Do I need dishwasher salt and rinse aid, or are all-in-one tablets enough?

The three products do three unrelated jobs and get muddled because they are sold together. Salt regenerates an ion-exchange resin inside the machine, which softens the incoming water. Rinse aid is a surfactant that makes water sheet off rather than bead, so items dry without spots. Detergent does the actual cleaning. An all-in-one tablet bundles detergent with a chemical water softener and a rinse aid, and the bundling is where the confusion starts.

Step by step

1. Find out how hard your water is.Your water company publishes the hardness for your postcode, usually in mg/l as calcium carbonate or in degrees Clark or German. Anything above roughly 200mg/l is hard, and most of the south and east of England is well above that. This single fact decides the whole question.
2. Check whether your machine has a salt reservoir at all.A screw cap in the base of the tub, usually next to the filter. If it is there, the machine has an ion-exchange softener and it needs salt to work. A few cheap machines have no reservoir, in which case tablets are all you have.
3. Set the machine's water hardness setting to match your supply.Almost nobody does this and it is in every manual. The setting controls how often the softener regenerates. Set too low it under-softens; set too high it wastes salt and can over-soften, which is one cause of etched glassware.
4. Fill the salt reservoir with dishwasher salt only.Coarse, very pure sodium chloride. Table salt contains anti-caking agents that clog the resin, and rock salt for paths contains grit. Top it up when the low-salt light comes on, or every four to six weeks in a hard water area.
5. Expect water to be in the salt reservoir and do not try to dry it.The reservoir is meant to hold brine. Pour the salt in through a funnel, let the displaced water overflow, wipe the threads and cap it firmly. Run a rinse cycle straight afterwards so no spilt salt sits in the tub.
6. Add rinse aid if items come out spotted or plastics come out wet.It lowers the surface tension so water sheets off instead of forming droplets that dry into marks. On a machine with condensation drying it makes a visible difference. On one with zeolite or heated drying it matters less.
7. Turn the rinse aid dose down if you see streaks or a slippery film.There is a dosage dial under the rinse aid cap, usually 1 to 6. Too much rinse aid leaves a rainbow film on glass and a slick feel on cutlery. Start in the middle and move one step at a time.
8. Turn both settings off deliberately if you have a whole-house softener.Softened mains water does not need the machine's softener, and running it anyway can over-soften the water and etch glass. Set the hardness setting to its lowest and stop adding salt, but keep using rinse aid.

Tips

• Buy salt in the largest bag you can store. It is one of the cheapest consumables in the house and running out is what causes the expensive damage.
• The salt and rinse aid warning lights on many machines are disabled when the machine is set to use all-in-one tablets. If your lights never come on, check that setting before concluding the reservoir is full.
• Salt does not go in the detergent drawer and detergent does not go in the salt reservoir. It sounds obvious; the two openings are close together and the mistake is common enough that manufacturers warn about it.

Common mistakes

• Relying on all-in-one tablets in a hard water area — The softening agent in a tablet treats the water in that cycle. It does not regenerate the machine's ion-exchange resin, so limescale still builds on the heating element — which is the failure that ends the machine's life.
• Using table salt because it is the same thing — It is not. Table salt contains anti-caking agents and often iodine, both of which foul the resin bed. Dishwasher salt is coarse and pure specifically so it dissolves slowly and leaves nothing behind.

Questions people ask