| Parameter | Drain Trigger | Partial or Full Drain? |
|---|---|---|
| TDS (total dissolved solids) | Above 2,500 ppm (or 1,500+ above fill water) | Partial 50% or full |
| CYA (cyanuric acid) | Above 100 ppm | Partial 50% usually sufficient |
| Calcium hardness | Above 600 ppm with scaling | Partial 50% or full |
| Chronic imbalance | pH/alk won't hold despite correct additions | Full drain recommended |
| Algae recurrence | Algae returns within days of treatment | Full drain + acid wash |
| General schedule | 5–7 years without a drain | Full drain |
Cyanuric acid (pool stabilizer) is the most common reason pools need draining. CYA cannot be removed chemically — the only way to lower it is to replace water. At above 80 ppm, CYA begins to suppress chlorine effectiveness. Above 100 ppm, the pool can become effectively unsanitizable at normal chlorine doses.
A 50% partial drain typically cuts CYA in half. If CYA was 120 ppm, a 50% drain brings it to roughly 60 ppm — a manageable level. This is why a full drain is rarely necessary for CYA alone.
Total dissolved solids measure everything dissolved in the water — calcium, salt, chemical byproducts, mineral residue. TDS accumulates continuously and never goes down on its own. High TDS causes:
TDS above 2,500 ppm (or more than 1,500 ppm above your source water baseline) is the general threshold for draining consideration. Saltwater pools run higher TDS naturally and use a different threshold.
Calcium hardness above 600 ppm causes chronic scaling on pool surfaces, plumbing, and equipment. Unlike some parameters, there is no chemical that removes calcium from pool water — only dilution works. Partial or full draining with softer fill water is the solution.
Before draining for high calcium, test your fill water's hardness. If your tap water is already 400 ppm, draining and refilling won't help much — you may need to blend with softened water or use a reverse osmosis mobile service instead of a full drain.
A partial drain (25–50% of pool volume) is the right choice when:
A full drain is appropriate when:
Fiberglass pools can "float" out of the ground if the water table is high. The surrounding groundwater exerts upward pressure (hydrostatic pressure) that the pool water normally counteracts. Always check the water table before draining a fiberglass pool — this is a job for a professional if you are unsure. Some fiberglass pools have a hydrostatic relief valve at the bottom drain.
Never fully drain a vinyl liner pool. The liner shrinks and stiffens when dry and may crack, wrinkle, or detach from the track. Partial drains of 25–30% are generally safe. For any repair that requires full draining, consult a liner specialist.
Plaster pools are the safest to drain fully. However, drain quickly if outdoor temperatures are above 85°F — exposed plaster can crack from heat and direct sun. Do not leave a plaster pool empty for more than 2–3 weeks in summer without covering it.
A standard garden hose flows at 8–12 gallons per minute:
| Pool Size | Gallons | Refill Time (1 hose) |
|---|---|---|
| Small | 10,000 gal | 14–21 hours |
| Medium | 15,000 gal | 20–30 hours |
| Large | 25,000 gal | 35–52 hours |
Water delivery trucks (6,000–9,000 gallons per load) can significantly reduce refill time. After refilling, test and balance all parameters from scratch — fresh water has very different chemistry than the depleted pool water.
PoolLens logs all your water parameters over time so you can see when TDS or CYA is trending toward drain territory — before it becomes an emergency. Track every partial drain and refill event with a timestamped log entry.
Open PoolLens Free →Plaster pools: generally safe if drained quickly in moderate temperatures. Fiberglass pools: risk of floating out of the ground if water table is high — consult a professional. Vinyl liner pools: never fully drain — the liner shrinks and may crack. Always know your pool type before draining.
With a standard garden hose (10 gpm): a 15,000-gallon pool takes 20–25 hours; a 25,000-gallon pool takes 35–40 hours. Water delivery trucks speed this up significantly. Always rebalance all chemistry after refilling.
A 50% partial drain is enough when the issue is high CYA (cut it in half), moderately elevated TDS, or elevated calcium hardness. A full drain is needed when multiple parameters are severely out of range, algae keeps returning, or the pool needs replastering.
TDS, CYA, and calcium accumulate over years. Eventually, the water becomes chronically difficult to balance, chlorine loses effectiveness at normal doses, and scaling damages equipment and surfaces. Most pools become unmanageable without at least a partial drain every 3–5 years.
Get a full water panel test — including TDS and CYA. These are not on basic test strips. If TDS is above 2,500, CYA above 100, or calcium above 600, draining is likely needed. If all other parameters are the problem (pH, alkalinity, chlorine), draining is rarely the answer.