Partial Drain and Dilution Decision
Purpose
Some numbers in a pool cannot be dosed downward. Stabilizer, calcium hardness, salt and dissolved metals only leave in the water that carries them, and the two ways shops get that wrong are opposite. One is selling a product for a parameter no product moves. The other is draining first and testing the fill water afterwards, which is how a technician removes half a pool and lands a few points from where they started. This procedure guarantees the parameter is one dilution can move, the fill water's own value is measured before the fraction is computed, the drain depth is gated on the shell and the groundwater, and the discharge has an approved destination before a pump starts.
Scope
Covers planned partial water replacement on residential and small semi-commercial pools to reduce a dissolved parameter: the decision, the arithmetic, the drain depth limit, discharge, refill and reconciliation.
It does not cover a full drain, which the drain and acid wash standard owns along with its hydrostatic gate, and it does not run on vinyl at all, where the liner shrinks and the vinyl liner standard owns the exclusion. It does not set stabilizer targets, which the cyanuric acid management standard owns, nor saturation targets, which the calcium hardness reference owns. Emergency dumping of an unrecoverable pool is the green pool recovery program.
Roles and responsibilities
| Role | Owns | Hands off |
|---|---|---|
| Lead technician | Fill water test, the fraction arithmetic, drain depth, discharge setup, reconciliation | Hands the achievable floor to the service manager BEFORE the customer hears a target, not after the water is gone |
| Service manager | Discharge approval on file, the customer conversation when dilution cannot reach the target | Hands the customer the fill water result, because that number is the one that limits what they can buy |
| Route technician | The trend that triggered this in the first place | Hands the last three readings of the parameter forward so the starting value is not a single test |
Procedure
Confirm the parameter is one dilution moves, and that it is out of range on more than one reading. Stabilizer, calcium hardness, salt, total dissolved solids and dissolved metals leave only with the water. pH, total alkalinity and chlorine do not need a drain and must not be used to justify one. Accept when the parameter is on the dilution list and two readings on separate days agree. Wrong is a drain sold off one strip reading, which is how a shop dumps water over a bad test. Hazard: none at this step, it is a decision made at the truck with a test kit.
Test the FILL water for the same parameter and compute the achievable floor before anything else. Draw a fill sample at the hose bib after running it to temperature and test it with the same kit. The floor you can reach by dilution is the fill water's own value, and the replacement fraction is the difference between current and target divided by the difference between current and fill. Accept when the fill value is recorded and the target sits above it with margin. Wrong is a target at or below the fill water's own number, which makes the required fraction one whole pool volume or more: stop, do not drain, and route the job to mobile reverse osmosis treatment or revise the target with the service manager. Hazard is a scald at a hot fill line, so run it before filling the vial.
Compute the fraction and the volume, and choose batch replacement over simultaneous drain-and-fill unless the shell forces it. A batch drain followed by a refill removes the computed fraction directly. Draining and filling at the same time mixes, so assuming complete mixing you need roughly 0.69 pool volumes to halve a parameter against 0.50 by batch, which is about a third more water for the same result. Accept when the fraction, the gallons, and the method are written on the ticket before a pump starts. Wrong is a volume estimated from the customer's memory: measure or take it from the property record, because every figure downstream is a percentage of it.
Gate the drain depth on the shell type and on standing groundwater, and split the job into batches rather than exceeding it. Auger a test hole near the shell and confirm it stands dry. Set a minimum remaining depth over the deepest point and do not go below it; a workable shop default is never below one third of the deep-end depth, tuned to your soils and your water table. Accept when the target level is marked on the wall before draining starts and the test hole is dry. Wrong is one large drain that beats the depth gate: split it into two smaller batches instead, and accept that two batches use more total water than one, because a floated shell is a total loss rather than a repair. On vinyl this step does not run at all.
Confirm the discharge destination in writing and set it up before the pump starts. Chlorinated, salted, low-pH pool water does not go to a storm drain, a creek or a neighboring property. Accept when the destination is a sanitary cleanout or another route the local sewer district or water authority has approved for this address, with the approval reference on the job, and the receiving system's flow limit is known and the pump throttled to it. Wrong is a hose to the street because the cleanout is awkward, which is a violation in most jurisdictions and a fine that follows the shop. Watch where the discharge lands even on an approved route: a hose left running at a slope undercuts it, and a hose across a walkway is a fall the customer takes, so route it and flag it.
Shut the pump down and lock it out, then drain to the marked level with the submersible on ground-fault protection. Open the pump disconnect and apply a lock and tag under 29 CFR 1910.147 before the level drops, because the hazard is the circulation pump running dry and the suction at fittings that will be exposed. Plug the submersible into ground-fault protected supply, keep the cord out of standing water on the deck, and never reach into the water to reposition it while it is energized. Accept when the level reaches the mark and the pump is unplugged before it is lifted. Wrong is a circulation pump left in auto that starts on a dropped level and cooks its seal in minutes.
Refill, restore the pump, retest, and reconcile the measured result against the prediction. Refill to between one third and one half up the skimmer mouth, remove the lockout only when the outlet covers are confirmed with every fastener and nobody is in the pool, restart from beside the strainer pot, and circulate at least one full turnover before sampling. Accept when the parameter reads within a reasonable margin of prediction and the pump primes with clear returns. Wrong is a reading far above prediction, which usually means an unmixed sample or a wrong volume: resample after another turnover before adding a batch. Then rebalance pH, alkalinity and sanitizer, because fill water arrives with its own numbers.
Three ways to take dissolved load out, and when to pick which
| Method | What it does | Water used | Main risk | Pick it when |
|---|---|---|---|---|
| Batch partial drain and refill | Removes the computed fraction directly, then replaces it | Lowest for a given reduction | Shell float and liner shrink at depth | Gunite shell, dry test hole, approved discharge, fill water well below target |
| Simultaneous drain and fill | Mixes as it runs, so reduction follows a decay rather than the fraction | About a third more for a halving, assuming complete mixing | Long run time, easy to lose track of level | The depth gate will not allow a real drawdown, or the shell is marginal |
| Mobile reverse osmosis | Filters dissolved solids out of the pool's own water and returns it | Little to none discharged | Vendor availability, longer on site | The fill water is at or above the target, or discharge is not permitted here |
The row that changes most shops' habits is the third. Once the fill water's own value sits at or above the target, dilution is arithmetically impossible no matter how much water you move, and the only route left is treating the water rather than replacing it. That is why step 2 comes before the arithmetic.
The record this produces
The water replacement record, attached to the property, because the fill water value and the discharge approval are reused on every future drain here.
Fields: the parameter, with the last three readings and their dates; the fill water value with the sample time and kit used; the target and the achievable floor; the pool volume and where it came from; the computed fraction and gallons, shown as the arithmetic rather than the result alone; method chosen with the reason; test hole location, depth and dry result; the marked minimum level and the level actually reached; discharge destination and approval reference; pump lockout on and off times; refill completion; and the post-turnover retest with the predicted value beside it.
The service manager reads the fill water value before promising a target on this property again. The next technician reads the discharge approval reference rather than re-negotiating it with the district. The office keeps the prediction and the measured result side by side, because that pair is what proves the job did what it was sold to do.
Worked pass: one pool, two out-of-range parameters, one of them unreachable
A 16 by 32 ft gunite plaster pool, roughly 20,000 gal from the property record. Stabilizer reads 110 ppm and calcium hardness reads 750 ppm, both confirmed on two separate days. Targets set by the service manager are 50 ppm stabilizer and 350 ppm calcium hardness.
Step 1: both parameters are on the dilution list and both are confirmed on two readings, so the gate passes for both.
Step 2 FAILS for calcium hardness. Fill water, sampled at the hose bib after running to temperature, reads calcium hardness 380 ppm and stabilizer 0 ppm. The required fraction for hardness is 750 minus 350, which is 400, divided by 750 minus 380, which is 370, giving 1.08 pool volumes. That is more than the whole pool, because the fill itself sits above the target. Stop rule taken: no drain is sized for hardness, and the service manager quotes mobile reverse osmosis with the fill water result attached.
Stabilizer passes the same gate. Fill reads 0 ppm, so the fraction is 110 minus 50, which is 60, divided by 110 minus 0, which is 110, giving 0.545. That is 54.5 percent of 20,000 gal, about 10,900 gal in a single batch.
Step 4 rejects the single batch. The test hole augered near the shallow-end wall stands dry over an hour, so draining is permitted, but drawing 54.5 percent out at once would take the water below the one-third deep-end mark. The job is split into two batches of 35 percent, and the extra water is the cost of the gate: 0.35 times 20,000 is 7,000 gal each, so 14,000 gal total against 10,900 gal in one batch.
Step 5: sanitary cleanout at the side of the house, district approval reference already on the property record, submersible throttled to the cleanout's stated limit, hose routed off the walkway and flagged.
Step 6: circulation pump disconnect opened and locked at 08:15 for batch one, submersible on a ground-fault protected receptacle, cord off the wet deck, unplugged before lifting.
Step 7: refilled to mid-skimmer, covers confirmed, lockout off at 14:40, restarted from beside the pot, one turnover before sampling. After batch one, stabilizer reads 72 ppm against a prediction of 110 times 0.65, which is 71.5 ppm. Batch two runs the following week; the prediction is 71.5 times 0.65, which is 46.5 ppm, and the measured value is 48 ppm. Within margin, so no third batch runs. pH, alkalinity and sanitizer are rebalanced, because 14,000 gal of fill water brought its own numbers.
References
- Local sewer district or water authority requirements for pool water discharge; chlorinated, salted or low-pH water to a storm drain or surface water is prohibited in most jurisdictions and approval is site-specific.
- 29 CFR 1910.147 for lockout of the circulation pump while the water level is drawn down, where the controlled hazard is pump motion and exposed suction fittings.
- NEC Article 680, in the edition your jurisdiction has adopted, for ground-fault protection on receptacles serving pool equipment, including a portable submersible pump used at the pool.
- See related: the PoolService cyanuric acid management standard, the calcium hardness and scale management reference, the drain and acid wash standard for a full drain, and the vinyl liner standard for why this procedure excludes vinyl.