Join Date: Nov 2007
Location: Lutterworth, UK
There are basically three types of 'water' that you can use in coolant.
De-ionised (note spelling). This has been filtered to avery high degree to remove particulates and molecules and then also ions of calcium, sodium, chlorides, fluorides, bromides, sulphates, carbonates etc., as well as H+ and OH- ions to give you H2O in its purest form.
Distilled. This has been boiled and the steam condensed to (mostly) remove all of the above elements, but will still contain some impurities.
Tap. In the UK this is pretty good stuff. Purity depends on locatin within the UK and time of year. Much, much purer than any Evian type product.
The recommendation to use distilled or de-ionised water comes from the practice that this prevents a 'furring up' of the channels as you tend to get in kettles. But guess what? Unless you have to top up your coolant a lot every day, then furring-up is not a problem. Paraflu, etc, will contain sufficient inhibitors, anti-foaming agents and surfactants to control any issues that tap water may bring up.
Hope this will eliminate some of the myths that surround coolant...
RIP 147 1.6 Lusso