If you use boiled, you're likely to get tiny bits of scale from the kettle in the iron...not good. Cheaper than distilled is "de-ionised" water...cheap as you like (Asda has it).
If it's a cheap steam iron (which you only expect to last a year or two) you might just as well use water straight from the tap (especially if you don't live in a hard water area).
Boiled (and preferably filtered) water would be slightly better but distilled water is available from motoring accessory stores and from old-fashioned hardware shops.
It is not a good idea to use de-ionised water in an iron. If you live in a hard water area, buy some distilled water from a motor accessory shop. De-ionised water is very pure and can cause problems due to the absence of minerals in it. I can't see why DI water should be cheaper than distilled water, as the process used to produce it (typically reverse osmosis) is nowhere near is simple as distillation.
Electrochem: "de-ionised water is very pure"...isn't that the idea? And, isn't distilled water very pure.
Cost-wise...doesn't distilling water use a lot of (expensive) energy?
Gingerbee: Distilling water does not use a great deal of energy and distilled water is nowhere near the purity of DI water. It is mostly based on electrical conductivity, which is dependent on the ionic strength of the solution. This is the main reason that you cannot measure the pH of DI water with a pH meter. Distilled water is nowhere near as pure as DI water. It is possible to use ion exchange resin to produce DI water, but it isn't cheap and needs to be regenerated, so cost wise it is cheaper to produce distilled water. Have a look at the information on here
Zeuhl...no, Halfords sell "battery top-up water", which is de-ionised water. Why? Cost. Contrary to statements above, de-ionised water is FAR cheaper to buy than distilled and is recommended for batteries AND irons.
And, as for the purity claims above, .....more tosh!
Do people still buy stuff from Halfrauds ? I'm shocked they are still in business the prices they charge. Go to any motor accessory shop for the same products at cheaper prices !
Smurfchops, do you have a condenser clothes dryer? The water in the collecting tray is what I use in my steam iron and topping up my car battery.
Gingerbee. I would like to know what qualifications you have in the sciences. I have worked in labs for over forty years and I can buy or produce distilled water for a lot less than I can produce DI water. I don't intend to spend ages demonstrating with price lists and explanations to someone who clearly is not interested in listening.
Electrochem: I really don't need to listen to you. I have a degree in Molecular Sciences (Warwick University) and have managed laboratories in the Water Industry for many years.