Salt water vs chlorine pool

Salt water or chlorine: what is the real difference?

Water passes through a salt cell, a stack of coated titanium plates in a clear housing that a control box feeds with a low-voltage current. The current splits the salt in the water to make chlorine, which sanitises as it would from a tablet and then turns back into salt. Nothing is used up but the cell itself; salt leaves the pool only when water does, through splashing, backwashing and rain.

The other route is a container. Stabilised tablets in a floater or a feeder, granules, or liquid chlorine, added by hand or by a dosing pump. Someone buys it, carries it and stores it, and the chlorine in the water rises when it is added and falls with sunlight and swimmers. Everything else in the pool is the same: circulation, filtration, a pH in band, a shock after a party or a storm.

What the water feels like

Salt pool water tastes of salt if you swallow some, but less than you expect: at about 0.4% it is a tenth as salty as the sea. Many swimmers find it softer on skin and hair, and it tends to carry less of the sharp smell that people call chlorine, which comes from chloramines, made when chlorine meets sweat and sunscreen, rather than from chlorine itself.

A chlorine pool kept in balance, with a filter that is working, smells of very little either. So feel is part of salt water's appeal and also a matter of taste; if you can swim in a friend's pool of each kind before you decide, do.

A week with each

With a salt cell you test pH and free chlorine every week, and the salt, the stabiliser and the cell's reading now and then. The cell makes chlorine only while the pump runs, so its output is a setting you adjust to the size of the pool and the season, and most control boxes have a boost for after a party. One habit differs: the cell's chemistry pushes pH up, so a salt pool needs acid more often than one fed with tablets, which are acidic and pull it down, and many owners dose the acid automatically.

With tablets or liquid you test the same two numbers and add the chlorine yourself. Tablets are the easy way and a trap: each stabilised tablet adds cyanuric acid, which does not evaporate, and after a few seasons chlorine stops performing at normal levels, as the maintenance guide warns. A salt cell adds none, so the stabiliser is set once and tested once a year.

A cell needs looking after in its own way. Hard water scales its plates, so check them now and then and clean them as the manual says, and a control box that shows low salt, low flow or a worn cell is telling you something that a tablet feeder cannot.

What each costs, without a price list

Salt costs more to install and less to run month by month, which is how the cost guide puts it, and that is the whole sum. The extra at the start is the cell, its control box, the plumbing and electrics to fit them, and the salt itself, 4 kg for every cubic metre: about 47 kg for a 13 × 8 ft (4 × 2.5 m) plunge pool, about 91 kg for a 20 × 10 ft (6 × 3 m) pool and about 175 kg for a 26 × 13 ft (8 × 4 m) one, which is two, four and seven 25 kg bags. Prices of cells and salt move, so ask a supplier for the ones for your volume; this page gives the quantities only.

Afterwards the monthly cost is salt to replace what splashing, backwashing and rain take out and the cell's electricity, against a tablet or a measure of liquid every week. The renewal arrives every five to seven years, when the cell's plates are spent. Size the cell from the water-line volume on your plan page, not the brochure's brim-full figure, and a step up if the pool is warm, used by many or left uncovered. A pool swum only from May to September has fewer months to earn the cell back than one a family runs from April to October. Both sums are yours to do with your own tariffs.

Stone, metal and finishes: what salt does to the surround

Water splashed on a deck dries to a crust. With chlorine that crust is harmless to most stone, though it is hard on grout, liners and covers; with salt it is salt, which is what weathers natural stone. As salt water moves through a porous stone such as travertine or limestone and dries, it leaves crystals at and below the surface that flake the stone, roughen it and lift sealers, and in a frost they do it faster. Sealing helps and has to be renewed, and dense stone, granite or porcelain, is far more tolerant. Of the copings the designer offers, travertine is the one this bites hardest and granite the one it barely touches; concrete lies in between and depends on its mix and its sealer.

In the splash zone salt also finds a weak steel: the ladder, the handrail and their sockets, light fittings, cover tracks, the screws in a skimmer lid. Ask for AISI 316 stainless; 304 freckles with rust spots within a couple of seasons, as the steps guide says, and a ladder anchored in the deck is expensive to change. Equipment in the plant room is out of the splash but not out of the salt, so ask each maker whether its pump, filter, heat pump and valves are rated for salt water. A warranty that excludes it is a reason to choose chlorine.

Plaster, tile and fiberglass generally cope with salt at pool levels, and a liner's maker will say. Ask how your finish holds up before you buy, because the construction guide tells you to. Then look at the garden: salty splash and backwash water do the lawn and the planting no good, and in some places the rules limit where salted water may be drained, so ask.

Cold water, and the months you do not swim

Most salt cells make little or no chlorine in cold water, below roughly 10–15 °C, and the maker's manual says where. In a pool swum in spring and autumn, or kept open through a mild winter, the cell stops being the sanitiser just when a cover is on and the water is still, so you need another route for those months: a tablet feeder or hand-dosed liquid. Owning both is no great hardship, and closing the pool for the winter works the same for a salt pool as for any other.

The cell also depends on the pump. It makes chlorine only while water flows, so a pump that stops, a blocked filter or a tripped flow switch means no sanitiser at all, where a pool on tablets just needs someone to dose it. That matters most when you are away.

What it decides in the plant room

The choice is made at the drawing board because the cell goes in the pipework. It sits in the return line after the filter and the heater, where the water is clean, in a straight run with room to take it out, with a flow switch, a bypass if its maker wants one, a tap and a drain for washing it, and mains power for its control box within reach of its cable. A pool planned for tablets needs none of that.

PoolPlanner draws the pool, its edge, steps and deck, not the plant room, so this belongs in the brief you give the builder: space on the wall near the return line, a spare earthed socket, and the cell sized from the water-line volume. If you are unsure, plan for the cell and fit it when you know; it is much cheaper than moving a pipe.

Plan the other route as well. Whatever you dose, the containers need a place of their own, and the labels say how to keep them: chlorine and acid apart, off the floor, upright and ventilated.

Which to choose

Choose a salt cell if the pool will be swum for most of the season, if you like steady water and no containers to carry, and if the surround is granite, porcelain or a sealed stone and the hardware is 316 stainless. Choose tablets or a dosing pump if the pool is small or used a few months a year, if the surround is natural stone you love, or if the budget is tight. Either way, settle it before the plumbing is drawn: the cell needs a place in the line and a socket, and a chlorine pool does not.