Most pool problems are solved with chemistry — not by draining. But some situations require replacing the water, either partially or fully. Understanding when a drain is genuinely necessary, how to execute it safely, and what to do after is essential knowledge for both pool service professionals and homeowners facing chemistry or structural issues.
| Problem | Partial Drain | Full Drain | Chemistry Fix Only |
|---|---|---|---|
| CYA 80–100 PPM | Yes (50% drain) | Not necessary | Possible (slow dilution) |
| CYA above 100 PPM | Yes (50–75% drain) | If 2+ partials failed | No — no chemical removes CYA |
| Calcium hardness 400–600 | Possibly (30% drain) | Not necessary | Yes (sequestering agents) |
| Calcium hardness above 800 | Yes (50% drain) | If partial insufficient | No |
| Persistent algae | Rarely needed | Severe cases only | Yes — almost always the first step |
| Resurfacing required | No | Yes | N/A |
| Structural repair | No | Yes | N/A |
The most common reason service professionals need to drain a pool is elevated CYA. Cyanuric acid accumulates over seasons of trichlor tablet use and can only be removed by dilution — there is no chemical treatment that reduces CYA. Once CYA exceeds 100 PPM, chlorine effectiveness is so significantly impaired that water replacement is the practical solution.
A partial drain removes and replaces 25–50% of the pool water. For a 20,000-gallon pool with CYA at 120 PPM, a 50% partial drain and refill brings CYA to approximately 60 PPM — within the acceptable range. Partial drains are:
The limitation of partial drains is that they preserve half of whatever problem exists in the old water. If the old water has 500 PPM calcium hardness, the post-partial fill will blend fresh water with 250 PPM hardness water — potentially landing at 300–350 PPM, which may still require treatment. For very high calcium or extreme TDS, a full drain provides a complete reset.
Never drain a fiberglass pool without consulting a pool contractor first. Fiberglass pools rely on water weight to counteract hydrostatic pressure from groundwater. If the water table is high in your area, an empty fiberglass pool can literally float out of the ground — a catastrophic and expensive failure. Consult with a professional before any full fiberglass drain.
Full drain risks by pool type:
After refilling, test the fresh water before adding any chemicals. Fresh tap water typically arrives with:
Balance alkalinity to 80–120 PPM first, then pH to 7.4–7.6, then calcium hardness to 200–400 PPM. Add CYA to 30–50 PPM for chlorine pools. Shock with liquid chlorine and run the filter for 24 hours before the first test confirmation.
Log the drain date, reason for draining, and all startup readings in PoolLens. A documented drain history tells you when the last full water replacement occurred, which helps predict when the next chemistry-driven drain might be needed — typically every 5–10 years for well-maintained pools.
Log drain dates, chemistry readings before and after, and all startup chemical additions in PoolLens. Free for pool service professionals — offline-first, no account required.
Open PoolLens Free →Drain a pool completely when CYA exceeds 100–150 PPM and partial drains have not brought it into range, when calcium hardness is above 800 PPM, when TDS is extremely high (5,000+ PPM), for major renovation or resurfacing, or for severe structural repair. Full drains are rare — partial drains resolve most chemistry issues more safely.
Full draining carries significant risks: fiberglass pools can float out of the ground if the water table is high, vinyl liners can shrink and crack, and gunite/plaster surfaces can check-crack if exposed to sun. Never fully drain a fiberglass pool without consulting a pool contractor about hydrostatic pressure in your area.
Use a submersible pump (also called a sump pump or trash pump) to pump water from the pool. Connect discharge hose to an approved drain location — most municipalities require discharge to a sanitary sewer, not storm drains. A 1.5-inch submersible pump drains a 20,000-gallon pool in 8–14 hours. Larger trash pumps (2–3 inch) drain faster.
Professional pool draining service costs $175–$350 for labor and equipment. Water refill cost depends on volume and local water rates: a 20,000-gallon pool typically costs $60–$150 in water utility costs. Add startup chemicals ($50–$150) for a total cost of $300–$650 for a full drain and refill.
A partial drain removes and replaces 25–50% of the pool water to dilute high CYA, calcium hardness, or TDS. It's safer than a full drain, preserves more of the existing water balance, and requires less chemical correction on refill. A partial drain is the appropriate first response to most chemistry issues requiring dilution.