Salt or chlorine: the salt water myth

At the point of sale the line is always the same: "with salt you can forget about chlorine". It is a very good line and it is false, and it is worth understanding exactly in what sense, because a salt system has genuine advantages that get lost behind a promise it cannot keep.

A salt pool is a chlorine pool. The difference is not in the disinfectant: it is in where it is made. In a conventional pool you buy chlorine and add it. In a salt pool, a unit makes it inside the pipework, out of salt dissolved in the water, while the pump runs.

What the cell actually does

Salt water passes through a cell with two electrodes and a direct current. The chloride in the salt is oxidised and forms, on contact with water, hypochlorous acid: precisely the same substance that disinfects when you tip hypochlorite in from a tub. Not a relative, not a substitute: the same molecule.

And there is a second leg of the cycle that explains the feeling that "salt never runs out": once the chlorine has done its work, it turns back into chloride. The salt is recycled. It is not consumed: it is lost, with splash-out, with filter backwashing, with rain spilling over the edge, and with every draining.

Why it does not taste of the sea

Because it is nowhere near. A salt pool works in the region of 3,000 to 4,000 mg/L of salt, whereas sea water sits around 35,000 mg/L: roughly a tenth. That is the whole reason for the soft feel of the water, and it is a real and perfectly legitimate advantage.

What salt genuinely buys you

• —Continuous dosing instead of spikes. Chlorine goes in a little at a time, rather than in a slug every two or three days. A steady level irritates less than one that rises and falls, and much of what people call "chlorine allergy" is in fact a reaction to the chloramines that appear when the level drops.
• —Less handling of product. Nobody carries or stores tubs of hypochlorite, which is a real domestic hazard and a source of accidents.
• —The feel of the water. It is subjective and it is true: at 3,000 mg/L water feels different, and plenty of people prefer it.
• —It satisfies, out of the box, what the rule asks of a dosing system. This is the strong argument and hardly anybody uses it.

What it genuinely costs

• —The cell is a consumable. It wears out, it scales up with calcium, and the day comes when it is replaced. It is the item missing from the initial comparison and the one that settles the ten-year cost. Ask in writing for the service life the manufacturer declares and the replacement price of that cell, not of a generic one.
• —It only produces while the pump runs. This is the operating trap of the system: if you cut pumping hours to bring the electricity bill down, you are cutting chlorine production too. The two adjustments are tied together and almost nobody knows it.
• —Acid. pH rises steadily and has to be corrected, so the saving on chlorine is partly offset by what is spent correcting pH.
• —Chloride is corrosive. At 3,000 mg/L, water attacks materials that stand up perfectly well in a conventional pool.
• —Draining gets complicated. Water at 3,000 mg/L of salt does not go on the garden: it salts the soil and kills the planting. With no sewer connection, discharge has to be solved beforehand, not afterwards.

What has to be measured in both alike

Not one of these duties changes because the pool runs on salt. They are the same for both, and the frequencies are the part most often breached:

The three lines you will hear

1. 1"It is chemical-free water." No. It is water with chlorine made on site and salt in solution, plus the acid you have to add to correct the pH the system itself pushes up.
2. 2"Nothing needs measuring, the unit handles it." The unit produces; the rule requires measuring and recording, and the unit does neither. A badly calibrated cell under-produces for weeks without saying so.
3. 3"It works out cheaper." Depends on the horizon. Less chlorine and more acid, more electricity, and a cell that gets replaced. The honest comparison runs to ten years and has the cell replacement price on the table.

How to decide

• —New pool, frequent family use, materials specified from the design stage: salt is a very good decision, and its best argument is continuous dosing.
• —A community of owners currently dosing by hand: salt settles the automatic dosing requirement in one move, but it calls for a prior review of ladders, fixings, coping and equipment. And it excuses not one single measurement.
• —An existing pool with stainless steel on show, natural stone coping or heating already installed: think twice. Here a conventional system, properly dosed through an automatic doser, is usually the sensible call.

The person who leads this line of work at the practice brings fifteen years of designing hydraulic systems and bodies of water, and the conclusion on this subject is always the same: salt is a dosing method, not a different chemistry. Choose it for what it actually does — dose without spikes and without handling product — and specify the materials accordingly. Choosing it for what the advertisement promises is how you arrive at a pitted ladder four years later.