Partial Drain and Water-Replacement Calculations

Water replacement is sometimes the practical way to reduce a dissolved constituent that cannot be removed reliably by ordinary filtration or chemical adjustment. The calculation is simple only when the physical system and chemistry assumptions are simple.

The fraction the math says to replace is not permission to drain to that level. Pool or spa structure, groundwater, liners, finishes, covers, equipment, discharge, source water, weather, and supervision can impose a much smaller limit or prohibit field draining entirely.

Single-cycle formula card

For one ideal cycle in which a fraction of fully mixed pool water is removed and replaced with source water:

Fraction replaced = (initial concentration − target concentration) ÷ (initial concentration − fill concentration)

Using symbols:

f = (C₀ − Cₜ) ÷ (C₀ − Cᶠ)

Then:

Replacement gallons = f × mixed-system gallons

The predicted final concentration can be checked with:

Cₜ = C₀ × (1 − f) + Cᶠ × f

Where:

• C₀ = verified initial concentration;
• Cₜ = desired target concentration;
• Cᶠ = verified fill-water concentration;
• f = fraction of water replaced, from 0 to 1.

If the result is below 0 or above 1, the target cannot be produced by one ideal replacement with that fill water and starting condition.

When the zero-fill shortcut works

If the fill water contains effectively none of the conservative constituent:

f = 1 − target ÷ initial

Example: lower CYA from 100 ppm to 50 ppm with fill water verified at 0 ppm CYA:

f = 1 − 50 ÷ 100 = 0.50

The ideal mathematical result is 50% replacement.

Do not use the zero-fill shortcut for calcium, alkalinity, salt, metals, nitrates, or another constituent that is present in the source water.

Fill-water-adjusted example

Suppose:

• pool CH = 1,000 ppm;
• fill-water CH = 250 ppm;
• target CH = 600 ppm;
• approved mixed pool volume = 15,000 gallons.

Fraction:

f = (1,000 − 600) ÷ (1,000 − 250)

f = 400 ÷ 750 = 0.5333

Ideal replacement:

0.5333 × 15,000 gal = 8,000 gal

Check:

1,000 × 0.4667 + 250 × 0.5333 = 600 ppm

The calculation does not establish that an 8,000-gallon drain is structurally or environmentally permissible.

Predict every material parameter

Use the same one-cycle mass-balance equation for each reasonably conservative dissolved constituent:

predicted final = initial × remaining fraction + fill × replacement fraction

At minimum, consider:

• CYA;
• calcium hardness;
• salt and TDS context;
• total alkalinity, recognizing acid-base and carbon-dioxide effects may require more than simple mass balance;
• borate if used;
• confirmed conservative metals or other constituents where the method is appropriate;
• source-water pH and the expected post-mixing pH through a proper equilibrium or demand model, not simple averaging;
• saturation index recalculated from predicted pH, corrected alkalinity, CH, temperature, CYA, salt or TDS, and borate.

Multi-constraint example

If a 60% replacement is calculated to reach the calcium target, use that same 60% to predict CYA, salt, TA, borate, and other values. Do not separately assume a 40% replacement for CYA and call both targets solved.

Choose one feasible replacement plan, predict all results, then design the post-fill sanitizer and balance plan.

Constituents that do not follow simple dilution reliably

Simple proportional replacement is weak or wrong for:

• pH: logarithmic and buffered; do not average pH values arithmetically;
• free chlorine or bromine: decays, reacts, and may be present in fill water;
• combined chlorine and oxidation demand: reacts during the process;
• total alkalinity: mixing can be approximated for mass balance in some cases, but pH, carbon dioxide, acid addition, and source chemistry affect the final state;
• metals: can oxidize, precipitate, stain, remain sequestered, or be introduced by plumbing;
• phosphates and specialty products: source, testing, precipitation, and treatment interactions matter;
• contamination: dilution alone is not an approved decontamination response;
• cloudiness, algae, bacteria, biofilm, or debris: filtration, sanitation, surfaces, and contamination procedures control;
• temperature: heat exchange occurs during fill and circulation.

Use an applicable chemistry model, product procedure, or qualified plan rather than forcing every observation into the conservative formula.

Repeated drain-and-refill cycles

If one authorized cycle replaces fraction f, the water is completely mixed, and the same fill concentration is used, after n cycles:

Cₙ = Cᶠ + (C₀ − Cᶠ) × (1 − f)ⁿ

Example with fill concentration zero, 100 ppm initial CYA, and two 25% cycles:

C₂ = 0 + (100 − 0) × (1 − 0.25)²

C₂ = 100 × 0.75² = 56.25 ppm

Two 25% cycles do not equal one 50% replacement because the second cycle removes some of the new water added during the first cycle.

Three 25% cycles would predict:

100 × 0.75³ = 42.19 ppm

Repeated cycles usually use more total water than one equivalent replacement, but they may be considered only when the structural, discharge, refill, water-supply, chemistry, and operational plan supports them.

Solve for equal cycle fraction

For n equal fully mixed cycles:

f = 1 − [(target − fill) ÷ (initial − fill)]^(1 ÷ n)

Reject the equation if the ratio is not physically meaningful or the resulting cycle exceeds the approved safe water-level change.

Simultaneous drain and fill

When source water enters while mixed water leaves, some new water can leave before the target is reached. The ideal continuous-mixing model is exponential rather than the single-cycle formula:

C = Cᶠ + (C₀ − Cᶠ) × e^(−added volume ÷ mixed-system volume)

This model can estimate planning demand but does not authorize simultaneous operation. Actual short-circuiting depends on inlet and outlet locations, circulation, density, temperature, plumbing, overflow, and mixing. Backflow, unattended fill, overflow, discharge, pump, structural, and chemistry risks still control.

Do not market simultaneous dilution as “no-drain” when water is still being discharged.

Rain and overflow are not free dilution assumptions

Rain adds low- or variable-mineral water, but actual chemistry change depends on:

• surface area and rainfall captured;
• cover position and cover pumping;
• overflow or deliberate lowering volume;
• whether overflow water was fully mixed;
• runoff contamination;
• initial and rainwater chemistry;
• timing relative to circulation and sampling.

Use the rain-volume and mass-balance method only with a defensible water-accounting model. The Rain and Weather Response Reference provides the field workflow.

Structural and hydrostatic gate

Before any partial drain, your company must know:

• gunite, plaster, fiberglass, vinyl, acrylic, elevated, on-grade, hillside, or other construction;
• current shell, liner, finish, tile, fitting, light, cover, plumbing, and deck condition;
• groundwater level or credible risk, recent and forecast rain, irrigation, drainage, nearby excavation, slope, and flood history;
• hydrostatic relief design and who is qualified and authorized to assess or operate it;
• minimum water level and maximum rate permitted by the exact pool or spa and cover manufacturer;
• whether exposed finish can dry, heat, freeze, stain, crack, shrink, float, bulge, or move;
• what water supports the liner, shell, walls, steps, fittings, and automatic cover;
• refill supply, rate, expected duration, supervision, and failure contingency.

Never assume a visible hydrostatic valve will function, that a main drain is a relief device, or that partial draining cannot float or deform a pool or spa.

Fiberglass and vinyl systems can be especially sensitive to water removal. Follow the exact shell or liner manufacturer and qualified installer. A customer waiver does not transfer structural authority to an unqualified technician.

Equipment and access gate

The approved plan must address:

• pump and automation shutdown or safe operating path;
• skimmer, suction, return, light, heater, salt cell, cleaner, autofill, cover, and feature minimum levels;
• protection from dry running, suction air, loss of prime, heater or cell operation without flow, and remote restart;
• hose, pump, cord, GFCI, trip, traffic, child, pet, gate, and neighboring-property exposure;
• discharge hose restraint, pump intake, entrapment, debris, erosion, and backflow;
• continuous or specified supervision and high- and low-level stop points;
• power, water-supply, pump, hose, weather, and access failure contingency.

Do not leave a draining or filling pool unattended because the calculation predicts a completion time.

Discharge authorization gate

Pool discharge rules vary by local sanitary district, municipality, watershed, regional Water Board, property, and water condition. Before discharge, obtain current written or recorded direction for the exact address and destination.

Assess:

• free chlorine or bromine and dechlorination requirement;
• pH;
• salt, TDS, borate, calcium, CYA, metals, copper algaecide, pesticides, and other products;
• filter backwash, D.E., sediment, algae, contamination, or cleaning waste;
• sanitary sewer approval, cleanout and flow limit;
• landscape soil capacity, salinity tolerance, saturation, erosion, slope, foundation, septic, well, neighbor, and runoff risk;
• storm drain, gutter, street, channel, creek, ocean, wetland, and surface-water prohibition or conditions;
• discharge permit, notification, sampling, or record requirement.

Do not infer that dechlorinated water is automatically legal for a storm drain. Do not infer that a sanitary cleanout is approved merely because one is present. Saltwater and filter-backwash restrictions can be different from ordinary pool water.

Source-water gate

Test the actual source that will refill the pool, including when a truck, well, softened supply, blend, or alternate tap is proposed.

Record:

• pH;
• TA;
• CH;
• CYA where relevant;
• salt or TDS context;
• disinfectant;
• iron, copper, manganese, nitrate, or other local concern when material;
• temperature;
• source identity and treatment system;
• flow, availability, cost, restriction, and expected variation.

If fill CH already exceeds the target, replacement cannot lower CH below the fill concentration under the simple model. If fill water creates an unacceptable saturation, metal, salt, or supply condition, choose another qualified strategy.

Water-level and volume planning

Where the pool or spa plan area is approximately constant near the operating level:

gallons per inch = surface area in ft² × 7.48052 ÷ 12

For a 500 ft² surface:

500 × 7.48052 ÷ 12 = 311.7 gallons per inch

Then:

planned level change in inches = planned gallons ÷ gallons per inch

This shortcut is invalid where the plan area changes materially, a shelf emerges, a spa or basin transfers, the shell flares, a cover displaces water, or leakage or fill occurs simultaneously.

The qualified structural limit overrides the calculated level change.

Execution record

Before water movement:

• Initial and fill tests confirmed
• Target and complete predicted chemistry approved
• Pool or spa volume, surface area, and water-level relationship documented
• Structural, groundwater, liner or fiberglass, finish, cover, and equipment plan accepted by qualified owner
• Discharge destination and conditions approved
• Fill supply, rate, duration, supervision, and contingency established
• Weather and site access acceptable
• Customer use restriction and communication complete

During movement:

• Start time, level, meter, and equipment state recorded
• Discharge remains in the authorized destination with no backup, erosion, runoff, or neighbor impact
• Pool, shell, liner, cover, fittings, and surrounding structure show no movement or distress
• Pump, hose, cords, access, autofill, and water supply remain controlled
• Planned stop level and rate are not exceeded
• Any interruption or scene change is recorded

After refill:

• Final water level and actual amount reconciled
• Equipment restarted under exact procedures and stable operation verified
• Water mixed for the required condition
• Direct chemistry retested with valid methods
• Saturation, sanitizer, CYA, salt, TA, CH, and other targets recalculated
• Actual result compared with prediction before another cycle or chemical plan
• Discharge equipment, hoses, access points, cover, and property secured

When actual result differs

Do not immediately repeat the planned fraction. Check:

• pool volume and connected-system assumption;
• actual gallons and water-level measurement;
• initial and fill test validity;
• complete mixing and sample timing;
• leak, autofill, rain, evaporation, splash, overflow, or simultaneous water movement;
• constituent reaction, precipitation, degradation, adsorption, or source variation;
• wrong pool or spa sample;
• unit or transcription error.

The mismatch is evidence. Correct the model and restore safe chemistry before choosing another action.

Common failures

• assuming fill concentration is zero;
• averaging pH like a conservative ppm value;
• adding separate replacement percentages for CYA and calcium instead of choosing one fraction and predicting both;
• treating two 25% cycles as one 50% cycle;
• calculating total pool gallons but draining an isolated spa or basin;
• using coping dimensions and guessed depth for gallons per inch;
• draining first and asking about groundwater later;
• sending dechlorinated salt water or backwash to a storm drain without local approval;
• leaving an autofill, hose, submersible pump, or automatic control unattended;
• refilling with hard or metal-bearing source water without testing;
• using customer permission as structural, environmental, or licensing authority.