Salt water pool basics

How a salt cell works

The salt chlorine generator (salt cell, or SWG) is a set of metal plates in the return line. A low-voltage current across the plates splits some of the dissolved salt (sodium chloride) and water, producing chlorine in the water — the same active chlorine you would get from a jug of liquid chlorine — plus hydrogen gas that bubbles away.

Here is the part that surprises new owners: the salt is not used up. When the chlorine has done its job, killing germs, oxidizing sweat or breaking down in sunlight, it ends up as chloride again. The salt goes round and round. It leaves the pool only with water: splash-out, backwashing, rain overflow, draining, and leaks. Evaporation takes water but leaves the salt behind.

So you add a big load of salt once, at start-up, and then small top-ups as water is replaced. Most of the season’s work is setting the cell’s output so the chlorine level holds.

cell:   salt + water → chlorine + hydrogen (gas)
use:    chlorine → chloride (salt) again
salt (lb) = gallons × 8.34 × ppm rise ÷ 1,000,000

What salt level to aim for

There is no universal number; each cell maker publishes its own range, and the warranty assumes you follow it.

Every 1,000 ppm of salt in 10,000 gallons is about 83 lb, so the arithmetic is simple. Pentair’s manual lists 300 lb per 10,000 gallons to go from zero to 3,600 ppm, which matches.

Worked examples with bag counts: how much salt for a 10,000 gallon pool, how much salt for a 15,000 gallon pool and how much salt for a 20,000 gallon pool. Use pool-grade salt: high-purity sodium chloride without anti-caking agents or iodine. Spread it over the shallow end with the pump running, brush until it dissolves, and follow the cell manual on how long to wait before reading salt.

Stabilizer and chlorine in a salt pool

An outdoor salt pool still needs stabilizer (cyanuric acid, CYA). Without it, midday sun destroys chlorine faster than a typical cell can make it. Salt pools often run CYA higher than pools dosed by hand, around 60–80 ppm, so more of the chlorine is held in its sun-resistant form and the cell can keep up at a modest setting.

With more stabilizer you need more free chlorine — but a cell supplies it continuously rather than in a once-a-day spike, so the hobbyist convention from Trouble Free Pool sets the salt-pool minimum at 5% of CYA instead of the 7.5% used for manual dosing: about 3.5 ppm at 70 ppm CYA. That is a forum convention, not a code; the CDC minimum with stabilizer is 2 ppm, and your cell manual may give its own CYA range. The full table is on what CYA and chlorine a salt water pool needs.

pH drift, and why salt pools buy acid

pH in a salt pool tends to creep upward, so acid becomes a regular purchase. The chlorine cycle itself is roughly pH-neutral — making chlorine from salt releases as much base as using that chlorine up releases acid. The upward drift comes from carbon dioxide leaving the water: the hydrogen bubbles from the cell, plus waterfalls, spillovers and jets, strip CO₂ out, and losing CO₂ raises pH. The higher the alkalinity, the more CO₂ the water carries and the faster it climbs.

• Test pH often, at least a couple of times a week; the CDC range is 7.0–7.8.
• Lower it with acid, in parts, with the pump running. How much muriatic acid to lower pH in a 10,000 gallon pool shows a typical dose; labels cap single additions (one says 16 fl oz per 10,000 gallons at a time).
• If pH keeps racing up, alkalinity is often on the high side. Lowering it a little slows the drift: see how much muriatic acid to lower alkalinity in a 10,000 gallon pool.
• Less splash, less drift. Running a waterfall only when you’re enjoying it keeps pH steadier.

Salt top-ups, and salt you didn’t mean to add

Retest salt after heavy rain, a big backwash or a partial drain, and after any long stretch of topping up with the hose. The cell’s own salt reading can drift, so check it against salt test strips now and then before adding a bag.

Liquid chlorine and bleach add salt too. Each ppm of chlorine from sodium hypochlorite leaves about 1.65 ppm of salt once it is used. If you supplemented a salt pool with an example 3 ppm of liquid chlorine a day for a month, that would add about 148 ppm of salt; over a few months, enough to push a pool from the middle of its range toward the top. Factor it in before adding a bag.

Shocking a salt pool

Many cells have a “boost” or “superchlorinate” mode that runs the cell at full output for a day. That helps after a party, but a cell can’t raise chlorine fast enough to clear algae. For a green or cloudy pool, use liquid chlorine to reach and hold the shock (SLAM) level — about 40% of CYA by the Trouble Free Pool method, so 28 ppm at 70 ppm CYA.

In a 15,000-gallon pool at 70 ppm CYA, going from 4 ppm to 28 ppm is about 2.88 gal (11.5 qt) of 12.5% liquid chlorine for the first dose alone. That is the cost of running stabilizer high, and one more reason to keep it from creeping past where it needs to be. How much liquid chlorine to SLAM a pool works through the method; how to shock a pool covers the rest.

Too much salt? Drain and refill

Salt doesn’t evaporate, filter out or break down, so the only way down is replacing water. With fresh fill water that has little salt in it, the share to replace is 1 − (target ÷ current). From 4,000 ppm down to Hayward’s 3,200 ppm that is 20% of the water — about 3,000 gallons in a 15,000-gallon pool. How much water to drain to lower salt works it out with your own fill-water reading.

What it costs to run

A salt pool trades the monthly chlorine bill for the cell’s electricity, the salt, more acid, and eventually a replacement cell. The pool running cost calculator has a salt-cell line; how much it costs to run a pool per month works through an example budget. For the whole picture, see pool running costs.