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Draining part of your pool

Calcium hardness and stabilizer have no chemical fix. Nothing you can pour in brings either one down, and the same goes for salt above what your cell wants. The only way to lower them is to replace some of the water with fresh water, a share at a time. Your plan says when that is the job and how many rounds to expect. This page is how to do one round safely, and when to stop and call someone.

A few hours a round, plus the refill and a day of circulation

When this applies

Your plan has a drain-and-refill step for calcium hardness, stabilizer or salt, or a step that says chlorine cannot do its job until the stabilizer comes down.

It is not the fix for cloudy or green water. Draining does not clear algae, and a pool taken too low can be damaged. That is a filter and chlorine job: see Cloudy, green and what to do about it.

  1. 1Test first, then read the planLog a full test, including calcium hardness and stabilizer, before you drain anything. The plan works out the rounds from those readings and from your fill water, and it may tell you that today is not the day: when the rest of your water is balanced, or when your tap water is as hard as the pool.Draining is the one correction you cannot take back. If the fill water is as hard as the pool, draining cannot win, and the plan says so instead of sending you to do it.
  2. 2Take out about a quarter, then stopDrain about a quarter of the water per round. The plan gives you a landmark: a number of inches down from the waterline, worked out from your pool’s depth, or about the middle of the skimmer when it does not know the depth. Put a strip of tape on the tile or wall at that mark before you start, so you can see when to stop.The water is part of what holds a pool in place. A vinyl liner drawn down too far can shift and may not seat back the same way, a fiberglass shell can lift with groundwater under it, and a concrete shell is partly held down by the water in it. A quarter at a time keeps every type well clear of that.
  3. 3Use a submersible pump, or the filter’s waste settingA submersible utility pump with a discharge hose is the simplest tool, and the only one for an above-ground pool, which has no main drain. On an in-ground pool with a multiport valve, the Waste setting sends water out through the backwash line without passing it through the filter. If you drain through the pool’s own pump, close the skimmer line and draw from the main drain once the level nears the skimmer, so the pump never sucks air. Switch off a heater and a salt cell before the level drops.A pool pump that runs dry loses its prime and can overheat quickly, and a heater or salt cell with no water moving through it can be damaged.
  4. 4Send the water where it is allowed to goCheck your city or county rules before you start. Many places want pool water sent to the sewer clean-out or spread on your own lawn, not into the street or a storm drain, and some ask you to let the chlorine drop first. Keep it off a neighbour’s property and away from a septic field. Water from a salt pool is hard on lawns and garden plants.Storm drains usually run straight to a creek or lake, and chlorinated or salty water harms what lives there. Many areas fine for it.
  5. 5Refill, circulate, retestRefill to the middle of the skimmer, run the pump until the new water is mixed through, which takes most of a day, then log a test.Fresh water lowers more than the reading you were after. It also dilutes chlorine, stabilizer, alkalinity and, on a salt pool, salt, so the next plan may ask for chlorine or salt. That is expected, and the engine works it out from the new test.
  6. 6Start another round only when the test says soIf the plan names more rounds, repeat from the top: test, drain about a quarter, refill, retest. Do not start the next round until the retest asks for it.Each round changes what the next one needs. On a stabilizer drain your chlorine target also falls as the stabilizer does, so the number you are aiming for moves between rounds.

When to stop and call a professional

  • Any drain beyond the rounds the plan names, or emptying the pool for any reason. A full drain is a professional job: a crew checks the groundwater first and knows when not to do it at all.
  • The ground around the pool is soggy, it has rained hard, or you know the water table is high. That is when shells lift and liners float.
  • A vinyl liner pulls away from the wall, floats or wrinkles as the level drops. Stop draining and call a liner professional before you refill.
  • You see a crack, a bulge, tiles coming loose or the floor moving.

Weather changes the job. If your water is cold and a freeze is coming, wait for a milder stretch: a drawdown exposes surface and plumbing that a full pool keeps covered. In real heat, start early or late in the day and refill as soon as the water is out. The plan adds both of those notes itself when your water temperature calls for them.

Where this comes from

  • packages/chemistry-engine/src/recommendations/drainPlan.ts: a quarter per round, retest between rounds, the fill-water and balanced-water cases, the surface cautions, and the cold and heat notes.
  • packages/chemistry-engine/src/content/chemicalGuidance.ts: the drain-and-refill and dilution-first steps, including the moving chlorine target.
  • packages/chemistry-engine/src/recommendations/generateRecommendation.ts: a drain removes salt, so the next plan may ask for some.

Keep learning

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