 ppm (water chemistry becomes unstable)
  • Calcium hardness above 500 ppm (scaling on equipment)
  • Salt above 5,000 ppm (corrosion risk on coping)
  • Persistent algae despite proper chemistry (sometimes a partial drain resets the water)

How much to drain

The math: drain percentage = (current value - target value) / current value.

To cut CYA from 120 ppm to 60 ppm: drain 50%. To cut TDS from 2,000 ppm to 1,000 ppm: drain 50%. The same fresh-water dilution formula applies to any concentration-based parameter.

Never fully drain an in-ground pool without professional consultation. Hydrostatic groundwater pressure can push an empty pool up out of the ground, cracking plaster or floating the entire shell. Partial drains (no more than 50% in a single event) are safe; full drains are a project for a service professional.

Step-by-step partial drain

1Test the parameter you’re trying to reduce.

Establish current ppm so you can calculate the drain percentage accurately.

2Mark the target water level.

For a 50% drain in a typical 4.5-foot-deep pool, target is 27 inches of water remaining (half the depth). Mark this with painter’s tape on the pool wall before draining so you don’t over-drain.

3Use a submersible pump OR the multiport WASTE setting.
  • Submersible pump: drop a sump pump into the deep end with a discharge hose running to your sewer cleanout or storm-water-appropriate location. Best for large drains.
  • Multiport WASTE: if you have a sand or DE filter, set the multiport to Waste and run the pump. Water exits through the backwash line. Watch the water level; the skimmer will lose suction before you fully drain, which is your natural stopping point.
4Monitor and stop at the target level.

Check every 30 minutes. A typical 20,000 gallon residential pool drains 50% in 4–8 hours with a submersible pump.

5Refill from the garden hose.

Run a hose from the spigot into the pool. Time varies by hose flow rate — usually 8–24 hours to refill a 50% drain.

6Test and rebalance.

After 24 hours of mixing, test all six chemistry parameters and rebalance per our chemical levels guide. Fill water often has lower alkalinity and calcium, so a drain-refill typically requires re-dosing those.

Refill chemistry pack

Durachlor alkalinity increaser

Durachlor Alkalinity Increaser

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Durachlor chlorine stabilizer

Durachlor Chlorine Stabilizer

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Taylor K-2006 test kit

Taylor K-2006 Test Kit

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Need help calculating exactly how much to drain for your target chemistry? Send PST Pool Supplies your test results and pool gallons and we’ll work out the math.

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